Preparation and application of series of mixed alkali metal pyrophosphate compounds and nonlinear optical crystals

Abstract

The invention provides a series of alkali metal pyrophosphate compounds, nonlinear optical crystals and a preparation method and application of the nonlinear optical crystals, a P2O7 group is combined with multiple alkali metal cations Li < + >, Na < + > and Rb < + >, three cases of mixed alkali metal pyrophosphate optical crystals LixRbyNa4-x-yP2O7 are obtained, and x = 1, 2y = 2 and 3. The crystal is composed of an isolated [P2O7] group and a [LiO4] tetrahedral group. The obtained crystal has the advantages of short ultraviolet cut-off edge, high hardness, good mechanical property, difficulty in fragmentation and easiness in processing. The series of mixed alkali metal pyrophosphate nonlinear optical crystals crystallized in the non-centrosymmetric space group obtained by the method can be used as a nonlinear optical device according to the application requirement of the device.

Description

Technical Field

[0001] The present invention relates to the field of nonlinear optics, and specifically to the synthesis of a series of mixed alkali metal pyrophosphate crystals and their application in nonlinear optical devices. Background Art

[0002] Deep ultraviolet nonlinear optical (NLO) crystals can be used in solid-state ultraviolet lasers through second harmonic generation (SHG) technology, and have important applications in laser micromachining, military confrontation, ultraviolet communications, information storage and other fields. The core requirement of this type of material is to have a non-centrosymmetric crystal structure to achieve effective second-order nonlinear optical effects, and at the same time, a wide optical band gap to ensure transmittance in the ultraviolet band. Traditional materials such as KBe2BO3F2 (KBBF) can achieve ultraviolet harmonic output, but their layered growth habits and highly toxic beryllium elements limit practical applications. For this reason, it is particularly important to explore new deep ultraviolet NLO crystals with excellent performance.

[0003] Phosphates, due to their wide UV transmittance range, have become a key system for exploring deep UV nonlinear optical crystals and have attracted widespread attention. Manipulating the cation sites in phosphate systems, particularly alkali metal cations with widely varying radii, not only increases the probability of obtaining noncentrosymmetric structures but also maintains a wide UV transmittance range. Therefore, phosphate systems with mixed alkali metal cation sites are considered ideal for developing new high-performance deep UV nonlinear optical crystals. Summary of the Invention

[0004] The present invention combines a variety of alkali metal cations and phosphate radicals to obtain a class of second-order nonlinear optical crystal mixed alkali metal pyrophosphates with excellent performance.

[0005] The present invention aims to provide a series of mixed alkali metal pyrophosphate compounds and a preparation method thereof, wherein the chemical formula of the series of mixed alkali metal pyrophosphate compounds is Li x Rb y Na 4-x-y P2O7, where x=1,2y=2,3.

[0006] The second object of the present invention is to provide a series of mixed alkali metal pyrophosphate crystals and a preparation method.

[0007] The third object of the present invention is to provide applications of a series of mixed alkali metal pyrophosphate nonlinear optical crystals.

[0008] The pyrophosphate compound described in the present invention refers to a crystal containing P2O7 anions and a compound containing both lithium and rubidium to illustrate the importance of different cations in mixed alkali metal occupancy to the controllable evolution of the structure;

[0009] According to the present invention, the mixed alkali metal pyrophosphate is lithium dirubidium pyrophosphate Li2Rb2P2O7, lithium rubidium tripyrophosphate LiRb3P2O7, or lithium rubidium sodium pyrophosphate LiRb2NaP2O7.

[0010] One of the purposes of the present invention is achieved in that:

[0011] The present invention aims to provide a series of mixed alkali metal pyrophosphate compounds, characterized in that the chemical formula of the series of mixed alkali metal pyrophosphate compounds is Li x Rb y Na 4-x-y P2O7 (x = 1, 2y = 2, 3). The mixed alkali metal pyrophosphate compounds Li2Rb2P2O7, LiRb3P2O7, and LiRb2NaP2O7 were prepared by a solid phase reaction method according to the following chemical reaction formula.

[0012] 1)Li2CO3+Rb2CO3+2NH4H2PO4→Li2Rb2P2O7+2NH3↑+3H2O↑+2CO2↑

[0013] 2)2LiF+Rb2CO3+2H3PO4→Li2Rb2P2O7+2H2O↑+2HF↑+CO2↑

[0014] 3)2LiOH+Rb2CO3+2H3PO4→Li2Rb2P2O7+4H2O↑+CO2↑

[0015] 4)Li2CO3+3Rb2CO3+4NH4H2PO4→2LiRb3P2O7+4NH3↑+6H2O↑+4CO2↑

[0016] 5)2LiF+3Rb2CO3+4H3PO4→2LiRb3P2O7+5H2O↑+2HF↑+3CO2↑

[0017] 6)2LiOH+3Rb2CO3+4H3PO4→2LiRb3P2O7+7H2O↑+3CO2↑

[0018] 7)Li2CO3+2Rb2CO3+Na2CO3+4NH4H2PO4→2LiRb2NaP2O7+4NH3↑+6H2O↑+4CO2↑

[0019] 8)2LiF+2Rb2CO3+Na2CO3+4H3PO4→2LiRb2NaP2O7+5H2O↑+2HF↑+3CO2↑

[0020] 9)2LiOH+2Rb2CO3+Na2CO3+4H3PO4→2LiRb2NaP2O7+7H2O↑+3CO2↑

[0021] The synthesis of lithium dirubidium pyrophosphate Li2Rb2P2O7, lithium rubidium tripyrophosphate LiRb3P2O7, and lithium rubidium sodium pyrophosphate LiRb2NaP2O7 is not limited to the above reaction formula. The alkali metal-containing compound can also be AOH, A2O, AF, ACl, ABr, ANO3, A2CO3, AHCO3, A2SO4 (A=Li, Na, Rb), and the phosphorus-containing compound can also be P2O5, H3PO4, AH2PO4, A2HPO4, A3PO4, A2H2P2O7, A4P2O7 (A=Li, Na, Rb).

[0022] The series of mixed alkali metal pyrophosphate compound single-phase polycrystalline powders are prepared by a solid-phase synthesis method, comprising the following steps: mixing a Li, Na, Rb-containing compound and a phosphorus-containing compound using a solid-phase reaction method to prepare a series of mixed alkali metal pyrophosphates, wherein the molar ratio of the elements Li, Na, Rb in the Li, Na, Rb-containing compound to the element phosphorus in the phosphorus-containing compound is in the range of 3.5-4.5:1.5-2.5; fully grinding the Li, Na, Rb-containing compound and the phosphorus-containing compound raw materials using an agate mortar to uniformly mix them; placing the ground mixture in a muffle furnace for low-temperature pre-calcination to remove moisture and gas from the raw materials; then slowly heating and continuously calcining; taking out and repeatedly grinding the mixture several times during the process; and finally cooling to room temperature; taking out and grinding the obtained series of mixed alkali metal pyrophosphate single-phase polycrystalline powders.

[0023] The alkali metal-containing compound comprises alkali metal carbonates, alkali metal nitrates, alkali metal sulfates, alkali metal oxalates, alkali metal borates, alkali metal phosphates, alkali metal halides, alkali metal oxides, and alkali metal hydroxides;

[0024] The phosphorus-containing compounds include ammonium dihydrogen phosphate, monohydrogen phosphate, dihydrogen phosphate, and hexafluorophosphate.

[0025] According to the present invention, the mixed alkali metal pyrophosphate crystallized in a non-centrosymmetric space group is used as a deep ultraviolet nonlinear optical crystal.

[0026] According to the present invention, the mixed alkali metal pyrophosphate crystallized in a non-centrosymmetric space group is used for solid-state ultraviolet lasers, such as laser micromachining, military confrontation, ultraviolet communication, information storage, etc.

[0027] The second object of the present invention is achieved in this way:

[0028] The present invention aims to provide a method for growing a series of mixed alkali metal pyrophosphate nonlinear optical crystals by a high-temperature melt method, a hydrothermal method or a solution method, wherein the chemical formula of the series of mixed alkali metal pyrophosphate nonlinear optical crystals is Li x Rb y Na 4-x-y P2O7, where x=1,2y=2,3.

[0029] Li2Rb2P2O7 has a molecular weight of 358.6, belongs to the orthorhombic system, space group Pca21, and the unit cell parameters are α=90°, β=90°, γ=90°, Z=17,

[0030] LiRb3P2O7 has a molecular weight of 437.1 and belongs to the orthorhombic system, space group Pnma, and unit cell parameters are α=90°, β=90°, γ=90°, Z=8,

[0031] LiRb2NaP2O7 has a molecular weight of 374.7 and belongs to the monoclinic system, space group P21 / c, and the unit cell parameters are α=90°, β=110.7040(10)°, γ=90°, Z=8,

[0032] The high temperature melt method for growing Li x Rb y Na 4-x-y P2O7, where x = 1, 2y = 2, 3, the specific operation is carried out as follows:

[0033] The reaction can be carried out by first heating, then holding, and then cooling: a series of mixed alkali metal pyrophosphate compound single-phase polycrystalline powders are mixed evenly with a flux, and the temperature is gradually increased to 600-650°C at a heating rate of 60-100°C / h with a temperature gradient of 50°C. Each temperature must be kept constant for at least 20 hours and then ground into a fine powder. The molten mixed solution is cooled to 500-550°C at a rate of 1-3°C / h, and the muffle furnace process is terminated to obtain a colorless single crystal, wherein the molar ratio of the series of mixed alkali metal pyrophosphate compound single-phase polycrystalline powders to the flux is 1:0-10;

[0034] Alternatively, a mixture of a Li, Na, Rb compound and a phosphorus-containing compound, or a mixture of a Li, Na, Rb compound and a phosphorus-containing compound and a flux is directly placed in a muffle furnace, and then heated to a temperature of 600-650° C., maintained at a constant temperature for a period of time to obtain a mixed melt, and then cooled to 500-550° C., wherein the molar ratio of the Li, Na, Rb compound and the phosphorus-containing compound to the flux is 3.5-4.5:1.5-2.5:0-10;

[0035] The reaction system can be reacted in a platinum crucible or an alumina crucible;

[0036] The flux mainly includes alkali metal salts, namely alkali metal carbonates, alkali metal nitrates, alkali metal sulfates, alkali metal oxalates, alkali metal borates, alkali metal phosphates, alkali metal halides and alkali metal oxides, alkali metal hydroxides, and at least one or more of monohydrogen phosphate, dihydrogen phosphate, hexafluorophosphate, lead oxide, and lead fluoride.

[0037] The hydrothermal growth of Li x Rb y Na 4-x-y P2O7, where x = 1, 2y = 2, 3, the specific operation is carried out as follows:

[0038] Adding a Li, Na, Rb-containing compound and a phosphorus-containing compound to a polytetrafluoroethylene liner of an autoclave, then adding 1-5 mL of deionized water or 1-5 g of a mineralizer, and thoroughly mixing the mixture to obtain a mixed solution, wherein the molar ratio of the Li, Na, Rb-containing compound to the phosphorus-containing compound is 3.5-4.5:1.5-2.5; tightening the lid of the polytetrafluoroethylene liner containing the mixed solution in the previous step, then loading the mixture into a corresponding autoclave, tightening the piston of the autoclave, placing the mixture in a thermostat, heating it to 150-200° C., maintaining the temperature for a period of time, and then cooling it to room temperature; finally, opening the autoclave, filtering the solution, and obtaining a series of transparent mixed alkali metal pyrophosphate crystals.

[0039] The solution-grown Li x Rb y Na 4-x-y P2O7, where x = 1, 2y = 2, 3, the specific operation is carried out as follows:

[0040] Add the Li, Na, Rb, and phosphorus compounds to a small glass vial, then add 10-30 mL of deionized water. Stir the solution until it becomes clear. Place the vial in an oil bath or on a heating plate at a constant temperature of 25-55°C. After a period of time, the series of mixed alkali metal pyrophosphate crystals will be obtained.

[0041] The third object of the present invention is achieved in this way:

[0042] The present invention found that mixed alkali metal pyrophosphate can be used as deep ultraviolet nonlinear optical crystal. The pyrophosphate ([P2O7] 4- ) and alkali metal ions can effectively break the symmetry and produce second-order nonlinear optical effects without the absorption loss caused by dd and ff electronic transitions; at the same time, the PO bond is a high-energy bond that gives the material a wide bandgap characteristic, making the material have a shorter ultraviolet cutoff edge. + / Na + / Rb + ) can balance the contradiction between wide bandgap and nonlinear effects. Compared with traditional materials containing Be and Pb, mixed alkali metal pyrophosphates also have advantages such as environmental friendliness and simple crystal growth process (large single crystals can be easily obtained using the flux method).

[0043] The preparation method and operation of the aforementioned series of mixed alkali metal pyrophosphate nonlinear optical crystals are simple, and the use of a muffle furnace and a heating process can achieve mass production to obtain high-purity products. The obtained crystals have the advantages of a relatively wide light transmission band, high hardness, good mechanical properties, not easy to break, and easy to process. The series of mixed alkali metal pyrophosphate nonlinear optical crystals crystallized in a non-centrosymmetric space group obtained by the method described in the present invention can be used as a nonlinear optical device according to the application requirements of the device, which includes a device that generates at least one beam of output radiation with a frequency different from the incident electromagnetic radiation after passing at least one beam of incident electromagnetic radiation through at least one nonlinear optical device, wherein the nonlinear optical crystal is Li x Rb y Na 4-x-y P2O7, where x=1,2y=2,3, see Appendix Figure 4 .

[0044] The present invention can also provide new ideas for the controllable transformation from thermodynamically stable centrosymmetric phase to functionalized non-centrosymmetric structure. + 、Na + , Rb + ) size differences and coordination diversity can induce lattice distortion, such as LiRb3P2O7 crystallizes in the centrosymmetric space group Pnma, while Li2Rb2P2O7 crystallizes in the non-centrosymmetric space group Pca21; secondly, the cation electric field can regulate [P2O7] 4- The orientation of the units changes from antiparallel (centrosymmetric) to uniform orientation (non-centrosymmetric). By constructing a cation size gradient and mixing alkali metal occupants, a controllable transition from a thermodynamically stable centrosymmetric phase to a non-centrosymmetric structure can be achieved, providing insights into the design of new nonlinear optical materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 The figure is the powder X-ray diffraction pattern of the compound Li2Rb2P2O7 crystal in the present invention.

[0046] Figure 2 It is the UV-visible-near-infrared diffuse reflectance spectrum of the compound Li2Rb2P2O7 crystal in the present invention.

[0047] Figure 3 This is the crystal structure diagram of the compound Li2Rb2P2O7 in the present invention.

[0048] Figure 4 This is a working principle diagram of the nonlinear optical device made in the present invention, where 1 is the laser, 2 is the emitted light beam, 3 is a series of nonlinear optical crystals of mixed alkali metal pyrophosphate crystallized in a non-centrosymmetric space group, 4 is the output light beam, and 5 is the filter. DETAILED DESCRIPTION

[0049] The technical solution of the present invention will be further described in detail below with reference to specific examples. However, the scope of protection claimed by the present invention is not limited to the scope described in the embodiments.

[0050] The starting materials used in the following examples were all commercially available products and did not require further purification.

[0051] Example 1:

[0052] Reaction formula: Li2CO3+Rb2CO3+2NH4H2PO4→Li2Rb2P2O7+2NH3↑+3H2O↑+2CO2↑, solid phase synthesis of Li2Rb2P2O7 compounds and crystals:

[0053] Li2CO3, Rb2CO3, and NH4H2PO4 raw materials were weighed and ground in a molar ratio of 1:1:2. After mixing evenly, they were transferred to a 20ml Al2O3 crucible and placed in a muffle furnace. The temperature was raised to 30°C and kept warm for 12 hours for pre-calcination to remove moisture and gas from the raw materials. Subsequently, the temperature was gradually raised to 550°C with a temperature gradient of 50°C. The temperature was kept constant for no less than 20 hours at each temperature and the sample was fully ground. Finally, the heating was stopped and the sample was cooled to room temperature with the furnace to prepare a white powder, which was the pure phase of Li2Rb2P2O7. The powder X-ray diffraction test was performed to obtain the sample powder X-ray diffraction pattern, see the attached figure. Figure 1 ;

[0054] The pure phase polycrystalline powder of the prepared compound lithium dirubidium pyrophosphate (Li2Rb2P2O7) is placed in a 70mm x 70mm Pt crucible. The mixed raw materials are heated to approximately 600°C at a heating rate of 60-100°C / h. After the raw materials are melted, the temperature is maintained for 10-12 hours to achieve a uniform concentration of the high-temperature solution in the Pt crucible. The solution is then slowly cooled at a rate of 1-3°C / h to obtain lithium dirubidium pyrophosphate crystals with the chemical formula Li2Rb2P2O7.

[0055] Example 2:

[0056] Reaction formula: 2LiF+Rb2CO3+2H3PO4→Li2Rb2P2O7+2H2O↑+2HF↑+CO2↑, hydrothermal synthesis of Li2Rb2P2O7 crystals:

[0057] LiF and Rb2CO3 raw materials are directly weighed in a molar ratio of 2:1 and added to the polytetrafluoroethylene liner of a 10mL high-pressure reactor. Then, 3mL of deionized water and 4ml of H3PO4 are added to mix them thoroughly to obtain a mixed solution. The polytetrafluoroethylene liner cap containing the mixed solution in the previous step is tightened, and then the mixture is placed in a clean and uncontaminated high-pressure reactor, and the reactor piston is tightened. The high-pressure reactor is then placed in an oven, heated to 180°C at a heating rate of 20°C / h, kept constant for 4 days, and then cooled to room temperature at a cooling rate of 3°C / h. The high-pressure reactor is opened and the solution containing the crystals is filtered to obtain lithium dirubidium pyrophosphate crystals.

[0058] Example 3:

[0059] Reaction formula: 2LiOH+Rb2CO3+2H3PO4→Li2Rb2P2O7+4H2O↑+CO2↑, solution synthesis of Li2Rb2P2O7 crystals:

[0060] LiOH and Rb2CO3 were weighed directly into a 30mL glass cup at a 2:1 molar ratio. 10mL of deionized water and 5mL of H3PO4 were then added. The solution was stirred until clear. The glass cup was then placed in an oil bath at 40°C. After 7 days, lithium dirubidium pyrophosphate crystals were obtained.

[0061] Example 4:

[0062] Reaction formula: Li2CO3+3Rb2CO3+4NH4H2PO4→2LiRb3P2O7+4NH3↑+6H2O↑+4CO2↑, solid phase synthesis of LiRb3P2O7 compounds and crystals:

[0063] Li2CO3, Rb2CO3, and NH4H2PO4 raw materials are weighed and ground in a molar ratio of 1:3:4. After mixing evenly, transfer them to a 20ml Al2O3 crucible and place it in a muffle furnace. The temperature is raised to 300℃ and kept warm for 12 hours for pre-calcination to remove moisture and gas in the raw materials. Then the temperature is gradually increased to 550℃ according to a temperature gradient of 50℃. The temperature must be kept constant for at least 20 hours at each temperature and fully ground. Finally, stop heating and wait for the sample to cool to room temperature with the furnace to prepare a white powder, which is the pure phase of LiRb3P2O7.

[0064] The pure phase polycrystalline powder of the prepared lithium rubidium pyrophosphate (LiRb3P2O7) compound is placed in a 70mm x 70mm Pt crucible. The mixed raw materials are heated to approximately 600°C at a heating rate of 60-100°C / hour. After the raw materials are melted, the temperature is maintained for 10-12 hours to achieve a uniform concentration of the high-temperature solution in the Pt crucible. The solution is then slowly cooled at a rate of 1-3°C / hour to obtain lithium rubidium pyrophosphate crystals with the chemical formula LiRb3P2O7.

[0065] Example 5:

[0066] Reaction formula: 2LiF+3Rb2CO3+4H3PO4→2LiRb3P2O7+5H2O↑+2HF↑+3CO2↑, hydrothermal synthesis of LiRb3P2O7 crystals:

[0067] LiF and Rb2CO3 raw materials are directly weighed in a molar ratio of 2:3 and added to the polytetrafluoroethylene liner of a 10mL high-pressure reactor. Then, 3mL of deionized water and 4ml of H3PO4 are added to mix them thoroughly to obtain a mixed solution. The polytetrafluoroethylene liner cap containing the mixed solution in the previous step is tightened, and then the mixture is placed in a clean and uncontaminated high-pressure reactor, and the reactor piston is tightened. The high-pressure reactor is then placed in an oven, heated to 180°C at a heating rate of 20°C / h, kept constant for 4 days, and then cooled to room temperature at a cooling rate of 3°C / h. The high-pressure reactor is opened and the solution containing the crystals is filtered to obtain lithium rubidium pyrophosphate crystals.

[0068] Example 6:

[0069] Reaction formula: 2LiOH+3Rb2CO3+4H3PO4→2LiRb3P2O7+7H2O↑+3CO2↑, solution synthesis of LiRb3P2O7 crystals:

[0070] LiOH and Rb2CO3 raw materials were weighed directly at a molar ratio of 2:3 and added to a 30mL glass cup. 10mL of deionized water and 5mL of H3PO4 were then added. The solution was stirred until clear. The glass cup was then placed in an oil bath at 40°C. After 7 days, lithium rubidium pyrophosphate crystals were obtained.

[0071] Example 7:

[0072] Li2CO3+2Rb2CO3+Na2CO3+4NH4H2PO4→2LiRb2NaP2O7+4NH3↑+6H2O↑+4C

[0073] O2↑, solid phase synthesis of LiK2RbP2O7 compounds and crystals:

[0074] Li2CO3, Rb2CO3, Na2CO3, and NH4H2PO4 raw materials are weighed and ground in a molar ratio of 1:2:1:4. After mixing evenly, transfer them to a 20ml Al2O3 crucible and place it in a muffle furnace. Raise the temperature to 300℃ and keep it warm for 12 hours to pre-sinter to remove moisture and gas from the raw materials. Then gradually increase the temperature to 550℃ with a temperature gradient of 50℃. Each temperature must be kept constant for at least 20 hours and fully ground. Finally, stop heating and wait for the sample to cool to room temperature with the furnace to prepare a white powder, which is the pure phase of LiK2RbP2O7.

[0075] The prepared pure phase polycrystalline powder of lithium sodium rubidium pyrophosphate (LiRb2NaP2O7) is placed in a 70mm x 70mm Pt crucible. The mixed raw materials are heated to approximately 600°C at a heating rate of 60-100°C / h. After the raw materials are melted, the temperature is maintained for 5-10 hours to achieve a uniform concentration of the high-temperature solution in the Pt crucible. The solution is then slowly cooled at a rate of 1-3°C / h to obtain lithium sodium rubidium pyrophosphate crystals with the chemical formula LiRb2NaP2O7.

[0076] Example 8:

[0077] 2LiF+2Rb2CO3+Na2CO3+4H3PO4→2LiRb2NaP2O7+5H2O↑+2HF↑+3CO2↑, hydrothermal synthesis of LiRb2NaP2O7 crystals:

[0078] LiF, Rb2CO3, and Na2CO3 raw materials are directly weighed in a molar ratio of 2:2:1 and added to the polytetrafluoroethylene liner of a 10mL high-pressure reactor. Then, 3mL of deionized water and 4ml of H3PO4 are added to mix them thoroughly to obtain a mixed solution. The polytetrafluoroethylene liner cap containing the mixed solution in the previous step is tightened, and then the mixture is placed in a clean and pollution-free high-pressure reactor, and the reactor piston is tightened. The high-pressure reactor is then placed in an oven, heated to 180°C at a heating rate of 20°C / h, maintained at a constant temperature for 4 days, and then cooled to room temperature at a cooling rate of 3°C / h. The high-pressure reactor is opened, and the solution containing the crystals is filtered to obtain lithium sodium rubidium pyrophosphate crystals.

[0079] Example 9:

[0080] Reaction formula: 2LiOH+2Rb2CO3+Na2CO3+4H3PO4→2LiRb2NaP2O7+7H2O↑+3CO2↑, solution synthesis of LiRb2NaP2O7 crystals:

[0081] LiOH, Rb2CO3, and Na2CO3 were weighed directly into a 30mL glass cup at a molar ratio of 2:2:1. 10mL of deionized water and 5mL of H3PO4 were then added. The solution was stirred until clear. The glass cup was then placed in an oil bath at 40°C. After 7 days, sodium lithium rubidium pyrophosphate crystals were obtained.

[0082] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and substitutions can be made without departing from the technical principles of the present invention. These improvements and substitutions should also be regarded as the scope of protection of the present invention.

Claims

1. The present invention relates to the preparation and application of a series of deep ultraviolet nonlinear optical crystal mixed alkali metal pyrophosphates. The series of mixed alkali metal pyrophosphate compounds are characterized by: The molecular formula of this series of alkali metal pyrophosphate compounds is Li x Rb y Na 4-x-y P2O7, where x=1,2y=2,3, and the molecular formula is Li2Rb2P2O7, LiRb3P2O7, LiRb2NaP2O7. 2.Li2Rb2P2O7 has a molecular weight of 358.6, belongs to the orthorhombic system, space group Pca21, and the unit cell parameters are α=90°, β=90°, γ=90°, Z=17, 3.LiRb3P2O7 has a molecular weight of 437.1, belongs to the orthorhombic system, space group Pnma, and unit cell parameters are α=90°, β=90°, γ=90°, Z=8, 4.LiRb2NaP2O7 has a molecular weight of 374.7, belongs to the monoclinic system, space group P21 / c, and the unit cell parameters are α=90°, β=110.7040(10)°, γ=90°, Z=8, 5. The method for preparing a series of mixed alkali metal pyrophosphate compounds according to claim 1, characterized in that: Prepared by solid phase reaction method: The solid phase reaction method for preparing Li x Rb y Na 4-x-y The specific operation of preparing the compound P2O7, wherein x=1,2y=2,3, is as follows: a compound containing Li, Na, Rb and a phosphorus-containing compound are uniformly mixed, ground, and then calcined in a muffle furnace, and subjected to multiple grindings to obtain a series of mixed alkali metal pyrophosphate compound single-phase polycrystalline powders, wherein the molar ratio of the elements Li, Na, Rb in the compound containing Li, Na, Rb to the element phosphorus in the phosphorus-containing compound is 3.5-4.5:1.5-2.5; The above-mentioned compound containing Li, Na, Rb includes at least one of AOH, A2O, AF, ACl, ABr, ANO3, A2CO3, AHCO3, A2SO4, wherein A=Li, Na, Rb; The phosphorus-containing compound includes P2O5, H3PO4 and phosphate; the phosphate includes at least one of NH4H2PO4, AH2PO4, A2HPO4, A3PO4, A2H2P2O7, and A4P2O7, wherein A=Li, Na, Rb.

6. The method for preparing a series of mixed alkali metal pyrophosphate crystals according to claim 1, characterized in that: A series of mixed alkali metal pyrophosphate crystals were grown using high-temperature melt, hydrothermal, and solution methods: a) The high temperature melt method for growing Li x Rb y Na 4-x-y The specific operation for preparing P2O7, where x=1,2y=2,3 crystals, is as follows: a series of mixed alkali metal pyrophosphate compound single-phase polycrystalline powders, or a mixture of a series of mixed alkali metal pyrophosphate compound single-phase polycrystalline powders and a flux, or a mixture of a Li, Na, Rb compound and a phosphorus-containing compound, or a mixture of a Li, Na, Rb compound and a phosphorus-containing compound and a flux, is heated to melt to obtain a mixed melt. The temperature is then slowly lowered to prepare a series of mixed alkali metal pyrophosphate crystals; b) The hydrothermal growth of Li x Rb y Na 4-x-y The specific operation for preparing P2O7, where x=1,2y=2,3 crystals, is as follows: a series of mixed alkali metal pyrophosphate compound single-phase polycrystalline powders, or a mixture of a series of mixed alkali metal pyrophosphate compound single-phase polycrystalline powders and a mineralizer, or a mixture of a Li, Na, Rb compound and a phosphorus-containing compound, or a mixture of a Li, Na, Rb compound and a phosphorus-containing compound and a mineralizer, is added to the polytetrafluoroethylene liner of an autoclave, and deionized water is added and mixed uniformly to obtain a mixed solution. The polytetrafluoroethylene liner is placed in the autoclave, and the autoclave is heated in an oven, then cooled to room temperature, and the solution containing the crystals is filtered to obtain transparent series of mixed alkali metal pyrophosphate crystals. c) The solution method for growing Li x Rb y Na 4-x-y The specific process for producing P2O7, where x=1,2y=2,3, is as follows: a series of mixed alkali metal pyrophosphate compound single-phase polycrystalline powders, or a mixture of the series of mixed alkali metal pyrophosphate compound single-phase polycrystalline powders obtained in claim 5 and a co-solvent, or a mixture of a Li, Na, Rb-containing compound and a phosphorus-containing compound, or a mixture of a Li, Na, Rb-containing compound and a phosphorus-containing compound and a co-solvent, is added to a glass bottle; deionized water is then added to dissolve the mixture, and the solution is stirred until uniform. The glass bottle is then exposed to air to evaporate the solution, thereby growing the series of mixed alkali metal pyrophosphate crystals.

7. The method for preparing a series of mixed alkali metal pyrophosphate crystals according to claim 6, characterized in that: The high temperature melt method for growing Li x Rb y Na 4-x-y P2O7 crystals, wherein x=1, 2y=2, 3, and the molar ratio of the series of mixed alkali metal pyrophosphate compound single-phase polycrystalline powder to the flux is 1:0-10; or the molar ratio of the Li, Na, Rb-containing compound and the phosphorus-containing compound to the flux is 3.5-4.5:1.5-2.5:0-10. The flux comprises an alkali metal salt, namely, at least one or more of AOH, A2O, AF, ACl, ABr, A3PO4, A3BO3, ANO3, A2CO3, AHCO3, A2SO4, NH4H2PO4, A2HPO4, AH2PO4, APF6, PbO, and PbF2, wherein A=Li, Na, Rb; The hydrothermal growth of Li x Rb y Na 4-x-y P2O7 crystals, wherein x=1, 2y=2, 3, the molar ratio of the series of mixed alkali metal pyrophosphate compound single-phase polycrystalline powder to the mineralizer is 1:0-5; or the molar ratio of the Li, Na, Rb-containing compound and the phosphorus-containing compound to the mineralizer is 3.5-4.5:1.5-2.5:0-10; the mineralizer includes at least one or more of AOH, A2O, AF, ACl, ABr, A3PO4, A3BO3, ANO3, A2CO3, AHCO3, A2SO4, NH4H2PO4, A2HPO4, AH2PO4, APF6, wherein A=Li, Na, Rb.

8. The method for preparing a nonlinear optical crystal belonging to a non-centrosymmetric space group in a mixed alkali metal pyrophosphate according to claim 6, characterized in that: The crystal is used as a deep ultraviolet nonlinear optical crystal.

9. The preparation method of the mixed alkali metal pyrophosphate according to claim 5, characterized in that: The difference in the crystal space groups provides a way to achieve controllable evolution from centrosymmetric to functionalized non-centrosymmetric structures.

10. Use of the non-centrosymmetric space group compound mixed alkali metal pyrophosphate nonlinear optical crystal according to claim 1, characterized in that: The crystal is used in laser micromachining, military confrontation, ultraviolet communication, information storage and other technologies.

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