Single crystal particle, powder, dispersion, slurry, sintered body, method for producing single crystal particle, and method for producing sintered body

Abstract

To provide single crystal particles capable of simultaneously exhibiting ferroelectricity, piezoelectricity, and ion conductivity.SOLUTION: The single crystal particles of the present invention comprise a compound containing lithium, lanthanum, titanium, and oxygen, and have a Ruddlesden-Popper type layered perovskite structure.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to single crystal particles, powder, dispersion, slurry, sintered body, a method for producing single crystal particles, and a method for producing a sintered body. [Background technology]

[0002] Oxides with a perovskite structure are known to be materials with various properties, such as ferroelectricity, piezoelectricity, and ionic conductivity, depending on their composition. For example, zinc zirconate titanate (PZT), which has ferroelectricity and piezoelectricity, is used as a dielectric in multilayer capacitors, piezoelectric elements, etc. Also, for example, lithium lanthanum titanate (LLTO), which has lithium ion conductivity, is used as a lithium ion solid electrolyte.

[0003] In Non-Patent Document 1, a polycrystalline substance exhibiting a wide range of properties such as ferroelectricity, piezoelectricity, and lithium ion conductivity was obtained, but single-crystal particles were not obtained. From the viewpoint of improving properties, the production of single-crystal particles is desired. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] Inorg. Chem. 2020, 59, 9718-9727 Summary of the Invention [Problem to be solved by the invention]

[0005] The present invention has been made in view of the above circumstances, and aims to provide single crystal particles that can simultaneously exhibit ferroelectricity, piezoelectricity, and ionic conductivity, powders, dispersions, slurries, and sintered bodies containing the single crystal particles, and methods for producing the single crystal particles and sintered bodies. [Means for solving the problem]

[0006] In order to solve the above problems, the present invention employs the following means.

[0007] (1) The single crystal particles according to one embodiment of the present invention are made of a compound containing lithium, lanthanum, titanium, and oxygen and having a Rudolfsden-Popper type layered perovskite structure.

[0008] (2) The single crystal particles according to (1) above are preferably plate-like particles having major surfaces extending in a direction intersecting the c-axis of the layered perovskite structure.

[0009] (3) A powder according to one aspect of the present invention contains a plurality of single crystal particles according to either (1) or (2) above.

[0010] (4) A dispersion according to one aspect of the present invention contains the powder described in (3) above.

[0011] (5) A slurry according to one aspect of the present invention contains the powder described in (3) above.

[0012] (6) A sintered body according to one aspect of the present invention includes a plurality of single crystal particles according to either (1) or (2) above.

[0013] (7) In the sintered body according to (6) above, it is preferable that the plurality of single crystal grains are c-axis oriented.

[0014] (8) The sintered body according to either (6) or (7) above has a pellet-like shape.

[0015] (9) The sintered body according to either (6) or (7) above, has a sheet-like shape.

[0016] (10) A method for producing single crystal particles according to one embodiment of the present invention is the method for producing single crystal particles according to either (1) or (2) above, in which a solution of a lithium compound, a lanthanum compound, and a water-soluble titanium complex is mixed with a solution of lithium hydroxide to obtain a solution, and the solution is heated to synthesize the particles.

[0017] (11) In the method for producing single-crystal particles according to (10) above, the heating is preferably carried out by solvothermal synthesis (hydrothermal synthesis when the solvent is water).

[0018] (12) In the method for producing single-crystal particles according to either (10) or (11), the heating is preferably carried out at a temperature of 170° C. or higher and 290° C. or lower for a time of 2 hours or higher and 72 hours or lower.

[0019] (13) A method for producing a sintered body according to one embodiment of the present invention is a method for producing a sintered body according to any one of (6) to (9), in which the powder according to (3) is uniaxially pressed and heated at a temperature of 1000°C or higher and lower than 1400°C for 1 hour or higher and 10 hours or lower.

[0020] (14) In the method for producing a sintered body according to (13), it is preferable that lithium carbonate is added to the powder before the heating. [Effects of the Invention]

[0021] According to the present invention, it is possible to provide single crystal particles that can simultaneously exhibit ferroelectricity, piezoelectricity, and ionic conductivity, powders, dispersions, slurries, and sintered bodies containing the single crystal particles, and methods for producing the single crystal particles and sintered bodies. [Brief explanation of the drawings]

[0022] [Figure 1] FIG. 1 is a perspective view of a single crystal particle according to one embodiment of the present invention. [Figure 2] FIG. 2 is a diagram showing the crystal structure (Rdolden-Popper type layered perovskite structure) of the single crystal particle of the embodiment. [Figure 3] 1A is a side view of the powder of single crystal particles according to the embodiment molded into a plate shape, and FIG. 1B is a cross-sectional view of the molded body at a predetermined position. [Figure 4](a) to (e) are SEM images of the surfaces of samples of Examples 12, 16, 18, 17, and 18. [Figure 5] 1 shows X-ray diffraction patterns of samples of Examples 12, 16, 18, 17, and 18. [Figure 6] 1(a) to 1(f) are SEM images of the sample surfaces of Comparative Example 1, Examples 5, and Examples 1 to 4. [Figure 7] (a) to (e) are SEM images of the surfaces of samples of Examples 6 to 9 and 12. [Figure 8] 1 shows X-ray diffraction patterns of samples of Comparative Example 1, Examples 5, 1 to 4, 6 to 9, and 12. [Figure 9] 1 is an SEM image of the surface of the sample of Example 14. [Figure 10] 1 is an X-ray diffraction pattern of a sample of Example 14. [Figure 11] 10(a) to 10(c) are SEM images of the surfaces of samples of Example 30 and Comparative Examples 3 and 4. [Figure 12] 1(a) to 1(c) are X-ray diffraction patterns of the samples of Example 30 and Comparative Examples 3 and 4. [Figure 13] STEM images of samples of Examples 12 and 18. (a) A low-magnification image of the main surface of the sample of Example 12. (b) and (c) High-magnification HAADF and ABF images of (a). (d) and (e) High-magnification HAADF and ABF images of a cross section perpendicular to the main surface of the sample of Example 18. [Figure 14] 10 is a loss spectrum obtained by EELS measurement of the sample of Example 18. [Figure 15] 1 is an AFM topography image of the sample of Example 19. [Figure 16] 10 is a phase curve obtained when a voltage is applied to the sample of Example 19. [Figure 17] 10 is a displacement curve obtained when a voltage is applied to the sample of Example 19. [Figure 18] 1 is an X-ray diffraction pattern of a sample of Example 21. [Figure 19] 10 is a complex impedance plot of the sample of Example 21. [Figure 20] 10 shows the frequency dependence of the complex dielectric constant of the sample of Example 21. [Figure 21] 1 is an X-ray diffraction pattern of a sample of Example 21. [Figure 22] 10 is a complex impedance plot of the sample of Example 21. [Figure 23] FIG. 23 is an enlarged view of a portion of the complex impedance plot of FIG. 22. DETAILED DESCRIPTION OF THE INVENTION

[0023] Hereinafter, single crystal particles, powders, dispersions, slurries, sintered bodies, methods for manufacturing single crystal particles, and methods for manufacturing sintered bodies according to embodiments of the present invention will be described in detail with reference to the accompanying drawings. Note that the drawings used in the following description may show characteristic portions enlarged for ease of understanding, and the dimensional ratios of each component element may not necessarily be the same as those in reality. Furthermore, the materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not limited thereto. Appropriate modifications may be made within the scope of the present invention.

[0024] [Single crystal particles] Fig. 1 is a perspective view of a plate-like single crystal particle (single crystal fine particle) 100 according to one embodiment of the present invention. Fig. 2 is a diagram showing the crystal structure of the single crystal particle 100.

[0025] The single-crystal particle 100 contains lithium (Li), lanthanum (La), titanium (Ti), and oxygen (O), and is represented as Li2La2Ti3O 10 It consists of Compound 101, which has a Rudolfsden-Popper type layered perovskite structure (RP layered perovskite structure) with the basic composition formula: Note that, as long as this crystal structure can be obtained, deviations from the stoichiometric ratio of Li, La, Ti, and O are acceptable, and different trace amounts of elements may be doped at each site.

[0026] The RP layered perovskite structure of this embodiment includes a layered unit structure 103 including a plurality of perovskite-type crystal structures 102 with their c-axis directions aligned, and a Li layer structure 104 aligned along a plane (c-plane) perpendicular to the c-axis direction. The layered unit structures 103 are stacked in the c-axis direction (here, vertically) with the Li layer structure 104 sandwiched between them.

[0027] Each perovskite-type crystal structure 102 is composed of Ti, O coordinated at the vertices of an octahedron centered on Ti, and La coordinated at the vertices of a cube centered on Ti. Li in the layer structure 104 is bonded to O that constitutes the crystal structure 102.

[0028] The single crystal particle 100 is a plate-like particle having a main surface 100a extending in a direction intersecting the c-axis of the RP layered perovskite structure, preferably in a direction perpendicular to the c-axis. The main surface 100a refers to a surface having a larger area than the other surfaces. It is assumed that the plate-like particle has two main surfaces 100a positioned opposite each other. Here, the main surface 100a is illustrated as being rectangular, but the shape and size of the main surface 100a are not particularly limited. Furthermore, the shape and size of one of the two main surfaces 100a may be the same as or different from the other main surface.

[0029] In the plate-like general shape of the single crystal particle 100, the aspect ratio (W / T) of the maximum diameter W of the main surface 100a to the thickness (width in the c-axis direction) T is preferably 10 or more and 1000 or less. The area of ​​the main surface 100a of the single crystal particle is not particularly limited, but may be, for example, 2500 nm 2 or more, 4μm 2 The thickness of the single crystal particle 100 is not particularly limited, but is, for example, 1 nm or more and 50 nm or less.

[0030] The single crystal particle 100 has piezoelectric properties, and when the sign of the voltage applied in the c-axis direction is changed, the phase is reversed by 180°. The piezoelectric properties can be evaluated using a scanning probe microscope in piezoelectric response microscope mode (PFM). For samples to be evaluated for piezoelectric properties, it is preferable to ultrasonically disperse the particles in ethanol, dry them, and then irradiate them with UV light to remove any contamination.

[0031] [Method of manufacturing single crystal particles] The single crystal particles of this embodiment can be produced mainly through the following two steps.

[0032] (Solution preparation process) A solution containing a lithium compound, a lanthanum compound, and a water-soluble titanium complex is obtained. The amount and concentration of the solution are preferably adjusted so that the composition ratio of the lithium, lanthanum, and titanium raw materials contained in the solution is in the range of 0.5:0.5:1 to 2.2:2.2:3.

[0033] Examples of lithium compounds that can be used include lithium halides such as LiNO3, CH3COOLi, and LiCl, and Li2SO4·H2O. Examples of lanthanum compounds that can be used include lanthanum halides such as La(NO3)3·6H2O, La(CH3COO)3·nH2O, and LaCl3·7H2O, and La2(SO4)3·9H2O.

[0034] The water-soluble titanium complex may be a compound that, after dissolving in a solvent such as water, dissociates the ligand from the titanium atom to form a bond between the titanium atom and an oxygen atom. Such a compound is preferably one in which the ligand is a hydroxycarboxylic acid.

[0035] Specific examples of hydroxycarboxylic acids include lactic acid, malic acid, citric acid, tartaric acid, glyceric acid, 2-hydroxybutyric acid, leucic acid (=2-hydroxy-4-methylpentanoic acid), quinic acid, mandelic acid (=2-hydroxy-2-phenylacetic acid), glycolic acid, etc.

[0036] Examples of water-soluble titanium complexes include titanium bis(ammonium lactate) dihydroxide (TALH) in which the ligand is lactic acid, (NH)[Ti(CHO)(CHO)(OO)O]·6HO in which the ligand is glycolic acid (HOCHCOOH), (NH)[Ti(CHO)(OO)]·8HO in which the ligand is citric acid ((CHCOOH)C(OH)COOH), and titanium complexes in which the ligand is malic acid (CHCHOH(COOH)) or tartaric acid ((CHOH)(COOH)).

[0037] A lithium hydroxide solution is added to the solution of the lithium compound, lanthanum compound, and water-soluble titanium complex. A portion of the lithium hydroxide solution may be replaced with sodium hydroxide. Specifically, approximately 0% to 12.5% ​​of the moles of lithium hydroxide contained in the solution may be replaced with sodium hydroxide. That is, the molar ratio of sodium hydroxide to lithium hydroxide may be approximately 0:10 to 7:1. From the viewpoint of promoting single crystallization, the moles of sodium hydroxide are preferably 15% or less of the moles of lithium hydroxide.

[0038] (Solution heating process) The prepared solution is heated to synthesize the compound. The synthesis solution can be prepared by solvothermal synthesis (hydrothermal synthesis when the solvent is water) by preparing a solution using an organic solvent such as ethanol or isopropanol or water and heating it. From the viewpoint of promoting single crystallization, this heating is preferably carried out at a temperature of 170°C or higher and 290°C or lower, more preferably at a temperature of 180°C or higher and 260°C or lower. From the same viewpoint, this heating is preferably carried out for a period of 2 hours or higher and 72 hours or lower, more preferably at a period of 12 hours or higher and 48 hours or lower.

[0039] After the above steps, a powder containing a plurality of the synthesized single crystal particles of this embodiment is obtained through centrifugation, washing, and drying. This powder is dispersed in a solution under predetermined conditions to obtain a dispersion or slurry containing the powder. For example, in preparing the slurry, a binder is first prepared. Examples of binders include a polyvinyl butyral binder, a diamine, and bis(2-ethylhexyl) adipate, each of which is prepared in a predetermined mass ratio relative to the solid electrolyte powder. Materials used for the binder are not limited to these. Next, the binder is dissolved in a solvent, such as a mixture of toluene and butanol in a predetermined volume ratio, to which the powder prepared above is added, followed by stirring and dispersion in a kneader. Thereafter, stirring is continued while reducing the pressure to, for example, about 0.2 kPa, and the slurry is concentrated to obtain the desired slurry. Materials used for the solvent are not limited to these. Furthermore, to prepare a dispersion, a surfactant such as oleic acid, oleylamine, or cetyltrimethylammonium bromide (CTAB) and a solvent such as toluene, mesitylene, ethanol, isopropanol, hexane, or acetone are mixed with the powder, and then ultrasonic dispersion or stirring is performed to obtain a dispersion containing the powder. Note that the materials used for the surfactant and solvent are not limited to these.

[0040] [Sintered body and its manufacturing method] The powder of single crystal particles synthesized through the above process is subjected to uniaxial pressing and heating (sintering) to obtain a sintered body containing a plurality of single crystal particles of this embodiment. In this sintered body, the lithium ion conductivity in the direction parallel to the c-plane is at least 2.7 times that in the direction perpendicular to the c-plane at room temperature. Furthermore, in this sintered body, the dielectric constant in the direction parallel to the c-plane is at least 1.2 times that in the direction perpendicular to the c-plane.

[0041] The lithium ion conductivity can be evaluated from a Nyquist plot measured using an impedance analyzer with gold electrodes deposited on both sides of the sintered compact. The dielectric constant can be calculated from the electrode area and sample thickness by measuring the frequency dependence of capacitance and loss with an impedance analyzer with gold electrodes deposited on both sides of the sintered compact.

[0042] Uniaxial pressing can be performed, for example, by pressing powder packed into a molding die (e.g., a cylindrical die with a diameter of 10 mm or a square die with dimensions of 5 mm x 20 mm) with a pressure of, for example, 0.1 kN or more and 60 kN or less.

[0043] 3(a) is a side view showing a state in which powder 10 containing single crystal particles 100 has been uniaxially pressed in the z-axis direction and formed into a plate-like shape. This uniaxial pressing causes the plurality of plate-like single crystal particles 100 contained in powder 10 to be controlled so that their main surfaces 100m and the c-plane 100c of the crystal structure are perpendicular to the pressing direction. By forming electrodes on either side of this sample in the z-axis direction in the figure, the properties in the direction perpendicular to the c-plane can be measured.

[0044] Figure 3(b) shows a cross-sectional view of the plate-shaped powder cut perpendicular to the main surface at a predetermined position (here, the position where the α-α line passes) after molding. By forming electrodes on either side of the x-axis (y-axis) of this sample in the figure, it is possible to measure the properties parallel to the c-plane.

[0045] The heating temperature is preferably, for example, in the range of 1000°C or higher and lower than 1400°C, and more preferably in the range of 1200°C or higher and 1300°C or lower. The heating time is, for example, preferably in the range of 1 hour or higher and 10 hours or lower, and more preferably in the range of 3 hours or higher and 4 hours or lower. The orientation of the plate-like single crystal particles 100 is maintained even after heating. Lithium carbonate may be added as an auxiliary agent to the powder after uniaxial pressing before heating. Adding lithium carbonate makes it possible to densify the sintered body to a relative density of 85% or higher without destroying the crystal structure.

[0046] The single crystal particles contained in the obtained sintered body are c-axis oriented in the direction of uniaxial pressure. This c-axis orientation is thought to occur as a result of the single crystal particles 100 being plate-shaped, and the orientation of the main surfaces 100m of the single crystal particles being automatically adjusted so that they fit into the compressed region of the powder 10. Therefore, the c-planes of the plate-shaped single crystal particles contained in the sintered body are aligned approximately parallel to the main surfaces of the plate-shaped sintered body.

[0047] The obtained sintered body can be processed by cutting, polishing, etc. in a predetermined manner to produce sintered bodies in various shapes such as pellets and sheets.

[0048] As described above, the single crystal fine particles of this embodiment are made of a compound having an RP layered perovskite structure, and therefore exhibit ferroelectricity and piezoelectricity in addition to ionic conductivity.

[0049] The single-crystal fine particles of this embodiment are plate-like particles, and the sintered body of this embodiment contains these plate-like particles. Therefore, the orientation of the plate-like particles can be easily controlled by uniaxial pressing during the manufacturing process of the sintered body of this embodiment, and the desired ionic conductivity, ferroelectricity, and piezoelectricity can be simultaneously achieved. [Example]

[0050] The effects of the present invention will be made clearer by the following examples. Note that the present invention is not limited to the following examples and can be practiced with appropriate modifications within the scope of the present invention.

[0051] (Examples 1 to 31) According to the above-described embodiment, a single-crystal particle sample was synthesized using the following procedure. Predetermined amounts of aqueous solutions of La(NO3)·6H2O, LiNO3, and TALH were mixed so that the lithium, lanthanum, and titanium composition ratios were between 0.5:0.5:1 and 2.2:2.2:3. This solution was then mixed with predetermined amounts of LiOH aqueous solution and H2O, and the mixture was placed in an autoclave (100 mL capacity) with a carbon-containing Teflon (registered trademark) inner tube (heat-resistant up to 290°C, manufactured by San-Ai Scientific Co., Ltd.). The solution in the autoclave was then stirred and held at 180°C to 260°C for 2 to 48 hours. The precipitate was then centrifuged and collected, followed by ultrasonic cleaning in ethanol, twice. The collected precipitate was dried in a vacuum dryer at 60°C to obtain a powder of plate-shaped single-crystal particles. In Example 18, 0.9518 g of La(NO3)·6H2O, 0.1514 g of LiNO3, 1.44 ml of a 50 wt% aqueous solution of titanium(IV) bisammonium lactatohydroxide (TALH), 7.2 ml of a 5 M aqueous solution of lithium hydroxide, and 51.36 ml of H2O were placed in an autoclave (capacity 100 ml) with a carbon-containing Teflon inner cylinder.

[0052] (Comparative Example 1) A powder of single crystal particles was obtained by the same treatment as in Example 1, except that the raw materials did not contain an aqueous solution of LiOH. However, the obtained single crystal particles were not plate-like particles.

[0053] (Comparative Example 2) A powder of single crystal particles was obtained by the same treatment as in Example 1, except that the heating time was changed to 1 hour. However, the obtained single crystal particles did not have an RP layered perovskite structure and did not become plate-like particles.

[0054] (Comparative Examples 3 and 4) A powder of single crystal particles was obtained by the same process as in Example 1, except that anatase and rutile TiO2 were used as the Ti source. However, the obtained single crystal particles did not have an RP layered perovskite structure and did not become plate-like particles.

[0055] Table 1 shows specific values ​​of the treatment conditions for each sample of Examples 1 to 31 and Comparative Examples 1 to 4.

[0056] [Table 1]

[0057] 4(a) to 4(e) are SEM images of the single crystal particles of Examples 12, 16, 18, 17, and 18, respectively. All of the single crystal particles, which were obtained by varying the composition ratio of lithium, lanthanum, and titanium contained in the raw materials, were plate-like particles with main surfaces. In this state, the orientation of the main surfaces was not biased in any particular direction. Many of the particles had angles close to 90 degrees. The particles smaller than the plate-like particles were excess lanthanum, and their number could be reduced by adjusting the composition ratio of the single crystal particles.

[0058] 5 is a graph showing X-ray diffraction patterns obtained for the single crystal particles of Examples 12 and 16 to 18. The bottom row shows Li2La2Ti3O 10 The X-ray diffraction pattern of La(OH)3 at the top is a typical X-ray diffraction pattern indicating a simple perovskite structure. The X-ray diffraction pattern of La(OH)3 at the top is a typical X-ray diffraction pattern indicating an RP layered perovskite structure.

[0059] All of the single crystal particles with different lithium, lanthanum, and titanium composition ratios exhibited X-ray diffraction patterns similar to that of the topmost La(OH)3. These results demonstrate that the samples of Examples 12 and 16 to 18 have an RP layered perovskite structure.

[0060] Figures 6(a) to (f) and Figures 7(a) to (e) are SEM images of single crystal particles from Comparative Example 1, Examples 5, 1 to 4, 6 to 9, and 12, respectively. All single crystal particles, which were prepared by varying the molar ratio of lithium hydroxide and sodium hydroxide used as raw materials, are plate-like particles with main surfaces. In this state, the orientation of the main surfaces is not biased in any particular direction. Many particles have an angle close to 90 degrees. Particles smaller than the plate-like particles are excess lanthanum, and their number can be reduced by increasing the molar ratio of lithium hydroxide.

[0061] 8 is a graph showing X-ray diffraction patterns obtained for the single crystal particles of Comparative Example 1, Examples 5, 1 to 4, 6 to 9, and 12. 0.35 La 0.55 The X-ray diffraction pattern of TiO3 is a typical X-ray diffraction pattern showing a simple perovskite structure. 10 is a typical X-ray diffraction pattern showing the RP layered perovskite structure.

[0062] As an example, the single crystal particles with different molar ratios of lithium hydroxide and sodium hydroxide were prepared. 10 On the other hand, the single crystal particles of the comparative example, which did not contain lithium hydroxide as a raw material, show the same X-ray diffraction pattern as the Li 0.35 La 0.55 This shows the X-ray diffraction pattern of TiO3. From this result, it can be seen that when the raw material contains lithium hydroxide, single crystal particles with an RP layered perovskite structure can be obtained. Conversely, when the raw material does not contain lithium hydroxide, single crystal particles with an RP layered perovskite structure cannot be obtained.

[0063] 9 is an SEM image of the single crystal particles of Example 14. The single crystal particles of Example 14 are also plate-like particles similar to the single crystal particles of the above examples.

[0064] FIG. 10 is a graph showing the X-ray diffraction pattern obtained for the single crystal particles of Example 14. The single crystal particles of Example 14 also show an X-ray diffraction pattern of an RP layered perovskite structure. From these results, it can be seen that single crystal particles having an RP layered perovskite structure can be obtained even if the heating temperature in the synthesis of the single crystal particles is reduced to 180°C. It can also be seen that single crystal particles having an RP layered perovskite structure can be obtained even if the heating time in the synthesis is reduced to 24 hours.

[0065] 11(a) to 11(c) are SEM images of the single crystal particles of Example 30 and Comparative Examples 3 and 4, respectively. The single crystal particles of Example 30 obtained using TALH as the titanium raw material are plate-like particles. In contrast, the single crystal particles of Comparative Examples 3 and 4, which use anatase titanium oxide and rutile titanium oxide as the titanium raw material, are not plate-like particles.

[0066] 12(a) to 12(c) are graphs showing the X-ray diffraction patterns obtained for the single crystal particles of Example 30 and Comparative Examples 3 and 4, respectively. The single crystal particles of Example 30 exhibit an X-ray diffraction pattern of an RP layered perovskite structure, whereas the single crystal particles of Comparative Examples 3 and 4 exhibit X-ray diffraction patterns of a compound of a different phase. These results show that there are limited compositions of titanium raw materials that can be selected when producing a compound with an RP layered perovskite structure, and that TALH is a suitable composition.

[0067] Figure 13 shows images obtained by STEM observation of the single crystal microparticles of Examples 12 and 18. Figure 13(a) is a low-magnification image of the main surface of the single crystal particle of Example 12. Figures 13(b) and 13(c) are high-magnification HAADF and ABF images, respectively, of Figure 13(a). Figures 13(d) and 13(e) are high-magnification HAADF and ABF images, respectively, of a cross section perpendicular to the main surface of the single crystal particle of Example 18.

[0068] Figures 13(b) and (c) show the c-plane of the RP layered perovskite structure, while Figures 13(d) and (e) show the planes parallel to the c-axis of the same structure. These observations reveal that the single crystal particles of Examples 12 and 18 are plate-like particles with major surfaces parallel to the c-plane of the RP layered perovskite structure. Furthermore, the width (maximum diameter) of the major surfaces of the single crystal particles averages approximately 387 nm, and the thickness of the single crystal particles averages approximately 9.4 nm, indicating that the aspect ratio of the width of the major surfaces to the thickness is at least approximately 41.

[0069] 14 is a graph showing the results of EELS measurement performed on the single crystal particles of Example 18. A peak due to lithium is observed in the loss spectrum in the loss energy range of 50 to 70 eV, indicating that lithium is present in the crystal structure of the single crystal particles of Example 18.

[0070] Figure 15 is an AFM topography image of the single crystal particle of Example 19. Figures 16 and 17 are phase curves and displacement curves, respectively, obtained when the voltage applied in the thickness direction to one plate-like single crystal particle shown in the AFM topography image of Figure 15 is changed to displace the thickness of the plate-like single crystal particle.

[0071] In the phase curve of Fig. 16, the phase changes by 180 degrees when a positive voltage is applied and when a negative voltage is applied. The voltage range in which the phase changes is -0.6V to 0.6V. Furthermore, in the displacement curve of Fig. 17, an inflection point is included in this voltage range in which the phase changes. These results demonstrate that the single crystal particles of Example 19 have piezoelectricity.

[0072] The powder of single crystal particles of Example 21 was uniaxially pressed at 10 kN and heated at 1200°C for 3 hours to produce a sintered body, which was then further processed into a pellet. Figure 18 is a graph showing the X-ray diffraction pattern obtained from this pellet-shaped sintered body. In the X-ray diffraction pattern obtained on a surface perpendicular to the pressing direction of this sintered body, a diffraction peak is present at

[0001] , where the diffraction intensity is high. In contrast, in the X-ray diffraction pattern obtained on a surface parallel to the pressing direction of this sintered body, no diffraction peak at

[0001] is observed. These results demonstrate that it is possible to control the orientation of the crystal structure in the sintered body obtained using the single crystal particles of Example 19.

[0073] 19 is a complex impedance plot of the lithium ion conductivity at room temperature for the sintered body of Example 21. The sintered body of Example 21 exhibits high lithium ion conductivity, with a value of 8.62 × 10 in the direction parallel to the c-plane. -8 S / cm, 3.23 × 10 in the direction perpendicular to the c-plane -8 S / cm. The lithium ion conductivity parallel to the c-plane is approximately 2.7 times higher than that perpendicular to the c-plane. This is thought to be because in the RP layered perovskite structure, the lithium layer structure extends parallel to the c-plane along the gaps between the layered unit structures, making it easier for lithium ions to conduct in the direction parallel to the c-plane.

[0074] Figure 20 is a graph showing the frequency dependence of the complex permittivity of the sintered body of Example 21 in the directions parallel to the c-plane and perpendicular to the c-plane. A comparison of the real parts of the complex permittivity reveals that the permittivity in the direction parallel to the c-plane is 1.2 times higher than the permittivity in the direction perpendicular to the c-plane. This result indicates that the sintered body of Example 21 is highly polarized in the direction perpendicular to the c-plane and has ferroelectricity.

[0075] The powder of single crystal particles of Example 21 was uniaxially pressed at 10 kN and heated under various conditions to produce sintered bodies. Specific heating conditions are shown in Table 2.

[0076] Figure 21 is a graph showing the X-ray diffraction patterns obtained for the sintered bodies produced under the conditions in Table 2. The bottom line shows Li2La2Ti3O 10 The X-ray diffraction pattern of the Li SiO2 layered perovskite structure is typical of the Ruddersden-Popper type. 0.3 La 0.55 The X-ray diffraction pattern of TiO3 is a typical X-ray diffraction pattern showing a simple perovskite structure.

[0077] Fig. 22 shows complex impedance plots measured in a direction parallel to the pressure axis of samples obtained by firing the powder of Example 21 under various conditions. Fig. 23 shows an enlarged view of a portion of the complex impedance plot in Fig. 22.

[0078] 21 to 23 reveal the following. At heating temperatures up to 1200°C, sintered bodies of single-crystal fine particles having an RP layered perovskite structure are obtained regardless of whether or not lithium carbonate is added. Furthermore, at heating temperatures around 1300°C, adding lithium carbonate results in a sintered body of single-crystal fine particles having an RP layered perovskite structure. However, not adding lithium carbonate results in a sintered body with a mixture of the RP layered perovskite structure and an impurity phase. Furthermore, at heating temperatures up to 1400°C, the RP layered perovskite structure is not obtained regardless of whether or not lithium carbonate is added. Table 2 lists the heating conditions, the ionic conductivity of the resulting sintered bodies, and the type of crystalline phase.

[0079] [Table 2]

[0080] The samples with RP layered perovskite structure and c-axis orientation have low ionic conductivity. In particular, the sample with Li2CO3 mixed at 1200°C for 3 hours has a conductivity of 10 -11is very low. This suggests that the Li occupancy rate is higher than in the unmixed sample because mixing Li2CO3 has the effect of compensating for the Li that volatilizes when heated, making it difficult for Li to conduct. The conductivity is one order of magnitude higher in the random orientation than in the c orientation, which is thought to be due to the ease with which Li ions conduct in the plane direction parallel to the c axis. When mixed with simple perovskite, the conductivity is significantly increased due to the contribution of the simple perovskite. The sample fired at 1400°C has a conductivity of 10 -4 However, pure perovskite LLTO (10 -3 The reason why the value is lower than the standard value is thought to be due to the presence of impurity phases. [Explanation of symbols]

[0081] 100··· Single crystal particles (plate-like particles) 100m...Main surface 100c...c side 100ab···c plane perpendicular 101...Compound 102···Perovskite-type crystal structure 103...Layered Unit Structure 104···Li layer structure 10...Powder

Claims

1. The present invention relates to single-crystal particles made of a compound containing lithium, lanthanum, titanium, and oxygen and having a Ruddersden-Popper type layered perovskite structure.

2. 2. The single crystal particle according to claim 1, which is a plate-like particle having a main surface extending in a direction intersecting the c-axis of the layered perovskite structure.

3. A powder comprising a plurality of single crystal particles according to claim 1 or 2.

4. A dispersion comprising the powder of claim 3.

5. A slurry comprising the powder of claim 3.

6. A sintered body comprising a plurality of single crystal particles according to claim 1 or 2.

7. The sintered body according to claim 6 , wherein the plurality of single crystal grains are c-axis oriented.

8. The sintered body according to claim 6 , which has a pellet-like shape.

9. The sintered body according to claim 6 , which has a sheet-like shape.

10. 3. The method for producing single crystal particles according to claim 1 or 2, A method for producing single crystal particles, comprising: mixing a solution of a lithium compound, a lanthanum compound, and a water-soluble titanium complex with a solution of lithium hydroxide to obtain a solution; and heating the solution to synthesize the single crystal particles.

11. The method for producing single-crystal particles according to claim 10 , wherein the heating is carried out using a solvothermal synthesis method.

12. The method for producing single-crystal particles according to claim 11, wherein the heating is carried out at a temperature of 170° C. or higher and 290° C. or lower for a time of 2 hours or higher and 72 hours or lower.

13. 7. A method for producing a sintered body according to claim 6, comprising: A method for producing a sintered body, comprising uniaxially pressing the powder according to claim 3 and heating it at a temperature of 1000°C or higher and lower than 1400°C for 1 hour or higher and 10 hours or lower.

14. The method for producing a sintered body according to claim 13 , wherein lithium carbonate is added to the powder before heating.

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