A-site ordered quadruple perovskite crystal and preparation method thereof

Abstract

The invention belongs to the technical field of functional materials, and relates to an A-site ordered quadruple perovskite crystal and a preparation method thereof, the chemical formula of the A-site ordered quadruple perovskite crystal is LaHg3Ti4O12, the crystal is a cubic crystal structure, and the space group is No.204. The invention further discloses a preparation method of the A-site ordered quadruple perovskite crystal. Through A-bit heterovalent doping, a brand new performance regulation and control dimension of electron doping is introduced, dependence on Pb < 2 + > lone pair electrons is avoided, and the technical defects that a ferroelectric driving mechanism is single and fragile and the performance regulation and control dimension is limited are overcome; the successful synthesis of LaHg3Ti4O12 solves the problems that the material system is single and the structure universality is not verified.

Description

Technical Field

[0001] This invention belongs to the field of functional materials technology and relates to an A-site ordered tetrap perovskite crystal and its preparation method. Background Technology

[0002] Among numerous perovskite derivatives, the A-site ordered tetrat perovskite AA′3B4O is particularly noteworthy. 12 Due to its unique 1:3 rock salt type A / A′ ordered structure, it can accommodate transition metal ions at the A′ and B sites, and generate strong coupling between spin, orbital and charge degrees of freedom through the ∠A′-OB path, thus exhibiting novel physical phenomena such as magnetoelectric coupling, colossal magnetoresistance, and charge transfer, making it one of the current hot topics in functional materials research.

[0003] However, a significant technical bottleneck has long existed in this field: the geometric configuration limitation of the A′ site. Traditional A-site ordered tetroxide perovskites, such as CaCu3Ti4O... 12 LaCu3Fe4O 12 In the above, the cation at the A′ position is usually Cu. 2+ or Mn 3+ Jahn-Teller ions, almost without exception, adopt a four-coordinated planar square configuration. This specific coordination environment greatly limits the types of ions that can be selected at the A′ site, thus restricting the diversity of the structure and further exploration and control of the properties of this type of material.

[0004] PbHg3Ti4O 12 (PHTO) is the first reported A-site ordered tetratonic perovskite to break through the aforementioned A′-site tetracoordination restriction. It possesses a unique A′-site coordination environment: the A′-site ion Hg in PHTO... 2+ Its large ionic radius (~1.1 Å) results in an approximately octet coordination configuration, rather than the traditional tetratet. This makes its crystal structure a "bridge" between simple perovskites (A-site 12 coordination) and typical A′-site 4 coordination tetratet perovskites. Ferroelectricity: PHTO is the first A-site ordered tetratet perovskite confirmed to exhibit a ferroelectric phase transition, in a highly symmetric cubic phase (space group 1). Im-3 ) and low-symmetry non-centrosymmetric phases (such as Imm2 The transition occurs between these two phases, with a Curie temperature as high as approximately 250 K. This ferroelectricity is thought to be related to Pb at the A site. 2+ 6s 2 The stereochemical activity of lone pair electrons is closely related.

[0005] However, it has the following drawbacks: 1. Limited Material System and Unverified Structural Universality: PHTO is the only A-site ordered tetragonal perovskite discovered to date with an A′-site octet coordination feature. This isolated case status means that this groundbreaking structural type remains only at the proof-of-concept stage, raising significant questions about its reproducibility and universality. Those skilled in the art cannot confirm whether this unique structure, intermediate between simple perovskites and typical tetragonal perovskites, can form a tunable material family, thus severely limiting the possibility of controlling its properties through chemical composition design.

[0006] 2. The driving mechanism of ferroelectricity is simple and fragile: the ferroelectricity of PHTO is mainly attributed to Pb at the A site. 2+ 6s of ions 2 The stereochemical activity of lone pair electrons. While this driving mechanism is effective, it is inherently fragile. Existing techniques show that when substituted with equivalent substitution (e.g., with Sr, which does not possess lone pair electrons), the activity is significantly reduced. 2+ Replacement of Pb 2+ When the A-site ion is altered, its ferroelectricity completely disappears. This indicates that the ferroelectricity of PHTO is heavily dependent on specific A-site elements, lacks universality, and greatly limits the scope for performance optimization and functional expansion through A-site engineering.

[0007] 3. Limited performance tuning dimensions and lack of effective means of electronic state control: In PHTO and its equivalent substitution systems, performance tuning mainly relies on altering the properties of the A-site ion itself (such as the strength of lone pair electrons). However, there is a lack of effective ways to directly control the electronic state of the B-site through heterovalent doping. For example, existing techniques have failed to demonstrate how to influence lattice dynamics (such as phonon modes) and induce new competing quantum orders by introducing charge carriers, thus making this material system significantly limited in exploring richer correlated electronic phenomena (such as electronic doping-induced phase transitions).

[0008] 4. Insufficient exploration of potential structural instabilities and novel properties: Although PHTO itself exhibits a clear structural phase transition, current techniques have not yet provided in-depth research on the lattice instabilities within this structural framework. In particular, it has not been revealed whether novel instability modes competing with ferroelectric instability can be induced in this system through electronic doping. This leads to a gap in our understanding of the potential hidden quantum order or other derivative properties of this type of material at low temperatures, hindering its application in next-generation quantum devices and other fields.

[0009] In summary, the core shortcomings of PHTO are that it is a unique but isolated "special case" with a single and easily disrupted driving mechanism for its core ferroelectric property, and it lacks the ability to be multidimensionally controlled through heterovalent doping, which greatly limits its application potential and development prospects in the fields of materials science and functional devices. Summary of the Invention

[0010] The main objective of this invention is to provide an A-site ordered tetrap perovskite crystal and its preparation method.

[0011] To achieve the above objectives, the specific technical solution is as follows: This invention provides an A-site ordered tetrap perovskite crystal with the chemical formula LaHg3Ti4O 12 The synchrotron radiation diffraction pattern of the crystal has characteristic peaks at 4.38, 7.66, 9.89, 11.69, 13.25, 14.72, and 15.93 at the 2θ angle, and the measurement error of the 2θ angle is ±0.2.

[0012] Furthermore, the crystal has a cubic crystal structure and a space group of . (No. 204).

[0013] This invention provides a novel A-site electron-doped, non-Pb-based A′-site octagonal quadruple perovskite material, LaHg3Ti4O. 12 The compound can be seen as another example of connecting simple perovskites with A-site ordered perovskites. The compound does not contain lead and represents the first attempt at electronic doping in this unique structure, as well as further exploration of the effect of electronic doping on the properties of the structure.

[0014] This invention introduces "electron doping," a novel dimension for performance regulation, through heterovalent doping at the A-site, thus overcoming the limitations imposed on Pb. 2+ Dependence on lone pairs of electrons. Technical principle: using La 3+ (No stereochemically active lone pair electrons) to replace Pb 2+ (Possessing highly stereochemically active lone pair electrons), it is essentially a hole doping process. This doping behavior produces two levels of technical effects: removing the single ferroelectric driving source: due to La 3+ LaHg3Ti4O, a novel material of this invention, lacks lone pairs of electrons. 12 It does not possess ferroelectric properties similar to PHTO, which precisely demonstrates the difference between its property regulation mechanism and that of PHTO. Introducing electronic state control degrees of freedom: This heterovalent doping alters the electronic structure of the system, allowing the material's physical properties (such as conductivity, magnetism, and lattice vibrations) to be effectively controlled through carrier concentration. This facilitates further chemical doping (such as in La...) 1- x A x Hg3Ti4O 12 This provides the possibility of finely tuning electronic correlation effects and lattice instabilities through A-site substitution.

[0015] This invention directly addresses the technical shortcomings of ferroelectricity, namely its singular and fragile driving mechanism and limited performance regulation dimensions. It breaks the dependence of PHTO on specific A-site ions, shifting the "switch" for performance regulation from the single factor of lone pair electrons to the more flexible and diverse carrier concentration.

[0016] This invention also provides a method for preparing the above-mentioned A-site ordered tetrap perovskite crystal, comprising the following steps: (1) Grind and mix La2O3, HgO and TiO2 to obtain a mixed powder; (2) Press the mixed powder in step (1) into a cylindrical blank, package it, and react it for 15-60 min at a pressure of 3-8 GPa and a temperature of 900-1200℃ to obtain A-site ordered tetragonal perovskite crystals.

[0017] This invention utilizes a high-temperature, high-pressure synthesis technique, employing trivalent rare earth ions (La) to synthesize... 3+ (Ionic radius 1.36 Å, coordination number 12) Replaces the divalent Pb ion in PHTO 2+ (Ionic radius 1.49 Å, coordination number 12), while retaining Hg with a large ionic radius at the A′ site. 2+ To maintain an eight-coordinate environment. The successful implementation of this substitution is based on the following technical principle: crystal field stability: La 3+ Ionic radius and Pb 2+ Comparable, able to stably occupy the A-site in the perovskite structure, satisfying geometric requirements; charge compensation mechanism: although the valence state of the A-site ion changes (from Pb... 2+ To La 3+ However, the charge balance of the entire compound is adaptively adjusted by the valence state of the Ti ion at the B site (from Ti). 3+ / Ti 4+ Ti with mixed valence state adjusted to stable 4+ This allowed the formation of a thermodynamically viable new compound, LaHg3Ti4O. 12 .

[0018] The improvement of this invention directly solves the problem of a single material system and unverified structural universality. LaHg3Ti4O 12 The successful synthesis of this material proves that the A′-octet quadruple perovskite structure is not a unique “special case” of PHTO, but a novel material platform with universal applicability and chemical design capabilities.

[0019] Furthermore, the molar ratio of La2O3, HgO and TiO2 is 1:6:8.

[0020] Furthermore, the La2O3 is pretreated by pre-calcining in air at 500-1000℃ for 2-12 hours.

[0021] La2O3 readily absorbs water and CO2. Before use, it can be pre-burned in air or oxygen to remove adsorbates, and then transferred to a desiccator or glove box for storage.

[0022] Furthermore, the purity of La2O3 is not less than 99.9%, the purity of HgO is not less than 99%, and the purity of TiO2 is not less than 99%. Preferably, the purity of La2O3 is 99.99%, the purity of HgO is 99.7%, and the purity of TiO2 is 99.9%.

[0023] Further, in step (1), the grinding and mixing method is hand grinding with an agate mortar or ball grinding, wherein the hand grinding time with the agate mortar is 30-90 min; the ball grinding time is 2-12 h; hand grinding with an agate mortar is preferred.

[0024] This invention preferably uses a low-energy hand-grinding method with an agate mortar and pestle to avoid overheating and contamination.

[0025] Further, in step (2), the pressure for pressing the mixed powder into a cylindrical blank is 2-6 MPa, and the holding time is 0.5-3 min.

[0026] Further, in step (2), the encapsulation uses an Au capsule or crucible, and the encapsulation uses a combination of press sealing and welding sealing, or cold pressing sealing; preferably, the Au wall thickness is 0.01-0.03 mm.

[0027] The purpose of sealing in this invention is to effectively isolate the reaction system, prevent the volatilization of substances, and inhibit Hg migration and contamination of high-voltage components.

[0028] Furthermore, in step (2), the reaction is allowed to cool naturally to room temperature after completion.

[0029] Compared with the prior art, the present invention has the following significant advantages: This invention achieves a breakthrough and expansion in material systems, providing a quadruple perovskite LaHg3Ti4O 12 This confirms that the A′-site octagonal coordination structure is a universal material platform, rather than an accidental and isolated phenomenon, laying a solid foundation for exploring similar materials with novel properties and expanding the research and application boundaries of perovskite materials.

[0030] This invention provides a more stable and diverse performance regulation mechanism, through La 3+ Replacement of Pb 2+ To fundamentally eliminate ferroelectricity to specific ions (Pb) 2+The dependence of stereochemically active lone pairs on electrons enhances the flexibility of material design and provides feasible structural templates and new pathways for developing environmentally friendly (lead-free) functional materials.

[0031] This invention introduces an electronic doping control dimension. The heterovalent doping strategy can introduce the key control parameter of controllable carrier concentration, which can continuously and precisely control the electronic structure of materials, thereby achieving precise control of the material's electrical transport properties (such as conductivity) and lattice stability (such as phonon modes).

[0032] This invention transforms the accidental discovery of quadruple perovskites into a designable perovskite material platform based on the A′-site eight-coordinate structure, and changes the performance regulation method from a single driving force to a multi-dimensional synergistic regulation of lead-free substitution and electronic doping, providing a material basis and theoretical reference for the exploration of novel quantum phenomena. Attached Figure Description

[0033] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0034] Figure 1 This is a schematic diagram of the crystal structure of LHTO according to the present invention; Figure 2 This invention presents the synchrotron radiation powder X-ray diffraction (SXRD) pattern of LHTO at room temperature and the Rietveld refinement results. Experimental observations: red crosses; calculated fitted curve: purple line; difference curve between the two: blue line; green lines indicate the Bragg diffraction peak positions allowed by the space group Im. Figure 3 These are high-angle annular dark-field scanning transmission electron microscope (HAADF) images of the LHTO of this invention; The upper right corner shows its Fourier transform image; the upper left corner shows a magnified view of a local atomic image, with the measured and indexed interplanar spacing marked (scale bar: 2nm). Figure 4 These are selected area electron diffraction (SAED) images obtained along the direction of the banded / striped crystal axis according to the present invention; Figure 5 This is a flowchart of the LHTO crystal preparation process according to the present invention; Figure 6 This invention relates to the change of dielectric constant with temperature; Figure 7 These are the cell parameters of this invention, ranging from 5 to 300 Kelvin; Figure 8 The second harmonic generation (SHG) signal (1000 nm) and fundamental wavelength peak (2000 nm) of the spectral data at room temperature are the spectral data of this invention. Figure 9 The present invention provides the SHG rotational anisotropy mode (solid dots) and theoretical fit (solid line) of the LHTO signal at 80 Kelvin. Figure 10 The electronic structure and lattice dynamics properties of this invention; Figure 11 PbHg3Ti4O 12 The present invention is LaHg3Ti4O 12 + δ The distribution of charge density difference (differential charge density) at the (100) crystal plane slice. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0036] Unless otherwise specified in the embodiments of the present invention, the techniques or conditions described in the literature in this field or the product instructions shall be followed; if the manufacturers of the reagents or instruments used are not specified, they are all conventional products that can be purchased through legitimate channels.

[0037] Example 1 This embodiment provides an A-site ordered tetrap perovskite crystal (LHTO) with the chemical formula LaHg3Ti4O. 12 Crystal structure diagram as shown Figure 1 As shown.

[0038] Figure 2 LaHg3Ti4O collected at room temperature was demonstrated. 12+δ Rietveld refined results of synchrotron radiation powder X-ray diffraction (SXRD) patterns of (δ < 0.5) (the inset in the upper right corner is a magnified view of the lower angle region); synchrotron radiation diffraction pattern 2 of the crystal. θ Characteristic peaks are observed at angles X = 4.38, 7.66, 9.89, 11.69, 13.25, 14.72, and 15.93, with an X ± 0.2° range. The refinement results indicate that LaHg3Ti4O... 12+δ The crystal has a cubic structure and a space group of . (No. 204). The refined structural parameters of LHTO are listed in Table 1. No abnormalities were observed in the occupancy factors of all atomic sites (including oxygen sites), indicating that LHTO has a stoichiometric composition.

[0039] like Figure 3 The aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) further confirms the cubic crystal structure of the LHTO sample. The measured lattice fringe spacing in the upper right inset is 0.268 nm, corresponding to cubic LaHg3Ti4O. 12+δ The (220) or (200) crystal planes of the phase. Furthermore, the selected area electron diffraction (SAED) pattern is as follows: Figure 4 As shown.

[0040] Table 1. LHTO refined structural parameters based on SXRD data collected at 300 K

[0041] like Figure 5 As shown, the preparation method of the A-site ordered tetrap perovskite crystal includes the following steps: S1 Raw Material Preparation: The La2O3 is pretreated before use: La2O3 is pre-calcined in air at 773–1273 K (500–1000 °C) for 2–12 hours to remove adsorbates, and then transferred to a desiccator or glove box for storage; in this embodiment, La2O3 is pre-calcined in air at 800 °C for 2 hours; the purity of the La2O3 is 99.99%, the purity of the HgO is 99.7%, and the purity of the TiO2 is 99.9%. S2 Mixing in the glove box: La2O3, HgO, and TiO2 are mixed in a molar ratio of 1:6:8 (corresponding to the nominal composition LaHg3Ti4O). 12.5 The mixture was ground by hand in an agate mortar and pestle for 60 minutes in an argon glove box (oxygen / water < 1 ppm) to obtain a mixed powder. S3 Pressed Ligand: The mixed powder is pressed into a cylindrical blank with a diameter of 6 mm by pressing at 4 MPa for 2 min, which matches the size of the high-pressure chamber. S4Au capsule sealing: The preform is filled into an Au capsule and sealed using a combination of compression sealing and welding sealing; in this embodiment, the Au wall thickness is 0.03mm; S5 cubic anvil high-pressure reaction: The Au capsule is loaded into the cubic anvil high-pressure device, the pressure is increased to the target pressure of 5GPa, and after the pressure stabilizes, the temperature is increased to 1200℃ and the reaction is carried out for 30 minutes. In this embodiment, a staged pressurization method is used: large and small pumps are mixed to uniformly increase the pressure to the set pressure.

[0042] S6 Cooling and depressurization: After the reaction is complete, turn off the heating power and allow it to cool naturally to room temperature while maintaining constant pressure; after the temperature drops to a safe range of 50°C, slowly depressurize to atmospheric pressure; S7 Sample Recovery: Remove the Au capsules and recover the LHTO sample.

[0043] S8 Post-processing and Testing: The LHTO sample was lightly ground into powder for testing. Room temperature synchrotron X-ray powder diffraction (SXRD) patterns were collected using a large Debye-Scherrer camera on the BL02B2 beamline (λ = 0.4201 Å) of SPring-8. The obtained SXRD patterns were refined using the GSAS / EXPGUI software package.

[0044] Scanning transmission electron microscopy (STEM) analysis was performed using a JEOL ARM200F instrument equipped with a condenser lens spherical aberration corrector (Cs corrector) at an operating voltage of 300 kV to study atomic-level microstructure and electronic structure.

[0045] Magnetic susceptibility and magnetization intensity were measured using a superconducting quantum interference magnetometer.

[0046] Dielectric constant measurements are performed using an impedance analyzer (e.g., Agilent 4294A) in the frequency range of 1000 Hz to 1 MHz.

[0047] The sample was ground into a 0.28 mm thick sheet, and a gold electrode was sputtered onto the sheet. Data was then acquired during cooling / heating cycles (10 to 300 Kelvin) in a cryostat.

[0048] Temperature-dependent X-ray diffraction experiments were performed using a Rigaku Ultima IV multi-purpose X-ray diffraction system with Cu Kα1 radiation (λ = 1.540598 Å) at a power setting of 40 kV and 30 mA.

[0049] For second harmonic generation (SHG) measurements, the sample was excited using a collinear optical parametric amplifier (240–4500 nm) (TOPAS-PRIME-HE). The incident laser was generated by a Chameleon Ti:Sapphire laser oscillator (approximately 100 femtoseconds, 1 kHz) with a center wavelength of 1000 nm. The resulting signal was collected using a WITec alpha 300RS+ Raman system with a 50× objective.

[0050] First-principles calculations were performed using density functional theory (DFT) implemented in the Vienna Ab initio Simulation Package (VASP).

[0051] from Figure 6 As can be seen from the data, the dielectric constant of LHTO exhibits a broad diffuse peak at approximately 90 K with significant frequency dispersion, showing relaxor ferroelectric characteristics.

[0052] from Figure 7 As can be seen, although no clear structural phase transition is observed, the temperature dependence curve of the lattice parameters shows a slope inflection near ~90 K, indicating potential low-temperature structural instability and corresponding to dielectric anomalies.

[0053] from Figure 8 As can be seen, a clear SHG peak of 2000 nm → 1000 nm was observed at 80 K and increased significantly with increasing laser power, proving the existence of a second harmonic signal (signs of non-centrosymmetry).

[0054] from Figure 9 As can be seen from the data, the LHTO pattern shows rotationally anisotropic SHG patterns, further supporting the existence of a short-range non-centrosymmetric polar structure.

[0055] from Figure 10 As can be seen from the DFT results, the electrons introduced by La substitution mainly occupy the Ti-3d orbitals and push the Fermi level into the conduction band, thus significantly altering the phonon spectrum (a soft mode appears only near the H point, suggesting low-temperature distortion and corresponding to the ~90 K anomaly). Simultaneously, it destabilizes the Imm2 ferroelectric phase in LHTO and spontaneously returns it to centrosymmetry. structure.

[0056] from Figure 11 From this, we can see that La 3+ Replace Pb 2+ It will weaken Pb at position A. 2+ The lone pair electron effect reduces (or even eliminates) ferroelectricity and makes the ferroelectric phase of LHTO less stable.

[0057] This invention successfully designed and synthesized for the first time a novel A-site electron-doped, specially coordinated quadruple perovskite material, LaHg3Ti4O. 12 This compound can be considered another example of connecting simple perovskites with A-site ordered perovskites; it overcomes the limitations of the closest existing technology, PHTO, and achieves effective control over the electronic states and lattice stability of this type of material, thereby inducing entirely new physical phenomena. Through temperature-dependent structural analysis, dielectric measurements, and first-principles calculations, the properties of LaHg3Ti4O3 were analyzed in depth. 12 The lattice dynamics of the La were observed, revealing heterovalent La 3+Substitution injects electrons into Ti ions at the B site, triggering lattice instability and suppressing the phase transition path previously dominated by ferroelectric instability. Theoretical calculations further confirm that this instability induces a soft phonon mode in the high-symmetry phase, while a new phonon mode (corresponding to the rotation of the oxygen octahedron) softens. This soft phonon mode suggests a new structural instability pathway to the nonpolar phase. Furthermore, A-site substitution weakens the A-site lone pair electron effect, thus making the ferroelectric phase more unstable. Our results indicate that, on this material platform, by synergistically controlling the A-site lone pair electron strength and the B-site electron doping concentration, it is possible to achieve cross-coupling between ferroelectricity and other electronic properties. This invention addresses the technical shortcomings of insufficient exploration of potential structural instabilities and novel properties. It not only confirms the existence of quantum instability competing with ferroelectric phase transitions in this type of material but also points to a research path for discovering new states of matter (such as new structural phase transitions and hidden quantum orders) by controlling the competing order, greatly enriching the fundamental physics and application potential of this type of material (e.g., potential applications in multi-state memory devices).

[0058] This invention provides a novel A-site electron-doped, specially coordinated quadruple perovskite material, LaHg3Ti4O. 12 First, it proves the platform's universality; second, it introduces a novel performance regulation lever through heterovalent doping; and finally, it uses this lever to reveal and regulate a new competitive quantum instability, systematically solving all the core defects of existing technologies.

[0059] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A tetrap perovskite crystal with A-site order, characterized in that, The chemical formula is LaHg3Ti4O 12 The synchrotron radiation diffraction pattern of the crystal has characteristic peaks at 4.38, 7.66, 9.89, 11.69, 13.25, 14.72, and 15.93 at the 2θ angle, and the measurement error of the 2θ angle is ±0.

2.

2. The A-site ordered tetrap perovskite crystal according to claim 1, characterized in that, The crystal has a cubic crystal structure and a space group of . No.

204.

3. A method for preparing an A-site ordered tetrap perovskite crystal as described in claim 1 or 2, characterized in that, Includes the following steps: (1) Grind and mix La2O3, HgO and TiO2 to obtain a mixed powder; (2) Press the mixed powder in step (1) into a cylindrical blank, package it, and react it for 15-60 min at a pressure of 3-8 GPa and a temperature of 900-1200℃ to obtain A-site ordered tetrad perovskite crystals.

4. The method for preparing A-site ordered tetrap perovskite crystals according to claim 3, characterized in that, The molar ratio of La2O3, HgO and TiO2 is 1:6:

8.

5. The method for preparing A-site ordered tetrap perovskite crystals according to claim 4, characterized in that, The purity of the La2O3 is not less than 99.9%, the purity of the HgO is not less than 99%, and the purity of the TiO2 is not less than 99%.

6. The method for preparing A-site ordered tetragonal perovskite crystals according to claim 4, wherein the La2O3 is pretreated by pre-calcining in air at 500-1000℃ for 2-12 hours.

7. The method for preparing A-site ordered tetrap perovskite crystals according to claim 4 or 6, characterized in that, In step (1), the grinding and mixing method is hand grinding with an agate mortar or ball grinding. The hand grinding time with the agate mortar is 30-90 min; the ball grinding time is 2-12 h; hand grinding with an agate mortar is preferred.

8. The method for preparing A-site ordered tetrap perovskite crystals according to claim 7, characterized in that, In step (2), the pressure for pressing the mixed powder into a cylindrical blank is 2-6 MPa, and the holding time is 0.5-3 min.

9. The method for preparing A-site ordered tetrap perovskite crystals according to claim 8, characterized in that, In step (2), the encapsulation uses an Au capsule or crucible, and the encapsulation uses a combination of press sealing and welding sealing, or cold press sealing.

10. The method for preparing A-site ordered tetrap perovskite crystals according to claim 8, characterized in that, In step (2), the reaction is allowed to cool naturally to room temperature after it is completed.

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