SnTiO3 materials, methods of making the same, use of the same as ferroelectric materials, and devices comprising ferroelectric materials

Abstract

The invention relates to a material of the formula SnTiO3 having a crystal structure comprising layers, wherein the layers comprise Sn(II) ions, Ti(IV) ions and corner- sharing O6-octahedra, the corner-sharing O6-octahedra form sub-layers, the Ti(IV) ions are located within 2 / 3 of the corner-sharing O6-octahedra to form corner-sharing TiO6-octahedra, the corner-sharing TiO6-octahedra form a honeycomb structure comprising hexagons within the sub-layers, the hexagons have Ti(IV)-voids within, the Sn(II) ions are located above and below the Ti(IV)-voids with respect to the sub-layers and the crystal structure satisfies at least one of the following features (i) and (ii): (i) the Sn(II) ions have a tetrahedral coordination shell, the tetrahedral coordination shell contains three O ions of the layer and an electron lone pair of the Sn(II) ion, the electron lone pair is located in a top position with respect to the three O ions of the layer, (ii) the layers are stacked in a specific manner.

Description

[0001] This application is a divisional application of application number 201980030377.2 filed on May 2, 2019, entitled "SnTiO3 material, preparation method thereof, use therein as ferroelectric material and device including ferroelectric material". Technical Field

[0002] This invention relates to tin titanate, having the formula SnTiO3 or SnTi 1-x M x Materials containing O3 (with M and x as defined herein), methods for their preparation, their use in ferroelectric elements, and devices including ferroelectric materials. Background Technology

[0003] Titanates have proven to be one of the most diverse material classes, with applications ranging from ferroelectrics and photocatalysts to thermoelectrics and batteries. In particular, SnTiO3 is a promising ferroelectric material.

[0004] Typically, the implementation of Sn(II) in solid-state chemistry remains challenging due to its low disproportionation temperature of approximately 350 °C, leading to Sn(II) and Sn(IV). However, the preparation of novel materials usually occurs via solid-state reactions at high temperatures. Therefore, the successful preparation of tin(II) oxide is rare. Currently, there is no method for preparing bulk SnTiO3.

[0005] In their paper, *Chemical Science* 6 (2015) 1, Sirajudeen P. et al., claimed to have prepared SnTiO3 with a perovskite-type structure via a co-precipitation peroxide method. In this method, the precipitate was calcined in a muffle furnace at 800 °C for 1 hour. This temperature is significantly higher than the disproportionation and oxidation temperatures of Sn(II). Therefore, under the given (oxidation) conditions, Sn(II) would be oxidized to Sn(IV), thus ruling out the formation of a Sn(II)-containing structure. Furthermore, the claimed formation of the material was not verified by the analytical methods described herein, such as X-ray or electron diffraction.

[0006] Kumada et al., Materials Research Bulletin 44 (2009) 1298, described the preparation of bulk tin(II) titanate with the formula Sn2TiO4. Y. Hosogi et al., Acta Physico-Chimica Sinica C 112 (2008) 17678, described the preparation of layered Sn-exchanged alkali metal titanates and niobates by solid-state reaction.

[0007] More particularly, R. Agarwal et al., Physical Review B 97 (2018) 054109, and S. Chang et al., Journal of Vacuum Science & Technology A 34 (2016) 01A119-2 describe the preparation of SnTiO3 thin films by atomic layer deposition and report on the ferroelectric properties of the deposited films. Chang et al. point out the lack of distinct peaks in the X-ray diffraction pattern of the SnTiO3 films, which prevents a clear conclusion on the structure of SnTiO3. Agarwal et al. use the method of Chang et al. and conclude from the X-ray diffraction pattern that the obtained SnTiO3 has an orthorhombic perovskite structure. However, the fitting of the X-ray diffraction pattern and the simulated pattern can not clearly elucidate the structure of the prepared SnTiO3 material.

[0008] T. Fix et al., Crystal Growth & Design 11 (2011) 1422 report on the preparation of a thin film material with a composition of SnTiO3 by pulsed laser deposition. The authors conclude from the X-ray diffraction pattern and TEM images that the material has a triclinic ilmenite-type structure. The material prepared by T. Fix et al. does not exhibit ferroelectric properties at room temperature. In this ilmenite-type structure, both Ti and Sn show an octahedral coordination sphere with a lattice parameter c of 1.456 nm, which is comparable to the c of 1.408 nm of ilmenite FeTiO3 itself. Thus, the structure reported by T. Fix et al. cannot be regarded as a layered structure, i.e. a structure comprising layers, in which the atoms within a layer have a shorter distance compared to the distance to the atoms of the adjacent layer.

[0009] Therefore, to the best of the inventors’ knowledge, a material of the formula SnTiO3 has only been prepared as a thin film by atomic layer deposition or pulsed laser deposition. The industrial applicability of these techniques is limited. More particularly, the growth of thick films by atomic layer deposition or pulsed laser deposition is a slow and expensive process, since it requires many cycles, and the total reaction time depends on the cycle time.

[0010] Furthermore, the structure of the material depends to a large extent on its preparation process. More particularly, the crystal structure and crystallinity of the material obtained by atomic layer deposition are known to be influenced to a large extent by the deposition conditions, the reactants and the substrate. The stability of the layer depends on the interaction with the substrate, and the separation of the layer from the substrate can lead to a change in the structure of the layer, or even to a complete failure. SUMMARY

[0011] The present invention aims at providing a method for the preparation of bulk SnTiO3. The method is preferably scalable and easy to apply industrially.

[0012] The inventors have found a new method for preparing bulk SnTiO3, comprising the steps of: (i) preparing a layered alkali titanate, (ii) ion-exchanging the layered alkali titanate with a tin(II) salt to obtain a tin-exchanged alkali titanate; and (iii) dehydrating the tin-exchanged alkali titanate in an oxygen-free atmosphere at a temperature below which disproportionation of tin(II) occurs and subsequently annealing, thereby obtaining SnTiO3.

[0013] The method of preparation of the present invention allows obtaining a new bulk SnTiO3 material, which can be obtained as a standalone material in high purity. Surprisingly, the method of the present invention allows preparing SnTiO3 and SnTi 1-x M x O3

[0014] a new structure of (wherein M and x are as defined herein), such as a ilmenite-type derived layered structure and a perovskite-type structure. The inventors have characterized these bulk SnTiO3 materials. The bulk SnTiO3 materials can have a non-centrosymmetric symmetry, which is a prerequisite for ferroelectricity. Thus, the materials of the present invention are expected to exhibit ferroelectricity.

[0015] The present invention thus also relates to the provision of bulk SnTiO3 materials and their use as ferroelectric elements.

[0016] The present invention is defined by the following aspects.

[0017] A first aspect of the present invention relates to a material of the formula SnTiO3, having a crystal structure comprising layers,

[0018] wherein

[0019] the layers comprise Sn(II) ions, Ti(IV) ions and edge-sharing O6-octahedra,

[0020] the edge-sharing O6-octahedra form sub-layers,

[0021] the Ti(IV) ions are located within 2 / 3 of the edge-sharing O6-octahedra, thereby forming edge-sharing TiO6-octahedra,

[0022] the edge-sharing TiO6-octahedra form a honeycomb structure within the sub-layers, the honeycomb structure comprising hexagons with Ti(IV)-voids within the hexagons,

[0023] the Sn(II) ions are located above and below the Ti(IV)-voids with respect to the sub-layers,

[0024] the Ti(IV) ions are optionally substituted by M,

[0025] M is one or more elements selected from the group consisting of elements of group 4 and group 14, and

[0026] The crystal structure fulfils at least one of the following characteristics (i) and (ii):

[0027] (i) the Sn(II) ion has a tetrahedral coordination sphere comprising three O ions of the layer and an electron lone pair of the Sn(II) ion, the electron lone pair being located in a top position with respect to the three O ions of the layer,

[0028] (ii) the layers are stacked such that each layer is translated with respect to each adjacent layer by a stacking vector S1 or a stacking vector S2,

[0029] the centers of adjacent hexagons form a parallelogram having one side with length x and one side with length y,

[0030] the stacking vector S1 is a combined translation of 2 / 3 x along the side having length x and 1 / 3 y along the side having length y,

[0031] the stacking vector S2 is a combined translation of 1 / 3 x along the side having length x and 2 / 3 y along the side having length y,

[0032] and the crystal structure comprises: layers which are translated with respect to adjacent layers by the stacking vector S1 ; and layers which are translated with respect to adjacent layers by the stacking vector S2.

[0033] Preferred embodiments of the material are described in dependent claims 2 to 5.

[0034] In a second aspect, the present invention relates to a material of formula SnTi03 having a tetragonal perovskite type crystal structure, wherein the Ti(IV) ions are optionally substituted by M, the M being one or more elements selected from the group consisting of elements of group 4 and group 14.

[0035] A third aspect of the present invention is a method for the preparation of SnTi03, the method comprising the following steps:

[0036] (1) reacting an alkali metal salt and titanium(IV) oxide to obtain a layered alkali metal titanate;

[0037] (ii) ion exchanging the layered alkali metal titanate with a tin(II) salt to obtain a tin-exchanged alkali metal titanate; and

[0038] (3) dehydrating the tin-exchanged alkali metal titanate in an oxygen-free atmosphere, followed by annealing in an oxygen-free atmosphere at a temperature below the temperature at which disproportionation of tin(II) occurs, thereby obtaining SnTi03,

[0039] wherein the titanium of the titanium (IV) oxide is optionally substituted by M, said M being one or more elements selected from the group consisting of the elements of group 4 and 14.

[0040] Preferred embodiments of the method are described in dependent claims 8 to 12.

[0041] In a fourth aspect, the present application relates to a material of formula SnTiO3, wherein Ti is optionally substituted by M, said M being one or more elements selected from the group consisting of the elements of group 4 and 14, obtainable by the preparation method according to the third aspect of the present application.

[0042] A fifth aspect of the present application relates to a device comprising a ferroelectric material, wherein the ferroelectric material comprises at least one material selected from the group consisting of the material according to the first aspect of the present application and the material according to the second aspect of the present application.

[0043] In a sixth aspect, the present application also relates to the use of a material according to the first and / or second aspect of the present application in a ferroelectric element. BRIEF DESCRIPTION OF DRAWINGS

[0044] Figure 1 a is a picture of a red SnTiO3 powder as a material according to the present application. Figure 1 b is a scanning electron micrograph of a material of the present application (scale bar 200 nm). Figure 1 c is a high resolution scanning transmission electron micrograph (HR-STEM) of a material of the present application (scale bar 2 nm) as an inset, and a contrast enhanced inset showing the position of titanium. Figure 1 d is a high resolution scanning transmission electron micrograph (HR-STEM) of a material of the present application (scale bar 3 nm) showing different stacking polytypes with the respective stacking vectors indicated.

[0045] Figure 2 shows five SnTiO3 polytypes representing different stacking sequences of a material according to the present application under ambient conditions.

[0046] Figure 3a is a packing diagram showing the stacking of the layers in the layered crystal structure of a material of the present application along the axis perpendicular to the layers (c-axis), and showing the honeycomb structure formed by the O6- octahedra, the position of the Ti(IV) ions within the octahedra, and the position of Sn(II) above and below the hexagons. The stacking vectors S1 and S2, and the basis vectors used to define the stacking vectors are shown in Figure 3a Figure 3b ​This diagram illustrates the stacking of layers in the layered crystal structure of the material of the present invention along an axis parallel to the layer, and shows a sublayer formed by O6-octahedrons, and Sn(II) ions above and below this sublayer. The positions of Ti(IV) ions are indicated by lowercase Greek letters, the positions of Sn(II) ions by bold lowercase Greek letters, and the positions of O ions by lowercase Latin letters. The positions of the entire layer are indicated by uppercase Latin letters.

[0047] Figure 4a The coordination layer of Sn(II) in the crystal structure of the material according to the present invention is shown. Figure 4b The coordination layer of Ti(IV) in the crystal structure of the material according to the invention is shown, which exhibits non-centrosymmetric symmetry.

[0048] Figure 5a The simulated electron localization function (isosurface 0.85) of the zero-valence electrons in the SnTiO3 material of the present invention is shown. Figure 5b The simulated electronic localization function of SnTiO3 is shown (isosurface 0.6), illustrating the direct Sn distribution between layers. 2+ The structure of isolated pairs.

[0049] Figure 6 The final Rietveld-refinement of the crystal structure of SnTiO3 according to the present invention is shown, employing different polymorphs to represent coherently scattering domains exhibiting different stacking orders. For clarity, the region where 2θ is above 30° is magnified (illustration).

[0050] Figure 7 The simulated X-ray powder diffraction pattern of SnTiO3 stacking sequence was compared with the measured X-ray powder diffraction pattern of the material of the present invention.

[0051] Figure 8 The results show the corresponding fit under static solid-state echo and quadrupole Carl-Purcell-Meiboom-Gill (QCPMG) conditions. 47,49 The 21.1T high-field solid-state NMR spectrum of Ti, assuming that the two central transitions have second-order quadrupole (2 nd (Order quadrupolar) interaction.

[0052] Figure 9 The following titanates are shown in Ti L 2,3Edge electron energy loss spectroscopy, from bottom to top: CdTiO3, FeTiO3, SnTiO3, PbTiO3 and SrTiO3.

[0053] Figure 10 Thermogravimetric analysis measurement of the inventive SnTiO3 material in air showing a weight increase of about 5 mass-% starting at a temperature of about 350 °C.

[0054] Figure 11 Illustration of the hypothetical stacking order in the SnTiO3 crystal structure with the basic stacking types AB (a), ABC (b), ACB (c) and with different ranges of twinning: ABC ACB (d) and ABCB (e).

[0055] Figure 12 Comparison of a simulated X-ray powder diffractogram of stacking faulted SnTiO3 showing a crystallographic coexistence of ABC- / ACB-stacking domains with AB-type stacking domains and a measured pattern of the inventive material. For the simulation the extension of the ABC- / ACB-stacking domains was kept constant while the extension of the AB-type stacking domains was gradually increased. The probability of a change from ABC-type to ABC-type is 0.95 (meaning to keep the same stacking type) and the probability of a change from AB-type to ABC / ACB-type is described by the parameter x.

[0056] Figure 13a , Figure 13b , Figure 13c and Figure 13d Comparison of a simulated X-ray powder diffractogram of stacking faulted SnTiO3 showing 120° twinning along the c-axis and a measured pattern of the inventive material. For the simulation the stacking fault probability s was gradually increased in different ranges, respectively. A range of 0 (a), 1 (b), 2 (c) and 3 (d) layers was applied. In other words, a larger range of twinning induces a larger domain of coherent scattering.

[0057] Figure 14 Final refinement of the measured X-ray powder diffractogram of the inventive material by using a polyphasic approach comprising five SnTiO3 polytypes representing different basic stacking types and twinning domains (see Figure 11 ), all polytypes having the same layer structure. SnO2 and K2Ti8O 17Also included as impurity phases. For clarity, the part starting at 2 theta of 30° is enlarged (inset). The calculated pattern is decomposed into the contributions of the individual phases. From top to bottom: refined pattern and superimposed calculated pattern, AB type, ABC type, ACB type, ABCB type, ABCACB type, K2Ti8O 17 and SnO2.

[0058] Figure 15 Measured X-ray powder diffraction patterns of (i) a SnTiO3 material prepared from K2Ti2O5(top pattern) and (ii) a SnTiO3 material prepared from Li2TiO3(bottom pattern) are shown. The reflections at 2 theta of 26.6°, 2 theta of 33.9° and 2 theta of 51.8° can be assigned to SnO2side phases.

[0059] Figure 16 Differential scanning calorimetry measurements of SnTiO3are shown, indicating a phase transition of SnTiO3during heating in the range of 50-80°C.

[0060] Figure 17 Temperature dependent X-ray powder diffraction patterns of a SnTiO3material cooled from room temperature 20 are shown in steps from -50°C to -120°C (from top to bottom), showing peak splitting in all (hk0) reflections (e.g. at ca. 2 theta of 16°, 2 theta of 24° or 2 theta of 28°).

[0061] Figure 18 Detailed temperature dependent X-ray powder diffraction patterns of a SnTiO3material are shown, showing peak splitting at 2 theta of 28° as the temperature is lowered from 293 K to 143 K, indicating a phase transition.

[0062] Figure 19 The peak splitting of the found peak at 2 theta of 28° (see inset and Figure 17 ) was evaluated and a phase transition at about 340 K was predicted by the fit.

[0063] Figure 20 Temperature dependent Raman spectra of SnTiO3are shown, showing the disappearance of the Raman bands at temperatures above 80°C (as indicated by the arrows), thus indicating a phase transition around this temperature.

[0064] Figure 21 Selected area electron diffraction of a SnTiO3material according to the first aspect of the present application is shown, showing a nearly hexagonal crystal structure with distances of and gamma = 119.2°.

[0065] Figure 22The energy versus volume curves of the DFT-calculations for 19 different structural models applied to SnTiO3 are shown, while the experimentally observed structural model is energetically favorable.

[0066] Figure 23 The enthalpy versus pressure diagram of the DFT-calculations for all 19 modeled structural types is shown. A phase transition from the ilmenite-type derived ABC / AB-stacking polytypes to the tetragonal perovskite structure type can be predicted at high pressures.

[0067] Figure 24 The X-ray powder diffraction patterns of SnTiO3 materials, wherein 0, 1 and 5 mol% of Ti are substituted by Zr, respectively, are shown, indicating a decrease in peak intensity at about 9.5°, 13.5°, 18.7°, 20° and 21° in 2Θ with increasing Zr content, while at the same time an increase in peak intensity at 12.8° and 20.6°, thus indicating an influence on the stacking sequence (see also Figure 7 ).

[0068] Figure 25a The packing of O ions in the material of the present invention with a layered structure is shown. Figure 25b The packing of O ions in the conventional ilmenite-type structure is shown. Apparently, in the material of the present invention, the O ions within a layer are close packed, while the O-O distance along the c-axis between different layers is larger than the O-O distance within the close packing of a layer ( Figure 25a ). In contrast, in the ilmenite-type structure, the O ions are also close packed, more specifically, in a hexagonal close packing in all three directions ( Figure 25b ).

[0069] Figure 26 is a flow chart showing the steps of the method of the present invention, and shows the step of reacting an alkali metal salt and titanium (IV) oxide (1); ion exchanging the layered alkali metal titanate with a tin (II) salt (2); dehydrating the tin-exchanged alkali metal titanate in an oxygen-free atmosphere (3a), followed by annealing in an oxygen-free atmosphere (3b).

[0070] Figure 27X-ray powder diffraction patterns of SnTiO3 exposed to maximum pressures of 18.1 GPa (solid line) and 20.8 GPa (dashed line), respectively, are shown. The high pressure data were obtained from the synchrotron at 0.28 A. The measurements were made in a Diamond Anvil Cell with neon gas as the pressure medium. The pressure was increased gradually from ambient pressure to the maximum pressure. The reflections at approximately 8.5°, 9.75° and 13.75° are assigned to crystalline neon. The peak positions marked in black (vertical lines) show the predicted intensities and peak positions of the tetragonal perovskite at ambient pressure. The X-ray powder diffraction patterns indicate that the material transforms to a high-pressure modification at least at 20 GPa. DETAILED DESCRIPTION

[0071] Material of formula SnTiO3

[0072] As mentioned above, the first aspect of the present application relates to a material of formula SnTiO3 having a crystal structure comprising layers,

[0073] wherein

[0074] the layers comprise Sn(II) ions, Ti(IV) ions and edge-sharing O6-octahedra,

[0075] the edge-sharing O6-octahedra form sub-layers, i.e. sub-layers within the layers,

[0076] the Ti(IV) ions are located within two thirds (2 / 3 or 66.66%) of the edge-sharing O6-octahedra, thereby forming edge-sharing TiO6-octahedra,

[0077] the edge-sharing TiO6-octahedra form a honeycomb structure within the sub-layers, the honeycomb structure comprising hexagons with Ti(IV)-voids within the hexagons,

[0078] the Sn(II) ions are located above and below the Ti(IV)-voids with respect to the sub-layers,

[0079] the Ti(IV) ions are optionally substituted by M,

[0080] M is one or more elements selected from group 4 and group 14 elements, and

[0081] the crystal structure fulfils at least one of the following characteristics (i) and (ii):

[0082] (i) the Sn(II) ions have a (distorted) tetrahedral coordination sphere comprising three O ions of the layer and an electron lone pair of the Sn(II) ion, the electron lone pair being located in a top position with respect to the three O ions of the layer.

[0083] (ii) the layer stack is formed such that each layer is translated relative to each adjacent layer by a stacking vector S1 or a stacking vector S2,

[0084] The centers of adjacent hexagons form parallelograms with one side having a length x and one side having a length y,

[0085] The stacking vector S1 is a combined translation of 2 / 3 x along the side having the length x and 1 / 3 y along the side having the length y,

[0086] The stacking vector S2 is a combined translation of 1 / 3 x along the side having the length x and 2 / 3 y along the side having the length y,

[0087] and the crystal structure comprises layers which are translated relative to adjacent layers by the stacking vector S1 and layers which are translated relative to adjacent layers by the stacking vector S2.

[0088] Generally, the crystal structure of the material of the present application can be determined by X-ray powder diffraction (XRPD). More particularly, a Stoe STADIP diffractometer (Mo Kα1 radiation, Ge (111) monochromator, Mythen detector) was used for powder diffraction measurements in Debye-Scherrer geometry and TOPAS 6.0 for structure refinement. For details see the section entitled "X-ray powder diffraction (XRPD)".

[0089] Symmetry

[0090] The crystal structure of the material according to the first aspect of the present application preferably has trigonal symmetry. The material according to the first aspect of the present application can also have a symmetry which is not centrosymmetric.

[0091] The a-b plane is the plane parallel to the layers and sub-layers of the crystal structure of the material according to the first aspect of the present application. The c-axis is the axis perpendicular to this plane.

[0092] Honeycomb structure

[0093] In the material according to the first aspect of the present application of the formula SnTi03, the crystal structure comprises layers, wherein the layers comprise Sn(II) ions, Ti(IV) ions and corner- sharing 06-octahedra (see Figure 3a ). In the following, the structure of such a layer is described in detail.

[0094] Each corner-sharing 06-octahedron is formed by 6 0 2- anions, the 0 2- anions are referred to as O ions in the following. AsFigure 3b As shown, edge-sharing O6-octahedrons form a sublayer within this layer. This sublayer forms a portion of the layer comprising Sn(II) ions, Ti(IV) ions, and edge-sharing O6-octahedrons, and is itself a layer. The edge-sharing O6-octahedrons are preferably arranged in the sublayer such that the center of each O6-octahedron lies in a plane. This center is the geometric center of the edge-sharing O6-octahedron. The plane parallel to the sublayer is designated as the ab plane, and the axis perpendicular to this plane is designated as the c-axis.

[0095] like Figure 25a As shown, in the material according to the first aspect of the invention, the O ions in the sublayers of the common-edge O6-octahedrons are closely packed. The sublayer comprises two atomic layers of closely packed O ions. The O ions in the sublayer do not form a close packing with the O ions in the adjacent layers. Specifically, the O ions in the sublayer do not form a hexagonal close packing with the O ions in the adjacent layers. Therefore, the distance between O ions in different layers along the c-direction is greater than the distance between O ions within a single layer. Conversely, in the ilmenite-type structure, O ions are closely packed in all three directions, specifically hexagonal close packing. Figure 25b ).

[0096] Ti(IV) ions are located within two-thirds (2 / 3 or 66.66%) of the edge-sharing O6-octahedra, thus forming edge-sharing TiO6-octahedra (see Figure 3 and...). Figure 4b The shared-edge TiO6-octahedrons form a honeycomb structure within the sublayers and are therefore parallel to the ab plane, as shown below. Figure 3a As shown. (Refer to...) Figure 3a In this paper, the honeycomb structure is defined as a structure composed of edge-sharing TiO6-octahedra, where, when viewed along the c-axis, the edge-sharing TiO6-octahedra in the layer form hexagons, such that each edge-sharing TiO6-octahedron forms part of three hexagons in that layer. In other words, each O6-octahedron in the sublayer that is not occupied by Ti(IV) ions is surrounded by six TiO6-octahedra. In the following text, the center of an O6-octahedron not occupied by Ti(IV) ions is referred to as a Ti(IV)-vacancy. Ti(IV) ions also form a honeycomb structure parallel to the ab plane. Therefore, in the ab plane, the Ti(IV)-vacancy in an edge-sharing O6-octahedron is surrounded by six Ti(IV) ions in an adjacent edge-sharing O6-octahedron.

[0097] The corner-sharing TiO6-octahedra can be distorted. For example, the corner-sharing TiO6-octahedra can be trigonally distorted. Trigonal distortion is a distortion along a three-fold axis of the TiO6-octahedra, wherein all TiO6-octahedra within the layer are distorted along parallel three-fold axes in the same direction.

[0098] Position of Ti(IV)

[0099] The Ti(IV) ions are located within two-thirds (2 / 3 or 66.66%) of the corner- sharing O6-octahedra, preferably at the center of the corner-sharing O6-octahedra, thereby forming corner-sharing TiO6-octahedra (see Figure 4b ). The center is the geometric center of the corner-sharing O6-octahedra.

[0100] The Ti(IV) ions can exhibit an ideal octahedral coordination layer or a distorted octahedral coordination layer. For example, the Ti(IV) ions can exhibit a trigonally distorted octahedral coordination layer. The trigonally distorted octahedral coordination layer is derived from the trigonal distortion of the TiO6-octahedra described above.

[0101] The Ti(IV) ions can be substituted by M, wherein M is one or more elements selected from group 4 and group 14 elements. Group 4 according to IUPAC nomenclature is a group of elements in the periodic table containing the elements titanium (Ti), zirconium (Zr) and hafnium (Hf). Group 14 according to IUPAC nomenclature is a group of elements in the periodic table consisting of carbon (C), silicon (Si), germanium (Ge), tin (Sn) and lead (Pb).

[0102] Up to 25 mol%, preferably up to 10 mol%, most preferably up to 5 mol% of the titanium can be substituted by M. M is preferably one or more elements selected from the group of Si, Ge, Zr and Hf, and more preferably one or more elements selected from the group consisting of Zr and Hf. Thus, the material according to the first aspect of the present invention can have the formula SnTi 1-x M x O3, wherein x is in the range of 0 and above to 0.25 and below. Typically, the substitution of Ti(IV) can cause a distortion of the TiO6-octahedra. Thus, the distortion of the TiO6-octahedra and the stacking polytype can be adjusted by the degree of substitution of Ti, in particular when Ti is substituted by one or both of Zr and Hf. Thus, the substitution of Ti positions by M influences the stacking order of the material of the present invention.

[0103] Titanium(IV)-vacancies

[0104] As described above, the Ti(IV) ions are located within 2 / 3 of the co- edge O6-octahedra of the sublayer, thereby forming co-edge TiO6-octahedra. The remaining 1 / 3 of the co-edge O6-octahedra is not occupied by Ti(IV) ions. The O6-octahedra in the sublayer that are not occupied by Ti(IV) ions are surrounded by six co-edge TiO6-octahedra. The space in the O6-octahedra that is not occupied by Ti(IV) ions is denoted as a Ti(IV)-vacancy. Thus, the Ti(IV)-vacancies are located within the hexagons of the honeycomb structure formed by the co-edge TiO6-octahedra. The Ti(IV)-vacancies are preferably located at the center (in the sense of the geometric center) of the O6-octahedra that are not occupied by Ti(IV). Thus, the Ti(IV)-vacancies are preferably located at the center of the hexagons of the honeycomb structure when viewed along the axis perpendicular to the layer (c-axis). Furthermore, the Ti(IV)-vacancies are preferably arranged in a plane (a-b plane) formed by the centers of the co-edge O6-octahedra of the sublayer.

[0105] Position of Sn(II)

[0106] The Sn(II) ions are located above and below the Ti(IV)-vacancies with respect to the sublayer within the hexagons along the axis perpendicular to the sublayer (c-axis). The Sn(II) ions are preferably located at the center of the hexagons of the honeycomb structure along the axis perpendicular to the sublayer (c-axis) and are arranged above and below the sublayer.

[0107] As Figure 4a shown, the Sn(II) ions have a trigonal pyramidal coordination sphere that includes three O ions of the layer and the Sn(II) ion at an apex position, and the distance of the Sn(II) ion to the nearest O ion of an adjacent layer is more than 1.5 times or more the distance of the Sn(II) ion to each of the three O ions of its trigonal pyramidal coordination sphere. The coordination sphere of the Sn(II) ion is as shown in Figure 2 The apex position is a position at the apex of a pyramid formed by three O ions and the Sn atom, wherein the base of the pyramid is formed by the three O ions.

[0108] Figure 4a It is shown that the coordination sphere of the Sn(II) ion can alternatively be described as a tetrahedral coordination sphere that includes three O ions of the layer and an electron lone pair of the Sn(II) ion at an apex position with respect to a plane formed by the three O ions of the layer. Thus, when the coordination sphere of the Sn(II) ion is described as a tetrahedral coordination sphere, the lone pair is considered as a fourth ligand of the Sn(II).

[0109] Thus, the Sn(II) coordination in the material of the present application is different from the ideal titanite type structure, in which the Sn(II) ions exhibit an octahedral coordination layer of O ions.

[0110] Stacking sequence

[0111] The material of formula SnTi03according to the first aspect of the present application comprises layers (see Figure 5a ), wherein the layers are arranged parallel to each other and are stacked along an axis perpendicular to the sub-layers formed by the O6- octahedra (c-axis).

[0112] The layers and their adjacent layers are stacked in such a way that a sufficient distance is provided between the lone pair of the Sn(II) ion and the nearest O ion of the adjacent layer (see Figure 5b ). Thus, the stacking order of the layers in SnTi03is determined by the position of the Sn(II) ion, in particular by the electronic lone pair of the Sn(II) ion. The layers are stacked such that in the ideal, un-twisted material, the Sn(II) ion of one layer has the same position as the Ti atom of the adjacent layer along the axis perpendicular to the sub-layers (c-axis). In the case of a twist, this can no longer be the case, such that the materials can have to be additionally described by a process-related feature, i.e. a feature related to the method of material preparation.

[0113] As mentioned above, in the ideal, un-twisted material as opposed to the twisted structure, the layers are stacked such that the Sn(II) ion of one layer has the same position as the Ti atom of the adjacent layer along the axis perpendicular to the sub-layers (c-axis). The stacking of the layers can be described by two stacking vectors S1 and S2. The stacking vectors describe the translation of a layer with respect to an adjacent layer. According to the honeycomb structure motif, six stacking vectors are theoretically possible, but due to the trigonal symmetry of the honeycomb only two of them are non-symmetrically equivalent. The layers in the crystal structure of the material according to the first aspect of the present application are stacked such that each layer is translated with respect to each adjacent layer according to stacking vector S1 or stacking vector S2.

[0114] The stacking vectors S1 and S2 are described by two basis vectors x and y (see Figure 3a ). The basis vectors x and y are defined by the edges of a parallelogram (see Figure 3a ). The parallelogram is formed by the centers (geometric centers) of adjacent hexagons of the honeycomb structure. The parallelogram has an edge of length x and an edge of length y.

[0115] Stacking vector S1 is a combined translation of 2 / 3 x along the edge of length x and 1 / 3 y along the edge of length y. Stacking vector S2 is a combined translation of 1 / 3 x along the edge of length x and 2 / 3 y along the edge of length y (see Figure 3a). Thus, S2 describes a translation in the opposite direction relative to S1. When viewed along the c-axis, the stacking vectors S1 and S2 describe a translation in the a-b plane relative to the adjacent layers. A translation according to S1 or S2 results in a translation of the individual Sn(II) ions of the layer such that, when viewed along the c-axis, the Sn(II) ions occupy the (un-deformed or ideal) positions on top of the Ti(IV) ions of the adjacent layers.

[0116] The stacking sequence can also be described by the order of the layers, e.g. ABC, wherein A, B and C are different individual layers (see Figure 2 ). The order of the individual layers is also referred to as the stacking sequence. When viewed along the axis perpendicular to the a-b plane (c-axis), the different individual layers A, B and C are translated relative to each other. All individual layers having the same index such as A, B or C are stacked along the axis perpendicular to the sub-layers formed by the common edge O6-octahedra (c-axis). In other words, all individual layers having the same index are not translated relative to each other along x or y.

[0117] In an ideal ilmenite-type structure, there is only one stacking vector pointing in one direction, either S1 or S2. Thus, in an ideal ilmenite-type structure, the overall stacking sequence reduces to ABC (or ACB), wherein A, B and C are three different individual layers. In contrast, the crystal structure of the material according to the first aspect of the present application comprises layers that are translated relative to adjacent layers according to stacking vector S1 and layers that are translated relative to adjacent layers according to stacking vector S2. Thus, crystal structures with various other stacking sequences such as AB, ABCB and ABCACB can be obtained (see Figure 2 and Figure 11 and Table 2). These different stacking sequences are also referred to as polytypes.

[0118] In the material according to the first aspect of the present application, the ratio of the occurrence of layers that are translated relative to adjacent layers according to stacking vector S1 to the occurrence of layers that are translated relative to adjacent layers according to stacking vector S2 is preferably in the range of 0.1 to 9, more preferably in the range of 0.25 to 4, even more preferably in the range of 0.5 to 2 and most preferably in the range of 0.8 to 1.3. The ratio can also be 1. As a result, a multitude of polytypes characterized by different stacking sequences can be obtained.

[0119] In other words, in an ideal ilmenite-type structure, the O ions are close-packed in all directions, i.e. in the a, b and c directions, in particular hexagonally close-packed (see Figure 25b ). In contrast, as shown in Figure 25a , in the material of the present application, only the O ions within an individual layer are close-packed and do not form a (hexagonal) close-packing with the O ions of the adjacent layers. Thus, the material of the present application does not exhibit a hexagonal close-packing of the O ions in the c-direction.

[0120] Independent material

[0121] The material according to the first aspect of the present invention is self-standing. Within the meaning of the present invention, self-standing means that the structure, in particular the crystal structure, of the material according to the first aspect of the present invention is stable by itself. In other words, the crystal structure and the macroscopic structure of the material according to the first aspect of the present invention can be maintained in the absence of a matrix or support layer stabilizing or supporting the material. Accordingly, the material according to the second aspect of the present invention is also self-standing.

[0122] The material according to the first and second aspect of the present invention can be provided as a thin layer having a thickness of 500 nm or less, preferably 200 nm or less, more preferably 10 nm or less, most preferably 0.5 to 5 nm.

[0123] Material of the formula SnTiO3 having a tetragonal perovskite-type crystal structure

[0124] As mentioned above, in the second aspect, the present invention relates to a material of the formula SnTiO3 having a tetragonal perovskite-type crystal structure, wherein the Ti(IV) ions are optionally substituted by M, M being one or more elements selected from the group of group 4 and group 14 elements. Up to 25 mol%, preferably up to 10 mol%, more preferably up to 5 mol% of the titanium can be substituted by M. Accordingly, the material according to the second aspect of the present invention can have the formula SnTi 1-x M x O3, wherein x is in the range of 0 or more and 0.25 or less. M is preferably one or more elements selected from the group of Si, Ge, Zr and Hf, more preferably one or more elements selected from the group consisting of Zr and Hf. Typically, substitution of Ti(IV) will cause distortion, in particular when Ti is substituted by one or both of Zr and Hf.

[0125] Process for the preparation of SnTiO3

[0126] A third aspect of the present invention is a process for the preparation of SnTiO3, the process comprising the following steps:

[0127] (1) reacting an alkali metal salt and titanium(IV) oxide to obtain a layered alkali metal titanate;

[0128] (2) ion-exchanging the layered alkali metal titanate with a tin(II) salt to obtain a tin-exchanged alkali metal titanate; and

[0129] (3) dehydrating the tin-exchanged alkali titanate in an oxygen-free atmosphere, followed by annealing in an oxygen-free atmosphere at a temperature below the temperature at which disproportionation of tin(II) occurs, to obtain SnTiO3, wherein the titanium in the titanium(IV) oxide is optionally substituted by M, M being one or more elements selected from group 4 and group 14 elements.

[0130] The process for the preparation of SnTiO3 according to the present invention is shown in the flow chart of Figure 26 , which shows the following steps: reacting an alkali salt and titanium(IV) oxide (1); ion exchanging the layered alkali titanate with a tin(II) salt (2); dehydrating the tin-exchanged alkali titanate in an oxygen-free atmosphere (3a), and subsequently annealing in an oxygen-free atmosphere (3b).

[0131] Preparation of layered alkali titanates

[0132] In step 1 of the process according to the present invention, a mixture of an alkali salt and titanium(IV) oxide is reacted to obtain a layered alkali titanate. The alkali salt and titanium(IV) oxide are preferably mixed by grinding or milling. The reaction temperature is preferably in the range of 400 to 800 °C, more preferably in the range of 500 to 700 °C, most preferably in the range of 550 to 650 °C, and the mixture is preferably heated for at least 1 hour, more preferably for at least 6 hours, most preferably for 12 hours. The reaction can be carried out in air or in an oxygen-free atmosphere, preferably in air. Generally, the reaction temperature depends on the reactants used and the desired target layered titanate. The reaction of K2CO3 and titanium(IV) oxide to form K2Ti2O5 is preferably carried out at 600 °C. The reaction of Li2CO3 and titanium(IV) oxide to form Li2TiO3 is preferably carried out at 700 °C.

[0133] The alkali salt used in step 1 for the preparation of the layered alkali titanate is preferably an alkali carbonate, more preferably K2CO3 or Li2CO3, most preferably K2CO3.

[0134] The titanium(IV) oxide used in step 1 for the preparation of the layered alkali titanate can be in the form of nanoparticles or fine powder, and can contain rutile and / or anatase. The content of anatase is preferably 50 wt% or more, more preferably 75 wt%. The titanium(IV) oxide is preferably titanium(IV) oxide nanoparticles, more preferably titanium(IV) oxide nanoparticles having an average particle size of 25 nm or less. The titanium(IV) oxide can be substituted by M, wherein M is one or more elements selected from group 4 and group 14 elements. Up to 25 mol%, preferably up to 10 mol%, most preferably up to 5 mol% of the titanium is substituted by M. M is preferably one or more elements selected from the group of Si, Ge, Zr and Hf, more preferably one or more elements selected from the group consisting of Zr and Hf.

[0135] The layered alkali titanate is preferably K2Ti2O5or Li2TiO3, more preferably K2Ti2O5.

[0136] Ion exchange

[0137] In step 2 of the process of the present application, the layered alkali titanate obtained in step 1 is ion exchanged with a tin(II) salt to obtain a tin-exchanged alkali titanate. The ion exchange is a cation exchange of the layered alkali titanate. More specifically, in the ion exchange, tin(II) ions completely or partially replace the alkali metal ions of the layered alkali titanate, thereby obtaining the tin-exchanged alkali titanate. Preferably, in the ion exchange, at least 95%, more preferably at least 99%, of the alkali metal ions are replaced by tin(II).

[0138] The tin(II) salt is not particularly limited and is preferably one or more salts selected from the group of tin(II) halides, tin(II) sulfate, tin(II) nitrate, tin(II) phosphate, tin(II) hydrogen phosphate, tin(II) acetate, tin(II) oxalate, tin(II) carbonate, and tin(II) hydrogen carbonate, more preferably tin(II) halide, even more preferably tin(II) halide hydrate, most preferably tin(II) chloride dihydrate.

[0139] The ion exchange in step 2 can be performed by mixing the layered alkali titanate and the tin(II) salt, preferably at a temperature below 60 °C, more preferably at a temperature in the range of 0 to 50 °C. Mixing in air at a temperature above 60 °C can lead to the formation of undesired SnO or SnO2. Mixing at a temperature below 0 °C can lead to a too low reaction rate of the ion exchange. Mixing in an oxygen-free atmosphere can be performed at a temperature below 90 °C, preferably below 70 °C, most preferably below 60 °C. The mixing can comprise grinding or milling the layered alkali titanate and the tin(II) salt. Preferably, the mixing is performed at least until a yellow, in particular lemon yellow, powder is formed.

[0140] Alternatively, the ion exchange in step 2 can be performed by combining the layered alkali titanate, the tin(II) salt, and an aqueous medium, dispersing the layered alkali titanate and the tin(II) salt to obtain a dispersion in the aqueous medium, and stirring the dispersion at a temperature below 70 °C. Higher temperatures can lead to the formation of undesired SnO or SnO2. Preferably, the dispersion is stirred at least until a yellow, in particular lemon yellow, powder is formed.

[0141] Dehydration and annealing

[0142] In step 3 of the process of the present application, the tin-exchanged alkali titanate obtained in step 2 of the process of the present application is dehydrated in an oxygen-free atmosphere, followed by annealing in an oxygen-free atmosphere at a temperature below the temperature at which disproportionation of tin(II) occurs, thereby obtaining SnTiO3.

[0143] Disproportionation of tin(II) to tin(0) and tin(IV) occurs at about 350°C. The disproportionation temperature depends on the reaction conditions, such as pressure, oxygen and water content of the reaction medium, and the presence of other substances that can catalyze disproportionation. Thus, the disproportionation temperature can also be higher than 350°C under specific reaction conditions.

[0144] Preferably, the dehydration is carried out until a water content of less than 5 wt%, more preferably less than 3 wt%, most preferably less than 1 wt%, based on the tin-exchanged alkali titanate, is reached.

[0145] The dehydration in an oxygen-free atmosphere preferably occurs under vacuum or in an inert gas stream. Suitable inert gases are nitrogen, argon, helium and mixtures thereof, preferably nitrogen. The vacuum is preferably a dynamic vacuum.

[0146] The dehydration in step 3 preferably occurs at a temperature of less than 250°C, more preferably less than 200°C. The dehydration can comprise a first dehydration step 3a at a temperature in the range of 100 to 150°C, and a subsequent second dehydration step 3b at a temperature in the range of more than 150 to less than 250°C, preferably in the range of more than 150 to 200°C. The first dehydration step 3a is preferably carried out for 0.25 to 5 hours, more preferably 1 to 4 hours, most preferably 2 to 3 hours. The second dehydration step 3b is also preferably carried out for 0.25 to 5 hours, more preferably 1 to 4 hours, most preferably 2 to 3 hours.

[0147] After the dehydration, the tin-exchanged alkali titanate is annealed in an oxygen-free atmosphere. The annealing in an oxygen-free atmosphere is preferably carried out under vacuum or in an inert gas stream. Suitable inert gases are nitrogen, argon, helium and mixtures thereof, preferably nitrogen. The vacuum is preferably a dynamic vacuum.

[0148] The annealing occurs at an annealing temperature in the range of 250 to 400°C, preferably in the range of 250 to 350°C, most preferably in the range of 275 to 325°C. The tin-exchanged alkali titanate is preferably annealed at the annealing temperature for 1 to 48 hours, more preferably 3 to 36 hours, most preferably 6 to 24 hours.

[0149] The dehydration and annealing in step 3 can be performed by continuously heating the tin-exchanged alkali titanate obtained in step 2 to the annealing temperature at a heating rate of 5°C / min or less, preferably 3°C / min or less, more preferably 1°C / min or less, in an oxygen-free atmosphere. The annealing temperature is preferably in the range of 250 to 400°C, more preferably in the range of 250 to 350°C, most preferably in the range of 275 to 325°C. The tin-exchanged alkali titanate is preferably annealed at the annealing temperature for 1 to 48 hours, more preferably for 3 to 36 hours, most preferably for 6 to 24 hours.

[0150] When the dehydration and annealing is performed by continuous heating, the heating rate is preferably adjusted to ensure sufficient dehydration before the heating temperature exceeds 250°C. Thus, when the dehydration and annealing is performed by continuously heating the tin-exchanged alkali titanate obtained in step 2 to the annealing temperature, the water content, based on the tin-exchanged alkali titanate, is preferably below 1 wt% before the temperature exceeds 250°C, more preferably below 0.5 wt% before the temperature exceeds 220°C, most preferably below 0.5 wt% before the temperature exceeds 200°C.

[0151] Washing and drying

[0152] The process of the present application can comprise an additional step 4 of washing and drying the SnTi03. The washing can be performed after the annealing, preferably after cooling to a temperature of 60°C or less. The washing step allows the removal of alkali metal salts formed from the alkali metal of the alkali titanate and the anion of the tin(II) salt. The solvent used in the washing step is a polar solvent or a mixture of polar solvents, preferably one or more solvents selected from the group of water, methanol, ethanol, isopropanol and acetone, most preferably water and / or ethanol. The washing step can comprise one or more steps of washing the SnTi03 obtained in step 3, wherein the same or different solvent or solvent mixture can be used in each step. The washing can comprise filtering, suspending, centrifuging or decanting the SnTi03 after adding the solvent.

[0153] After washing, the SnTi03 is subsequently dried. The drying can occur at a temperature in the range of 20 to 250°C, preferably in the range of 50 to 150°C, most preferably in the range of 70 to 120°C. The drying can be static drying, drying in a gas stream or drying under vacuum.

[0154] Inducing phase transition

[0155] Generally, as will be described in detail below, the temperature after annealing and the cooling will influence the structure of the material of the present application due to the phase transition point being in the temperature range of 30 to 100 °C. Therefore, the phase transition can be induced during cooling of the SnTi03 material of the present application after annealing, during washing or during drying. The phase transition can also be induced by a separate subsequent step after annealing, for example by heating the material to a temperature above the phase transition point and subsequent cooling.

[0156] It was found that the phase transition occurs in the range of 30 to 100 °C, preferably in the range of 35 to 80 °C, most preferably in the range of 40 to 60 °C, as the phase transition point is in this temperature range. For the following considerations, the term low-temperature phase refers to the thermodynamically stable phase below the phase transition point and the term high-temperature phase refers to the thermodynamically stable phase above the phase transition point.

[0157] The high-temperature phase is considered to have a higher symmetry compared to the low-temperature phase. Inducing the phase transition allows for the preparation of SnTi03 in the low-temperature phase having a non-centrosymmetric structure. More specifically, inducing the phase transition allows for the preparation of SnTi03 having a layered crystal structure comprising a non-centrosymmetric layer. Therefore, it is expected that a SnTi03 material can be obtained which exhibits ferroelectric properties.

[0158] For inducing the phase transition to form the low-temperature phase, the material is preferably cooled from a temperature above the phase transition point to a temperature below the phase transition point at a cooling rate of 0.1 °C / min or below. For stabilization of the high-temperature phase, the material is preferably cooled to room temperature at a rate of 1 °C / min or above, which is also referred to as quenching, or directly from the annealing temperature of step 3 of the present application or any other temperature above the phase transition point to room temperature. If the high-temperature phase is induced by a separate subsequent step, the material of the present application can be induced, for example, by annealing at a temperature above the phase transition point until the phase transition is completed and subsequent cooling to room temperature at a rate of 1 °C / min or above. Washing and drying can also be performed after the phase transition has been induced.

[0159] Generally, the crystal structure of the material of the present application can be determined and the low-temperature phase and the high-temperature phase can be distinguished by X-ray powder diffraction (XRPD). For details, please refer to the section entitled “X-ray powder diffraction (XRPD)”.

[0160] High pressure phase transition

[0161] The method of the present application can comprise an additional step of exposing the SnTi03 to a pressure of 4 GPa or above, preferably 6 GPa or above, more preferably 8 GPa or above, to obtain SnTi03 having a perovskite-type structure, in particular a tetragonal perovskite-type crystal structure. SnTi03 having a perovskite-type structure can exhibit ferroelectric or piezoelectric properties. In this additional step of exposing the SnTi03 to pressure, the pressure is even more preferably 15 GPa or above, most preferably 20 GPa or above.

[0162] Before releasing the pressure, the SnTiO3 having a perovskite-type structure obtained by exposure to high pressure can be quenched, thereby suppressing the phase transition during and after the pressure release. The quenching is preferably performed by cooling the SnTiO3 having a perovskite-type structure from the annealing temperature to a temperature at which the perovskite-type structure is stable at an average cooling rate of 1 °C / s or more, more preferably 2 °C / s or more, most preferably 5 °C / s or more. In the context of the present invention, a phase is considered to be stable if it is thermodynamically stable, or if the phase transition to a thermodynamically more stable phase is kinetically suppressed.

[0163] Figure 27 X-ray powder diffraction patterns of SnTiO3 exposed to maximum pressures of 18.1 GPa (solid grey line) and 20.8 GPa (dashed line), respectively, are shown. The data sets were obtained from synchrotron radiation at 0.28 A. The measurements were performed in a Diamond Anvil Cell using neon gas as pressure medium. The pressure was gradually increased from ambient pressure to the maximum pressure. The reflections at about 8.5°, 9.75° and 13.75° are assigned to crystalline neon. The marked peak positions (vertical solid black lines) show the predicted intensities and peak positions of the tetragonal perovskite at ambient pressure.

[0164] Both diffraction patterns show the typical shift of the peak positions due to the compression of the lattice parameters of the relatively soft material. The X-ray powder diffraction pattern of SnTiO3 exposed to a maximum pressure of 18.1 GPa is similar to the pattern of the interlayered SnTiO3. The small increase in intensity at about 6.1° of the pattern at 20.8 GPa is indicative of the formation of a phase different from the interlayered SnTiO3 according to the first aspect of the invention. If one assumes a peak shift due to the high pressure, the peak positions are in very good agreement with the highest intensity reflections of the predicted tetragonal perovskite as indicated by the vertical lines. However, the very small overall intensity does not allow to assign any further reflections from the obtained data set. Thus, the data indicate that the material transforms to a high-pressure modification at a pressure of about 20 GPa. A (pressure) quenching of the structure at ambient conditions would then likely result in the stable tetragonal modification. Figure 6

[0165] Material obtainable by the preparation method

[0166] ​The production method according to the third aspect of the present application produces a material according to the first or second aspect of the present application, in particular according to the first aspect of the present application. In addition, the method of the present application can result in a material in which the Ti(IV) ions, the ions M which can partially substitute the Ti(IV) ions and / or the Sn(II) ions exhibit a distorted coordination shell and / or do not occupy the central position of the entire structure (e.g. within the octahedron and hexagon, respectively). Such SnTi03 and SnTi 1-x M x The SnTi03 and SnTi 1-x M x O3 materials exhibit Sn(II) ions between close-packed (sub)layers of O ions consisting of two atomic layers of O ions and 2 / 3 of the octahedral voids of the (sub)layer of O ions being filled with Ti(IV) ions, respectively. These materials are a further aspect of the present application themselves. They have a crystal structure of low symmetry and it is not possible to define their crystal structure beyond the above definition except for the process by which they are manufactured. The materials defined additionally by their method of manufacture are the subject of the fourth aspect of the present application.

[0167] In the fourth aspect, the present application relates to a material of the formula SnTi03, wherein Ti is optionally substituted by M, M being one or more elements selected from the group of elements of group 4 and group 14, obtainable by the production method according to the third aspect of the present application. M is preferably one or more elements selected from the group of Si, Ge, Zr and Hf, and more preferably one or more elements selected from the group consisting of Zr and Hf. Thus, the material can have the formula SnTi 1-x M x O3, wherein x is in the range of 0 and 0.25.

[0168] The material of the formula SnTi03, obtainable by the production method according to the third aspect of the present application, can have a layered crystal structure comprising a non-centrosymmetric layer. Thus, it can be expected that the material exhibits ferroelectric properties.

[0169] Ferroelectric properties

[0170] The materials according to the first and second aspect of the present application and the material produced by the production method according to the third aspect of the present application can have a non-centrosymmetric symmetry and thus can be expected to exhibit ferroelectric properties.

[0171] Ferroelectricity is based on the coupling of the electric dipoles inside the material to the material lattice. Thus, a change in the lattice leads to a change in the spontaneous polarization. Two parameters that influence the size of the material lattice are force and temperature. The generation of surface charges in response to an external stress applied to the material is called piezoelectricity. The change in the spontaneous polarization of a material in response to a change in temperature is called pyroelectricity. There are 21 classes of non-centrosymmetry, of which 20 are piezoelectric. Of the piezoelectric classes, 10 are pyroelectric, i.e. they have a spontaneous polarization that changes with temperature. Of the pyroelectric materials, some are ferroelectric. Ferroelectric crystals usually show several transition temperatures and domain structure hysteresis, similar to ferromagnetic crystals.

[0172] The material according to the second aspect of the present application can also exhibit ferroelectricity.

[0173] In order to enhance the ferroelectric properties of the material of the present application, a phase transition can be induced in the material.

[0174] Due to its potential ferroelectricity, the material of the present application can be used as a ferroelectric material in a ferroelectric element or device.

[0175] Device comprising a ferroelectric material

[0176] As mentioned above, the fifth aspect of the present application relates to a device comprising a ferroelectric material, wherein the ferroelectric material comprises at least one material selected from the group of a material according to the first aspect of the present application, a material according to the second aspect of the present application, and a material according to the fourth aspect of the present application.

[0177] The device comprising a ferroelectric material can be a device selected from the group of a ferroelectric capacitor, a ferroelectric tunnel junction element (FTJ), a multiferroic element, a piezoelectric body for ultrasonic imaging, an actuator, an electro-optic material for data storage applications, a thermistor, a switch such as a transformer or a ferroelectric dielectric impedance, an oscillator, a filter, a light deflector, a modulator and a display, etc.

[0178] A ferroelectric capacitor is a capacitor based on a ferroelectric material. In contrast, a conventional capacitor is based on a dielectric material. Ferroelectric capacitors can be used as data storage devices in digital electronic devices. More particularly, ferroelectric capacitors can be used as part of a ferroelectric random access memory (ferroelectric RAM).

[0179] In a ferroelectric random access memory, a ferroelectric layer is used instead of a dielectric layer to achieve non-volatility of the stored data, so that information can be maintained even when the power is off.

[0180] Ferroelectric capacitors can also be used as tunable capacitors in analog electronic devices. For example, a ferroelectric capacitor can be used as a voltage controlled capacitor.

[0181] Ferroelectric tunnel junction elements include a ferroelectric layer between metal electrodes, where the thickness of the ferroelectric layer is in the nanometer range and small enough to allow tunneling of electrons.

[0182] Multiferroic elements are materials in which magnetic and ferroelectric ordering within the material are coupled.

[0183] Thermistors are resistors whose resistance depends on temperature, where the temperature dependence is much more pronounced than for standard resistors.

[0184] Actuators are machine components responsible for moving and controlling mechanisms or systems.

[0185] Use in ferroelectric elements

[0186] These aspects of the invention relate to the use of the materials according to the first and second aspects of the invention, and the materials according to the fourth aspect of the invention, in ferroelectric elements. Within the meaning of the invention, a ferroelectric element is any element or component whose function is based on ferroelectricity.

[0187] Use as high-k dielectrics

[0188] In a final aspect, the invention relates to the use of the materials according to the first and second aspects of the invention, and the materials according to the fourth aspect of the invention, as high-k dielectrics.

[0189] The term high-k dielectric refers to a material having a high dielectric constant κ compared to silicon dioxide. High-k dielectrics are used in semiconductor manufacturing processes, where they are typically used to replace a silicon dioxide gate dielectric or another dielectric layer of a device.

[0190] High dielectric constants require polarizability over a wide frequency range. For most applications, the microwave region is of most interest. Here, materials with less strongly bound valence electrons (e.g. lone pairs) and intrinsic dipoles due to structural distortions (e.g. distorted TiO6-octahedra) are advantageous. BaTiO3and Pb(Zr,Ti)O3are considered high-k candidates due to their high dielectric constants, which typically peak in a phase transition. Promising phase transitions are also found in SnTiO3 Figure 17 and Figure 19 ). In addition, SnTiO3offers advantages for thin film processing due to its pronounced two-dimensional character, i.e. its layered structure.

[0191] Examples and calculations

[0192] Preparation

[0193] As mentioned above, avoiding the disproportionation of Sn(II) to Sn(0) and Sn(IV) is experimentally challenging. To circumvent this, a layered potassium titanate precursor was synthesized to enable a low-temperature ion exchange reaction from potassium to tin. First, a mixture of K2CO3 and TiO2-nanoparticles (P25) was heated in air for 12 hours up to 600 °C. Then, the formed layered potassium titanate precursor K2Ti2O5 was ground together with SnCl2*2H2O until a lemon yellow powder was formed. This powder was transferred to an ampoule and dehydrated in two steps: 2.5 hours at 130 °C and then 2.5 hours at 200 °C under dynamic vacuum. Finally, the powder was annealed at 300 °C for 24 hours under static vacuum until SnTiO3 was formed. To remove KCl, the obtained SnTiO3 powder was washed twice with H2O and once with ethanol. A uniform red powder was obtained Figure 1 a ) Platelets with lateral dimensions of several hundred nanometers and axial dimensions of several tens of nanometers were formed Figure 1 b

[0194] Characterization

[0195] Elemental analysis

[0196] Table 1 shows the elemental analysis results of different washed batches of the material obtained by the above preparation method using inductively coupled plasma atomic emission spectroscopy. For the elemental analysis, a VARIAN VISTA RL CCD Simultaneous ICP-AES was used. The elemental analysis results confirm the stoichiometric SnTiO3.

[0197] Table 1 ICP-AES elemental analysis of SnTiO3 of different washed batches.

[0198]

[0199] Thermogravimetric analysis

[0200] Thermogravimetric analysis measurements of the material obtained by the above preparation method in air show a weight increase of the material at about 350 °C (see Figure 10 ). Thus, the oxidation of SnTiO3 to SnO2 and TiO2 in air indirectly proves the oxidation state of Sn of +II. The obtained weight increase of ~ 5 wt% corresponds to the theoretical value of ~ 7 wt%. This small deviation can be explained by partially oxidized surface species. ​

[0201] Scanning transmission electron microscopy (STEM)

[0202] The sample was prepared from an ultrasonic mixture of the powder sample with pure ethanol. This mixture was drop cast onto a copper grid with a perforated carbon film. Low-loss EELS acquisition and Cs correction of STEM images were acquired using a FEI Titan Themis 60-300 equipped with an aberration corrector for the probe-forming lens system, a high-brightness field emission (XFEG TM ) source, a monochromator and a high-resolution electron energy-loss spectrometer (Gatan 966 GIF) operating at 3000 kV.

[0203] EELS data were acquired in monochromated STEM point mode with an energy resolution of 0.2 eV. A dispersion of 0.1 eV was used for the spectrum acquisition. Sub-pixel scanning and short acquisition times were used to avoid electron beam-induced sample damage.

[0204] STEM revealed a lamellar structure, confirming the macroscopic platelet morphology Figure 1 d ). Apparently, the structure is severely stacking faulted (as indicated by the two stacking vectors S1 and S2) and does not show regular superstructure ordering types.

[0205] X-ray powder diffraction (XRPD)

[0206] A Stoe STADIP diffractometer (Mo Kα1 radiation, Ge (111) monochromator, Mythen detector) was used for powder diffraction measurements in Debye-Scherrer geometry, and TOPAS 6.0 for structure refinement.

[0207] Due to the severe stacking faulted nature of the structure Figure 1 a and 1b STEM in

[0208] The initial structural model of bulk SnTi03derives from a prediction made by T. Fix et al., Crystal Growth & Design 11 (2011) 1422. According to this prediction, SnTi03was expected to crystallize in the ilmenite-type structure.

[0209] The diffraction patterns of the material according to the first aspect of the present application were successfully modelled by the superposition of different stackings of hypothetical polytypes (see Table 2). In total, five different polytypes representing five different stacking sequences were considered.

[0210] In all five crystal structures, the layer is described by two tin sites, two titanium sites and six oxygen sites. The coordinate axes were refined in real space to maintain the same layer configuration within one polytype and between all polytypes. In addition, the z-coordinates of three oxygen sites were also constrained. The only parameters refined separately for each polytype were: scale factors to account for the different relative frequencies of the stacking patterns, and crystal size parameters to account for the different extensions of the coherent scattering domains.

[0211] The assumption that variations in the stacking sequence of the SnTiO3 layers neither cause variations in the layer composition nor deviate from the basic trigonal layer symmetry is further supported by electron energy loss spectroscopy (EELS) and density functional theory (DFT). The Rietveld refinement of SnTiO3 was performed by the program TOPAS 6.0 2017, Bruker AXS. SnO2and K2Ti8O 17 occurred as a secondary impurity phase and was included in the refinement (see Figure 6 ).

[0212] Table 2 represents the refined lattice parameters of the five polytypes of SnTiO3 with different stacking sequences under ambient conditions.

[0213]

[0214]

[0215] Therefore, the five hypothetical polytypes were created in a pseudotrigonal cell with the space group P1 and constrained lattice parameters determined by a LeBail-Fit as described in A. Le Bail et al., Materials Research Bulletin 23 (1988) 447 with peak profiles modelled by the fundamental parameter method described in R.W. Cheary et al., Journal of Applied Crystallography 25 (1992), 109.

[0216] The refined atomic coordinates of a single layer are listed in Table 3, Figure 6 showing the final correlated Rietveld refinement results.

[0217] Table 3 Refinement atomic coordinates of a layer in the SnTiO3 crystal structure under ambient conditions.

[0218]

[0219] a To create the stacking polytypes, the stacking vectors given in eq. 1 have to be used to add layers.

[0220] (eq. 1)

[0221] b The z coordinate is given for a unit cell containing one layer, i.e. To create different stacking patterns, the z coordinate has to be divided by the total number of layers N in the unit cell.

[0222] The crystal structure of SnTiO3 is composed of layers of edge-sharing TiO6- octahedra perpendicular to the c-axis. The z-direction corresponds to the c-axis. Within the layers, 1 / 3 of the Ti sites are empty, which creates a honeycomb-like lattice Figure 3a ). The Sn 2+ The ions are located directly above and below the Ti-vacancies. Table 4 gives the atomic distances.

[0223] Table 4 Atomic distances in SnTiO3 under ambient conditions.

[0224]

[0225] The stacking of the material according to the first aspect of the present application was further investigated using the DIFFaX program as described in M. M. J. Treacy et al., Proc. R. Soc. Lond. A: Math. Phys. Sci. 433 (1991) 499. The XRPD pattern of SnTiO3 with stacking faults was simulated by using the DIFFaX-routine method in a recursive model. For the simulation, the refined layer structure was used. A Pseudo-Voigt function was used to simulate the peak profile.

[0226] Various fault models using different combinations of the stacking vectors S1 and S2 were simulated. The crystallographic intergrowth between the ABC-type (stacking vector: S1), the ACB-type (stacking vector: S2) and the AB-type stacking (stacking vectors: S1 and S2 in an alternating way) was simulated by extending the coherently stacked sections using a 4x4 transition probability matrix with variable parameters x and y (see Table 5).

[0227] Table 5 Transition probability matrix with associated stacking vectors for the simulation of the crystallographic intergrowth between the ABC-, AB- and ACB-type. The extension of the AB-stacked sections is described by the parameter x, the transition between the ABC / ACB-type and the AB-type stacked sections is described by the parameter y.

[0228]

[0229]

[0230] Additional simulations were performed in which each transition from S1 stacking to S2 stacking was considered to be a possible source of a range of stacking faults, such that after a stacking fault occurred, the minimum number of layers must be fault-free. This was implemented by a 2i x 2i transition probability matrix (see Table 6), where i-1 is the range of stacking faults, and s, a variable parameter, is the probability of an additional stacking fault after the minimum range of stacking faults.

[0231] In each case, a set of simulations was performed in which only one parameter was systematically varied.

[0232] Table 6 Transition probability matrix with associated stacking vectors for simulations with a range (i-1) of stacking faults. The sharpness of the range is described by the parameter s.

[0233]

[0234] X-ray powder diffraction patterns were simulated for the basic stacking types ABC Figure 7 ), ACB and AB, and compared to the measured X-ray powder diffraction pattern of SnTiO3. It is clear that the measured pattern exhibits more reflections than each of the individual basic stacking types. However, a superposition of the three basic stacking types exhibits a good similarity to the measured pattern, especially with respect to the characteristic "triplets" at 2 theta of approximately 12.5°, 21° and 26°. Respectively, the "outer" two reflections (104) and (015) are related to ABC / ACB type stacking (either S1 or S2 only), while the "central" reflection (103) is related to AB type stacking (S1 and S2). It should be noted that it is very difficult to distinguish between ABC type and ACB type stacking from X-ray powder diffraction patterns, as the simulated X-ray powder diffraction patterns Figure 7 ) show the same number of reflections at the same positions, only with different intensities of some reflections. It can be concluded that in the microstructure of SnTiO3, there are domains of coherent scattering of each of these stacking types.

[0235] Using further DIFFaX simulations, it is clear that AB type stacking cannot fully describe the stacking fault nature Figure 13a to Figure 13d ). If each transition of the stacking vector from S1 to S2 (and vice versa) is considered to be a stacking fault in terms of twinning, then each twinned domain can have a certain extension. Therefore, each stacking fault has a range. Various simulations of twinning using different ranges Figure 13a to Figure 13d). When the range is set to 0, then the twin is statistically pure occurring, thus the increasing probability of stacking faults leads to a large broadening of the reflections Figure 13a ). A further increase of the probability of stacking faults finally leads to a transition of the ABC / ACB type XRPD pattern to an AB type pattern. By increasing the range, the broadening of the peaks due to an increasing probability of stacking faults is less pronounced Figure 13b to Figure 13d ). In the simulated pattern, also a triplet of characteristic peaks at about 9, 12-14, 20-21 and 26-27° in 2-theta appears. The maximum similarity between the measured and the simulated XRPD pattern is obtained by a range of 2 Figure 13c ). Thus, due to the very small coherent scattering domains, the central reflections will have a rather broad profile, which is not observed in the powder pattern. Only by additionally considering a twin range larger than zero and the corresponding stacking types (ABCB and ABCACB), the clear reflection profile can be reproduced.

[0236] Since the DIFFaX simulation indicates the presence of multiple coherent scattering domains, showing a uniform stacking sequence, the microstructure of the sample can be described by a superposition of multiple phases. Therefore, in the Rietveld refinement Figure 14 ), the measured XRPD pattern is refined by using separate phases of SnTiO3, each phase representing a different stacking sequence. The same layer structure is used for each phase. The only parameter refined individually is the grain size parameter used to describe the extension of the coherent scattering domain. In the Rietveld refinement, in addition to the SnTiO3 phases describing the basic stacking sequences ABC, ACB and AB, two phases ABCB and ABCACB are used describing the twin domains. In addition, SnO2 and K2Ti8O 17 are included as impurity phases.

[0237] The grain size, i.e. the extension, of the coherent scattering domain of the AB type stacking is refined to low values. Therefore, for the SnTiO3 phase of the AB type stacking, the calculated pattern Figure 14 ) decomposed into the individual components shows broad reflections. This is in agreement with the systematic DIFFaX study. The other SnTiO3 polytypes show rather sharp reflections, indicating a larger extension of the related coherent scattering domain. In summary, the calculated patterns of all SnTiO3 polytypes considered gain sufficient intensity, which means that each stacking sequence is present in significant amounts in the microstructure. The contribution Figure 6 ) of the impurity phases to the measured pattern is almost negligible (calculated content of SnO2: 1.3(1) wt- %, calculated content of K2Ti8O 17 : 3.1(1) wt- %).

[0238] The measured X-ray powder diffraction pattern of the material according to the first aspect of the application was satisfactorily described by a polyphase approach (R-wp = 10.54%, where R-wp is the weighted profile R-factor, which is the difference index of Rietveld refinement used to judge the quality of the Rietveld fit). The largest difference between the calculated and measured pattern could be observed at the 110 reflection (2theta of about 16°), which indicates that modelling by the individual relative twin domain size alone is not fully sufficient. However, an extension of the approach to more phases would lead to an over-parameterization of the refinement. Since the 110 reflection is not affected by stacking faults, the results of the refinement can be considered as a semi-quantitative description of the SnTi03 microstructure.

[0239] Density functional theory data

[0240] To further elucidate the stacking fault nature of the structure, DFT calculations of the relative stability of different stacking types were performed: (1) ABC / ACB type, (2) AB type, (3) ABCB type and (4) ABCACB type. Independently of the applied exchange-correlation functional (LDA, GGA, HSE06), the differences of all applied structure models were only smaller than 0.14 kJ mol -1 (LDA; PBE: 0.06 kJ mol -1 ; HSE06: 0.01 kJ mol -1 ). These extremely close energy minima nicely illustrate that all stacking patterns will occur at finite temperatures.

[0241] The volume per formula unit (V FU ) obtained with the HSE hybrid functional is about in good agreement with the V obtained from the Rietveld refinement. The V FU increases from about in the common ilmenite type structure (e.g. FeTi03) to in SnTi03, which is in line with the observed increase from about in SrTi03 to Figure 2 in PbTi03. While the lone pairs in PbTi03 are constrained within two layers of the corner-sharing Ti06-octahedra, the lone pairs in SnTi03 form a van der Waals gap as described in R.H.S. Winterton, 11 Contemporary Physics (1970), 559, thus occupying more space (see also the ELF in

[0242] Electron energy loss spectroscopy (EELS)

[0243] Figure 9Ti L 2,3 edges. Ti-L 2,3 edges show a characteristic splitting of the octahedral crystal field, thus confirming the TiO6- octahedra as the main structural motif. Specifically, it is known that the e G -level (at about 460 eV) is sensitive to the tetragonal distortion in PbTiO3. In contrast, the trigonal distortion of corundum-type Ti2O3 and other ilmenite-type materials has no influence on the e G -level. Since no peak splitting is observable for SnTiO3, it can be concluded that the potential distortion of the TiO6- octahedra is constrained to D 3D symmetry. Therefore, it is reasonable to refine only the z-coordinates of the atomic sites.

[0244] Ti-L 2,3 Detailed analysis of the Ti-L G splitting, the CFS was measured between peak C and peak D. Interestingly, upon introduction of the lone pair, the value of the perovskite-type structure decreases drastically from 2.36 eV in SrTiO3 to 1.91 eV in PbTiO3, while in the ilmenite-type derived structures, the value increases from about 2.06 eV in FeTiO3 to 2.27 eV in SnTiO3, while for both structure types, the e G -peak decreases, while the full width at half maximum (FWHM) increases

[40] . This broadening has been explained by the differentiation of the hybridization of the orbitals involved and thus of the transition probabilities

[41] .

[0245] High field solid state nuclear magnetic resonance spectroscopy (ss-NMR)

[0246] The Ti solid-state NMR spectra were acquired on a Bruker Avance II-900 instrument (magnetic field 21.1 T) at a Larmor frequency of 50.73 MHz 47, 49 Ti solid-state NMR spectra. A Bruker 4 mm low-gamma MAS probe and a Bruker 7 mm single channel MAS probe were used. In 47,49 The external reference in the Ti spectra was done with a 1 M solution of TiCl4 in CCl4 (from 49 The high frequency signal of Ti was set to 0 ppm) and solid SrTiO3 was used as a second reference. In the liquid reference samples 49Non-selective π / 2 pulses were calibrated on Ti. Solid-echo (π / 2-τ-π / 2-τ-acq) was used, as described in EL Hahn 80, Physical Review (1950) 580, and QCPMG (π / 2-τ1-(-τ2-π-τ3-acq)) as described in FH Larsen et al., Journal of Magnetic Resonance 131 (1998) 144. n FID was collected using [method name missing]. For each sample, cyclic delays ranging from 1 s to 6 s, depending on the sample and signal complete relaxation, were tested individually. Depending on the sample, 2000 to 20000 scans were acquired. Spectra were obtained in a single frequency shift.

[0247] As described in D. Massio et al., Chemical Magnetic Resonance, 40 (2002) 70 and F.A. Perras et al., Solid State NMR, 45 (2012) 36, analytical simulations of experimental spectra were performed using the DMFit and QUEST simulation packages. If possible, the isotropic chemical shift δ was first fitted. iso Quadrupole constant C Q and the four-pole asymmetric parameter η Q The MAS spectrum was obtained. These parameters were then used in the simulation of the static powder map to obtain the span Ω and square root k, and the Euler angles were typically set to Euler angles obtained from quantum chemical calculations.

[0248] Modeling titanates 47,49 The calculations of the Ti electric field gradient and NMR shielding tensor were performed using available crystallographic data. The density functional theory calculations based on plane waves were performed using the NMR module of the CASTEP DFT code described in J.R. Yates et al., Physical Review B 76 (2007) 024401, which employs a Gauge Including Projector Augmented Wave (GIPAW) and is part of the Biovia MaterialsStudio simulation and modeling package (2017 edition). This method is specifically designed for extended lattice structures of crystalline materials. The Perdew-Burke-Ernzerhof (PBE) functional, along with the generalized gradient approximation (GGA), was used in all calculations, as described in J.P. Perdew et al., Physical Review Letters 77 (1996) 3865 and J.P. Perdew et al., Physical Review Letters 78 (1997) 1396. Geometric optimization was performed using a PBE functional, with the following convergence tolerance parameters set: Energy: 10 -5eV / atom, max force: 0.03 eV / A, max stress: 0.05 GPa, max displacement: The Euler angles (a, b, g) relating the EFG tensor PAS to the CSA tensor PAS were extracted using the program EFG-Sheffield.

[0249] High-field solid-state NMR spectroscopy confirmed the distortion of the TiO6octahedra (Fig. 4 and Table S7). The quadrupole constant C Q of 7.29 MHz was obtained for SnTiO3, which indicates that the local distortion of the Ti sites is playing a role. However, a comparison with other compounds with ilmenite-type structure, such as CdTiO3or ZnTiO3(Fig. S9 and Table S7), shows very large C Q values, some of which exceed 15 MHz. Thus, surprisingly, the role of the lone pair in SnTiO3is opposite to the strong increase observed from the unstrained SrTiO3to the strained perovskite PbTiO3

[44] Since the electric field gradient (EFG) is a function of the cell volume, V FU from FeTiO3to SnTiO3can balance the role of the lone pair. The still relatively high experimental C Q value together with η Q = 0.0 indicates an axial symmetric distortion along the rotation axis higher than C2. Periodic structure DFT calculations indicate a value of C Q of about 6.04 MHz. Although the calculated value is 1.2 MHz smaller than the experimental one, the agreement here is much better than for the other models. This discrepancy should be related to the effects induced by the stacking faults and the large variations of the Ti positions in the octahedra. We note that it was not possible to distinguish between the different Ti sites even at very high magnetic fields of 21.1 T.

[0250] In the following, specific embodiments of the invention will be compiled:

[0251] (1) A material of the formula SnTiO3having a crystal structure comprising layers,

[0252] wherein

[0253] the layers comprise Sn(II) ions, Ti(IV) ions and edge-sharing O6octahedra,

[0254] the edge-sharing O6octahedra form sub-layers, i.e. sub-layers within the layers,

[0255] the Ti(IV) ions are located within 2 / 3 of the edge-sharing O6octahedra, thereby forming edge-sharing TiO6octahedra,

[0256] The shared-edge TiO6-octahedrons form a honeycomb structure within the sublayer. The honeycomb structure includes hexagons, each containing Ti(IV)- vacancies.

[0257] The Sn(II) ions are located above and below the Ti(IV)- vacancies relative to the sublayer.

[0258] The Ti(IV) ions may optionally be substituted with M.

[0259] M is one or more elements selected from groups 4 and 14, and

[0260] The Sn(II) ion has a tetrahedral coordination layer containing three O ions and the lone pair of electrons of the Sn(II) ion, with the Sn(II) ion located at the top relative to the three O ions of the layer.

[0261] (2) A material with the formula SnTiO3, having a crystal structure comprising layers.

[0262] in

[0263] The layer comprises Sn(II) ions, Ti(IV) ions, and edge-sharing O6-octahedra.

[0264] The shared-edge O6-octahedrons form sublayers, that is, sublayers within each of the aforementioned layers.

[0265] The Ti(IV) ions are located within 2 / 3 of the edge-sharing O6-octahedron, thereby forming an edge-sharing TiO6-octahedron.

[0266] The shared-edge TiO6-octahedrons form a honeycomb structure within the sublayer. The honeycomb structure includes hexagons, each containing Ti(IV)- vacancies.

[0267] The Sn(II) ions are located above and below the Ti(IV)- vacancies relative to the sublayer.

[0268] The Ti(IV) ions may optionally be substituted with M.

[0269] M is one or more elements selected from groups 4 and 14, and

[0270] The layers are stacked such that each layer is translated relative to its adjacent layer according to stacking vector S1 or stacking vector S2.

[0271] The centers of adjacent hexagons form a parallelogram, with one side having length x and the other side having length y.

[0272] The stacking vector S1 is a combined translation of 2 / 3 x along an edge having length x and 1 / 3 y along an edge having length y,

[0273] The stacking vector S2 is a combined translation of 1 / 3 x along an edge having length x and 2 / 3 y along an edge having length y,

[0274] and the crystal structure comprises layers that are translated relative to adjacent layers according to stacking vector S1 and layers that are translated relative to adjacent layers according to stacking vector S2.

[0275] (3) A material of formula SnTi03 having a crystal structure comprising layers,

[0276] wherein

[0277] The layers comprise Sn(II) ions, Ti(IV) ions and edge-sharing O6-octahedra,

[0278] The edge-sharing O6-octahedra form sub-layers, i.e. sub-layers within each of the layers,

[0279] The Ti(IV) ions are located within 2 / 3 of the edge-sharing O6-octahedra, thereby forming edge-sharing TiO6-octahedra,

[0280] The edge-sharing TiO6-octahedra form a honeycomb structure within the sub-layers, the honeycomb structure comprising hexagons having a Ti(IV)-void within,

[0281] The Sn(II) ions are located above and below the Ti(IV) voids relative to the sub-layers,

[0282] The Ti(IV) ions are optionally substituted by M,

[0283] M is one or more elements selected from group 4 and group 14 elements, and

[0284] The Sn(II) ions have a trigonal pyramidal coordination sphere comprising three O ions of the layer and the Sn(II) ion in a position at the apex, and the distance of the Sn(II) ion to the nearest O ion of an adjacent layer exceeds 1.5 times or more the distance of the Sn(II) ion to each of the three O ions of its trigonal pyramidal coordination sphere.

[0285] (4) The material according to any one of items 1 to 3, wherein the Ti(IV) ions are located at the center of the edge-sharing O6-octahedra, such that the Ti(IV) ions are located in one plane.

[0286] (5) The material according to any one of items 1 to 4, wherein the ratio of the recurrence of the shift of the layers according to stacking vector S1 relative to the adjacent layers to the recurrence of the shift of the layers according to stacking vector S2 relative to the adjacent layers is preferably in the range of 0.1 to 9, preferably in the range of 0.25 to 4, more preferably in the range of 0.5 to 2, and most preferably in the range of 0.8 to 1.3.

[0287] (6) The material according to any one of items 1 to 5, wherein the crystal structure is non-centrosymmetric.

[0288] (7) The material according to any one of items 1 to 6, wherein the O ions of the edge- sharing O6-octahedra within the sub-layers are close packed and do not form a hexagonal close packing with the O ions of the adjacent layers.

[0289] (8) The material according to any one of items 1 to 7, wherein the Ti(IV)- vacancy is surrounded by six edge-sharing TiO6-octahedra.

[0290] (9) The material according to any one of items 1 to 8, wherein the crystal structure is trigonal.

[0291] (10) The material according to any one of items 1 to 9, wherein the material has the formula SnTi 1-x M x O3, wherein x is in the range of 0 and 0.25 above and below, and M is as defined in item 1.

[0292] (11) The material according to any one of items 1 to 10, wherein M is one or more elements selected from the group consisting of Si, Ge, Zr and Hf, and preferably one or more elements selected from the group consisting of Zr and Hf.

[0293] (12) The material according to any one of items 1 to 11, having an ilmenite-type derived structure.

[0294] (13) The material according to any one of items 1 to 12, wherein the material is free-standing.

[0295] (14) A material of the formula SnTiO3 having a tetragonal perovskite-type crystal structure, wherein Ti(IV) ions are optionally substituted by M, M being one or more elements selected from group 4 and group 14 elements.

[0296] (15) The material according to item 14, wherein the material has the formula SnTi 1-x M x O3, wherein x is in the range of 0 and 0.25 above and below, and M is as defined in item 14.

[0297] (16) The material according to item 14 or 15, wherein M is one or more elements selected from the group consisting of Si, Ge, Zr and Hf, and preferably one or more elements selected from the group consisting of Zr and Hf.

[0298] (17) A method for the preparation of SnTiO3, said method comprising the steps of:

[0299] (1) reacting an alkali metal salt and titanium (IV) oxide to obtain a layered alkali metal titanate;

[0300] (2) ion-exchanging the layered alkali metal titanate with a tin (II) salt to obtain a tin-exchanged alkali metal titanate;

[0301] (3) dehydrating the tin-exchanged alkali metal titanate in an oxygen-free atmosphere, followed by annealing in an oxygen-free atmosphere at a temperature below which disproportionation of tin (II) occurs, thereby obtaining SnTiO3,

[0302] wherein the titanium ions of the titanium (IV) oxide are optionally substituted by M, M being one or more elements selected from group 4 and group 14 elements.

[0303] (18) The method according to item 17, said method comprising an additional step 4 of washing and drying SnTiO3.

[0304] (19) The method according to item 17 or 18, wherein the reaction in step 1 is carried out at a temperature in the range of 400 to 800 °C, preferably at 500 to 700 °C, most preferably at 550 to 650 °C.

[0305] (20) The method according to any one of items 17 to 19, wherein the alkali metal salt used in step 1 is an alkali metal carbonate, preferably K2CO3.

[0306] (21) The method according to any one of items 17 to 20, wherein up to 25 mol% of the titanium of the titanium (IV) oxide used in step 1 is substituted by M.

[0307] (22) The method according to any one of items 17 to 21, wherein M is one or more elements selected from the group of Si, Ge, Zr and Hf, and preferably one or more elements selected from the group consisting of Zr and Hf.

[0308] (23) The method according to any one of items 17 to 22, wherein the layered alkali metal titanate is K2CO3 or Li2CO3.

[0309] (24) The method according to any one of items 17 to 23, wherein the ion exchange in step 2 comprises mixing the layered alkali titanate and the tin(II) salt, preferably at a temperature below 60 °C.

[0310] (25) The method according to item 24, wherein the mixing comprises grinding or milling the layered alkali titanate and the tin(II) salt.

[0311] (26) The method according to any one of items 17 to 25, wherein the ion exchange in step 2 comprises:

[0312] dispersing the layered alkali titanate and the tin(II) salt to obtain a dispersion in the aqueous medium, and

[0313] dispersing the layered alkali titanate and the tin(II) salt to obtain a dispersion in the aqueous medium, and

[0314] stirring the dispersion at a temperature below 70 °C.

[0315] (27) The method according to any one of items 17 to 26, wherein the dehydration in step 3 occurs at a temperature below 250 °C, preferably below 200 °C.

[0316] (28) The method according to any one of items 17 to 27, wherein the dehydration in step 3 results in a water content below 5 wt.%, preferably below 3 wt.%, most preferably below 1 wt.%, based on the tin-exchanged alkali titanate.

[0317] (29) The method according to item 27, wherein the dehydration in step 3 comprises a first dehydration step 3a at a temperature in the range of 100 to 150 °C, and a subsequent second dehydration step 3b at a temperature in the range of above 150 to below 250 °C, preferably in the range of above 150 to 200 °C.

[0318] (30) The method according to any one of items 17 to 29, wherein the annealing in step 3 occurs at a temperature in the range of 250 to 400 °C, preferably in the range of 250 to 350 °C, most preferably in the range of 275 to 325 °C.

[0319] (31) The method according to any one of items 17 to 30, wherein the dehydration and the annealing in step 3 are performed by continuously heating the tin-exchanged alkali titanate obtained in step 2 to the annealing temperature at a heating rate of 5 °C / min or less, preferably 3 °C / min or less, more preferably 1 °C / min or less, in an oxygen-free atmosphere.

[0320] (32) The method according to any one of items 17 to 31, wherein the material has the formula SnTi1-x M x O3, wherein x is in the range of 0 or above and 0.25 or below.

[0321] (33) The method according to any one of items 17 to 32, comprising the additional step of exposing SnTiO3 to a pressure of 4 GPa or above, preferably 6 GPa or above, more preferably 8 GPa or above, even more preferably 15 GPa or above, most preferably 20 GPa or above, to obtain SnTiO3 having a perovskite-type structure.

[0322] (34) The method according to item 33, comprising the additional step of quenching SnTiO3 having a perovskite-type structure before releasing the pressure, such that the phase transition during and after pressure release is suppressed.

[0323] (35) A material of the formula SnTiO3, wherein Ti is optionally substituted by M, M being one or more elements selected from the group consisting of the elements of group 4 and 14, obtainable by the preparation method of items 17 to 32.

[0324] (36) The material according to item 35, wherein the material has the formula SnTi 1-x M x O3, wherein x is in the range of 0 or above and 0.25 or below.

[0325] (37) The material according to item 35 or 36, wherein,

[0326] the material comprises a close-packed sublattice of O ions, the close-packed sublattice consisting of two atomic layers of O ions,

[0327] Ti(IV) ions are located in 2 / 3 of the octahedral voids of the close-packed sublattice of O ions, and

[0328] Sn(II) ions are located between each adjacent close-packed sublattice of O ions.

[0329] (38) A material of the formula SnTiO3, wherein Ti is optionally substituted by M, M being one or more elements selected from the group consisting of the elements of group 4 and 14, obtainable by the preparation method of items 33 or 34.

[0330] (39) The material according to item 38, wherein the material has the formula SnTi 1-x M x O3, wherein x is in the range of 0 or above and 0.25 or below.

[0331] (40) A device comprising a ferroelectric material, wherein the ferroelectric material comprises at least one material selected from the group of materials of items 1 to 13 and materials of items 14 to 16.

[0332] (41) The device according to item 40, wherein the ferroelectric material is the material of items 14 to 16.

[0333] (42) Use of the material of any one of items 1 to 13 in a ferroelectric element.

[0334] (43) Use of the material of any one of items 14 to 16 in a ferroelectric element.

[0335] (44) Use of the material of any one of items 35 to 37 in a ferroelectric element.

[0336] (45) Use of the material of item 38 or 39 in a ferroelectric element.

[0337] (46) A material of formula SnTiO3or SnTi 1-x M x O3, wherein M is one or more elements selected from the group consisting of group 4 and group 14 elements, and x is in the range of 0 or more and 0.25 or less,

[0338] and wherein the material comprises a close-packed layer of O ions consisting of two atomic layers of O ions,

[0339] Ti(IV) ions are located in 2 / 3 of the octahedral sites of the close-packed layer of O ions, and

[0340] Sn(II) ions are located between each adjacent close-packed sublayer of O ions.

[0341] (47) Use of the material of any one of items 1 to 13 as a high-k dielectric.

[0342] (48) Use of the material of any one of items 14 to 16 as a high-k dielectric.

[0343] (49) Use of the material of any one of items 35 to 37 as a high-k dielectric.

[0344] (50) Use of the material of item 38 or 39 as a high-k dielectric.

Claims

1. A material with the formula SnTiO3, said material having a crystal structure comprising layers, in The layer comprises Sn(II) ions, Ti(IV) ions, and edge-sharing O6-octahedra. The shared-edge O6-octahedrons form a sublayer. The Ti(IV) ions are located within 2 / 3 of the edge-sharing O6-octahedron, thereby forming an edge-sharing TiO6-octahedron. The shared-edge TiO6-octahedrons form a honeycomb structure within the sublayer. The honeycomb structure comprises hexagons, within which Ti(IV)- vacancies are present. The Sn(II) ions are located above and below the Ti(IV)- vacancies relative to the sublayer, and The crystal structure satisfies at least one of the following characteristics (i) and (ii): (i) The Sn(II) ion has a tetrahedral coordination layer comprising three O ions of the layer and lone pairs of electrons of the Sn(II) ion, wherein the lone pairs of electrons are located at the top relative to the three O ions of the layer. (ii) The layers are stacked such that each layer is translated relative to each adjacent layer according to stacking vector S1 or stacking vector S2. The centers of adjacent hexagons form a parallelogram, with one side having length x and the other side having length y. The stacking vector S1 is a combined translation of 2 / 3x along an edge with length x and 1 / 3y along an edge with length y. The stacking vector S2 is a combined translation of 1 / 3x along an edge with length x and 2 / 3y along an edge with length y. And the crystal structure includes: The layer that is translated relative to the adjacent layer according to the stacking vector S1; and the layer that is translated relative to the adjacent layer according to the stacking vector S2.

2. The material according to claim 1, wherein, The Ti(IV) ions are substituted with M, which is one or more elements selected from Group 4 and Group 14.

3. The material according to claim 1 or 2, wherein, The ratio of the reproduction of a layer according to the stacking vector S1 relative to the translation of an adjacent layer to the reproduction of a layer according to the stacking vector S2 relative to the translation of an adjacent layer is in the range of 0.1 to 9.

4. The material according to claim 1 or 2, wherein, The O ions of the shared-edge O6-octahedrons within the sublayer are closely packed, but do not form a close packing with the O ions of adjacent layers.

5. The material according to claim 1 or 2, wherein, The Ti(IV)- vacancy is surrounded by six TiO6- octahedra sharing the same edge.

6. The material according to claim 2, wherein, The material has the formula SnTi 1-x M x O3, wherein x is in the range of greater than 0 to less than 0.25, and M is defined according to claim 2.

7. An apparatus comprising a ferroelectric material, wherein, The ferroelectric material comprises at least one material selected from the group consisting of materials according to any one of claims 1 to 6.

8. Use of the material according to any one of claims 1 to 6 in ferroelectric components.

9. Use of the material according to any one of claims 1 to 6 as a high-k dielectric.

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