PIEZOELECTRIC SINGLE CRYSTAL M3RE(PO4)3 AND THE PREPARATION METHOD AND APPLICATION THEREOF

Abstract

A crystal is of a non-centrosymmetric structure and belongs to the −43 m point group of the cubic crystal system. M denotes an alkaline earth metal, which can be Ba, Ca, or Sr, and RE denotes a rare earth element, which can be Y, La, Gd, or Yb. The growth method of the M.sub.3RE(PO.sub.4).sub.3 crystal comprises steps as follows: (1) polycrystalline material synthesis: MCO.sub.3, RE.sub.2O.sub.3, and phosphorous compound are used as raw materials and blended according to the stoichiometric proportions; then, the phosphorous compound is further added to be excessive; the raw materials are sintered twice to obtain the M.sub.3RE(PO.sub.4).sub.3 polycrystalline material; (2) polycrystalline material melting; (3) Czochralski crystal growth. The M.sub.3RE(PO.sub.4).sub.3 crystal prepared by the invention is a high-quality single crystal.

Claims

1. A piezoelectric single crystal with the general formula of M.sub.3RE(PO.sub.4).sub.3, where M denotes an alkaline earth metal and RE denotes a rare earth element; the said single crystal is of a non-centrosymmetric structure and belongs to the −43m point group of the cubic crystal system.

2. The said piezoelectric single crystal according to claim 1, characterized in that the said alkaline earth metal is Ba, Ca, or Sr.

3. The said piezoelectric single crystal according to claim 1, characterized in that the said rare earth element is Y, La, Gd, or Yb.

4. The said piezoelectric single crystal according to claim 1, characterized in that the M.sub.3RE(PO.sub.4).sub.3 piezoelectric single crystal is one from the following: Ba.sub.3Y(PO.sub.4).sub.3 single crystal, Ba.sub.3La(PO.sub.4).sub.3 single crystal, Ba.sub.3Yb(PO.sub.4).sub.3 single crystal, Ca.sub.3Gd(PO.sub.4).sub.3 single crystal, Sr.sub.3Y(PO.sub.4).sub.3 single crystal, Sr.sub.3La(PO.sub.4).sub.3 single crystal, and Sr.sub.3Gd(PO.sub.4).sub.3 single crystal.

5. The said piezoelectric single crystal according to claim 4, characterized in that the structure parameters of the single crystals are as follows: Ba.sub.3Y(PO.sub.4).sub.3 single crystal: the −43m point group of the cubic crystal system; I-43d space group; lattice parameters: a=b=c=10.4655 Å; density=4.553 g/cm.sup.3; Ba.sub.3La(PO.sub.4).sub.3 single crystal: the −43m point group of the cubic crystal system; I-43d space group; lattice parameters: a=b=c=10.521 Å; density=4.77 g/cm.sup.3; Ba.sub.3Yb(PO.sub.4).sub.3 single crystal: the −43m point group of the cubic crystal system; I-43d space group; lattice parameters: a=b=c=10.459 Å; density=5.149 g/cm.sup.3; Ca.sub.3Gd(PO.sub.4).sub.3 single crystal: the −43m point group of the cubic crystal system; I-43d space group; lattice parameters: a=b=c=9.857 Å; density=3.9 g/cm.sup.3; Sr.sub.3Y(PO.sub.4).sub.3 single crystal: the −43m point group of the cubic crystal system; I-43d space group; lattice parameters: a=b=c=10.0814 Å; density=4.093 g/cm.sup.3; Sr.sub.3La(PO.sub.4).sub.3 single crystal: the −43m point group of the cubic crystal system; I-43d space group; lattice parameters: a=b=c=10.192 Å; density=4.3 g/cm.sup.3; Sr.sub.3Gd(PO.sub.4).sub.3 single crystal: the −43m point group of the cubic crystal system; I-43d space group; lattice parameters: a=b=c=10.114 Å; density=4.526 g/cm.sup.3.

6. A yttrium strontium phosphate single crystal with the chemical formula of Sr.sub.3Y(PO.sub.4).sub.3, which is of a non-centrosymmetric structure, belongs to the −43m point group of the cubic crystal system, and with lattice parameters: a=10.0814 Å, b=10.0814 Å, c=10.0814 Å, α=β=γ=90°. It has no phase transition from room temperature to the melting point.

7. The said yttrium strontium phosphate single crystal according to claim 6, characterized in that the melting point of the said yttrium strontium phosphate single crystal is basically at 1850° C., and the single crystal has no phase transition from room temperature to the melting point; upon spectral analysis, the said yttrium strontium phosphate single crystal has high optical transmittance of >80% in 480 nm-4100 nm band; or, upon measurement with the impedance method, the said yttrium strontium phosphate single crystal has an effective piezoelectric constant d.sub.14=6-10pC/N; or upon measurement and calculation with the impedance method, the said yttrium strontium phosphate crystal has an effective electromechanical coupling factor k.sub.14=10-30%.

8. The said piezoelectric single crystal according to claim 1, wherein the said M.sub.3RE(PO.sub.4).sub.3 piezoelectric single crystal is prepared by the following steps as follows: (i) polycrystalline material synthesis the raw materials MCO.sub.3, RE.sub.2O.sub.3, and phosphorous compound are accurately weighed by stoichiometric proportions according to the general formula M.sub.3RE(PO.sub.4).sub.3; the phosphorous compound is further added to exceed the mass percent (based on the stoichiometric proportion of the phosphorus compound in the formula) by 1.5-10%; the well-weighed raw materials are ground and mixed evenly for the first sintering (the sintering is preferred to be conducted in a ceramic crucible); the sintering temperature is maintained at 800° C.-950° C. and constant for 10 to 15 hours; after the first sintering, the raw materials are cooled down, ground, and evenly mixed; then, they are pressed into round cake-like blocks for the second sintering; the sintering temperature is maintained at 1200-1400° C. and constant for 24 to 48 hours; finally, the M.sub.3RE(PO.sub.4).sub.3 polycrystalline material is obtained through the solid-state reaction; (2) polycrystalline material melting the M.sub.3RE(PO.sub.4).sub.3 polycrystalline material synthesized in step (1) is placed in the iridium crucible of a single crystal growing furnace; before that, the furnace has been evacuated and filled with protective gas nitrogen or argon; the polycrystalline material is heated up with the medium-frequency induction heating method till melting; it is cooled down to coagulate after melting thoroughly and then heated up again to fully melt; this process is repeated several times to drain the bubbles from the melt; then, the melt is overheated 10-20° C. for 0.5-2 hours to obtain the homogeneously melted M.sub.3RE(PO.sub.4).sub.3 melt; (3) czochralski crystal growth an iridium rod or an M.sub.3RE(PO.sub.4).sub.3 crystal is used as the seed crystal and lowered to the melt level of the M.sub.3RE(PO.sub.4).sub.3 to have the bottom perpendicular to and just in contact with the melt for single crystal growth; the technological conditions of the single crystal growth are as follows: growth temperature: 1800-1950° C.; pulling rate during necking of the seed crystal: 1-8 mm/h; pulling rate during shouldering: 0.2-1 mm/h; pulling rate while performing constant diameter growth: 0.5-1 mm/h. The crystal is then extracted after growing to the desired size.

9. The said piezoelectric single crystal according to claim 8, characterized in that after extracting, the crystal is maintained at a constant temperature for 0.5-1 h in the thermal field and then cooled down at the rate of 5-30° C./h to room temperature to obtain the M.sub.3RE(PO.sub.4).sub.3 piezoelectric single crystal; after being taken out from the single crystal growing furnace, the M.sub.3RE(PO.sub.4).sub.3 crystal is annealed at 1200-1400° C.; the time of annealing is preferred to be 24-48 hours.

10. The said piezoelectric single crystal according to claim 8, characterized in that, in step (1), the said phosphorous compound is NH.sub.4H.sub.2PO.sub.4 or P.sub.2O.sub.5; the total mass of the phosphorus compound is to exceed its mass percent by 3-6%.

11. The said piezoelectric single crystal according to claim 8, characterized in that, in step (1), the volume fraction of the nitrogen or inert gases is 90-95%; in step (3), the crystal growth undergoes four stages: necking, shouldering, performing constant diameter growth, and extracting crystal; during the necking stage, the pulling rate is controlled between 1 and 8 mm/h; after the diameter of the seed crystal is narrowed to 0.5-2.0 mm, the single crystal begins to be cooled down slowly at 0.5-5° C./h for shouldering; during the shouldering stage, the pulling rate is reduced to 0.2-1 mm/h; when the shoulder diameter reaches the desired size, the crystal is heated up or cooled down at the rate of 0-5° C./h and maintained at the temperature of 1800-1950° C. to perform constant diameter growth; after being pulled to the desired height, the crystal starts to be extracted.

12. The said piezoelectric single crystal according to claim 8, characterized in that the technological conditions of extracting the crystal are as follows: the temperature is raised slowly at the rate of 10-30° C./h; when the bottom of the crystal has a tendency of inward shrinkage, the pulling rate is increased to 5-20 mm/h to pull the crystal away from the melt.

13. The said yttrium strontium phosphate single crystal according to claim 6, wherein the yttrium strontium phosphate single crystal is prepared by the following steps: the Sr-containing compound, Y-containing compound, and P-containing compound are mixed and sintered to obtain the yttrium strontium phosphate polycrystalline material; the polycrystalline material is heated up to melt and repeats the cooling coagulation and high-temperature melting process several times to obtain a homogeneously melted yttrium strontium phosphate melt; according to the chemical formula Sr.sub.3Y(PO.sub.4).sub.3, based on the stoichiometric proportions of the said Sr-containing compound, Y-containing compound, and P-containing compound in the raw materials, the mass of the P-containing compound is added to exceed its mass percent by 2.5-7.5%; an iridium rod or a yttrium strontium phosphate single crystal is used as the seed crystal; the bottom of the seed crystal is made just in contact with the melt level of the yttrium strontium phosphate for single crystal growth with the Czochralski method; the technological conditions of the single crystal growth are as follows: growth temperature: 1700-1850° C.; pulling rate during necking of the seed crystal: 2-5 mm/h; pulling rate during shouldering: 0.2-2 mm/h; pulling rate while performing constant diameter growth: 0.2-1 mm/h; the crystal is extracted after it grows to the desired size; after that, it is maintained at a constant temperature in the thermal field for 0.5-1 h and then cooled down at a rate of no more than 30° C./h to room temperature to obtain the yttrium strontium phosphate single crystal.

14. The said yttrium strontium phosphate single crystal according to claim 13, characterized in that the said Sr-containing compound, Y-containing compound, and P-containing compound are separately selected from at least one of the corresponding oxides, carbonates, phosphates, halides, nitrates, oxalates, and borates containing such elements, and they are not allowed to be halides at the same time; the Sr-containing compound is preferred to be selected from the corresponding oxides, carbonates, halides, sulfates, or nitrates; the Y-containing compound is preferred to be selected from the corresponding oxides, nitrates, or phosphates; the P-containing compound is preferred to be selected from the corresponding oxides or phosphates.

15. The said yttrium strontium phosphate single crystal according to claim 13, wherein the raw materials are sintered twice at high temperatures after being mixed; the well-weighed raw materials are ground and mixed evenly for the first sintering; the sintering temperature is maintained at 800° C.-950° C. and constant for 10 to 15 hours to decompose and remove CO.sub.2, NH.sub.3, and H.sub.2O; after the first sintering, the raw materials are cooled down, ground and mixed evenly; then, they are pressed into round cake-like blocks for the second sintering; the sintering temperature is maintained at 1200-1400° C. and constant for 20 to 40 hours; finally, the yttrium strontium phosphate polycrystalline material is obtained through the solid-state reaction.

16. The said yttrium strontium phosphate single crystal according to claim 13, characterized in that the yttrium strontium phosphate polycrystalline material repeats cooling coagulation and high-temperature melting 3-4 times after it is melted; preferably, after melting and repeating the cooling coagulation and high-temperature melting process several times, the yttrium strontium phosphate polycrystalline material is overheated 10-20° C. for 0.5-2 hours.

17. The said yttrium strontium phosphate single crystal according to claim 13, characterized in that the crystal growth process comprises four stages: necking, shouldering, performing constant diameter growth, and extracting crystal. Preferably, during the necking stage, the pulling rate is controlled between 2 and 5 mm/h; after the diameter of the seed crystal is narrowed to 0.5-2.0 mm, the single crystal starts to be cooled down slowly at 0.5-5° C./h for shouldering; during the shouldering stage, the pulling rate is reduced to 0.2-2 mm/h; when the shoulder diameter reaches the desired size, the crystal is heated up or cooled down at the rate of 0-5° C./h to perform constant diameter growth; during the constant diameter growth, the pulling rate is 0.4-0.7 mm/h, and the rotational rate is 6-8 r/min. After being pulled to the desired height, the crystal is extracted; the method of extracting the crystal is as follows: the temperature is raised slowly at the rate of 10-30° C./h; when the bottom of the crystal has a tendency of inward shrinkage, the pulling rate is increased to 5-20 mm/h to pull the crystal away from the melt.

18. The said yttrium strontium phosphate single crystal according to claim 13, characterized in that the yttrium strontium phosphate single crystal growth method also comprises an annealing treatment step; the annealing treatment step is as follows: the yttrium strontium phosphate single crystal is taken out after it is fully grown and placed in a high-temperature muffle furnace for annealing treatment at 1200-1400° C.; the time of annealing is 24-48 hours.

19. The said yttrium strontium phosphate single crystal according to claim 6, characterized in that the single crystal can be used as laser host crystal, nonlinear frequency conversion crystal, and high-temperature piezoelectric crystal.

20. The said yttrium strontium phosphate single crystal according to claim 9, characterized in that the nonlinear optical functional devices comprise laser frequency converters, optical parameter amplifiers, optical parameter oscillators or Raman frequency converters; the piezoelectric devices include piezoelectric resonators, oscillators, filters, piezoelectric transducers, piezoelectric pressure sensors, acoustic transducers, or ultrasonic sensors.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0096] FIG. 1 is the XRD phase diagram of the M.sub.3R(PO.sub.4).sub.3 single crystal.

[0097] FIG. 2 shows the picture of the Ba.sub.3Y(PO.sub.4).sub.3 single crystal grown in Embodiment 1.

[0098] FIG. 3 shows the picture of the Ba.sub.3La(PO.sub.4).sub.3 single crystal grown in Embodiment 2.

[0099] FIG. 4 shows the picture of the Ca.sub.3Gd(PO.sub.4).sub.3 single crystal grown in Embodiment 3.

[0100] FIG. 5 shows the transmission spectrum of the Ba.sub.3Y(PO.sub.4).sub.3 single crystal in Embodiment 1.

[0101] FIG. 6 shows the dielectric spectrum of the Ba.sub.3Y(PO.sub.4).sub.3 single crystal in Embodiment 1.

[0102] FIG. 7 shows the picture of the product in Comparative Example 1, a Ba.sub.3Y(PO.sub.4).sub.3 polycrystal obtained by following the stoichiometric proportions.

[0103] FIG. 8 shows the picture of the product in Comparative Example 2, a Ca.sub.3Gd(PO.sub.4).sub.3 single crystal with poor optical properties.

[0104] FIG. 9 shows the picture of the Sr.sub.3Y(PO.sub.4).sub.3 single crystal grown in Embodiment 5.

[0105] FIG. 10 shows the X-ray diffraction spectrogram of the Sr.sub.3Y(PO.sub.4).sub.3 single crystal.

[0106] FIG. 11 shows the picture of the product grown with the method in Comparative Example 3.

[0107] FIG. 12 shows the 2090 nm frequency doubling data of the Sr.sub.3Y(PO.sub.4).sub.3 single crystal in Embodiment 5. An AgGaS.sub.2 single crystal is used as a control. The horizontal coordinate is the particle size of the yttrium strontium phosphate single crystal and AgGaS.sub.2 single crystal samples, and the vertical coordinate is the relative intensity.

[0108] FIG. 13 shows the transmission spectrum of the Sr.sub.3Y(PO.sub.4).sub.3 single crystal in Embodiment 5.

[0109] FIG. 14 shows the dielectric spectrum of the Sr.sub.3Y(PO.sub.4).sub.3 single crystal in Embodiment 7 with the XY cutting orientation.

[0110] FIG. 15 shows the phase angle plots of the impedance generated by the piezoelectric effect of the Sr.sub.3Y(PO.sub.4).sub.3 single crystal with the ZX cutting orientation in Embodiment 9.

[0111] FIG. 16 shows the Sr.sub.3Y(PO.sub.4).sub.3 single crystal in Comparative Example 4.

[0112] FIG. 17 shows the picture of the Ba.sub.3Yb(PO.sub.4).sub.3 single crystal grown in Embodiment 11.

[0113] FIG. 18 shows the X-ray diffraction spectrogram of the Ba.sub.3Yb(PO.sub.4).sub.3 single crystal in Embodiment 11.

[0114] FIG. 19 shows the dielectric spectrum of the Ba.sub.3Yb(PO.sub.4).sub.3 single crystal in Embodiment 11.

[0115] FIG. 20 shows the resistivity plot of the Ba.sub.3Yb(PO.sub.4).sub.3 single crystal in Embodiment 11.

EMBODIMENTS

[0116] The invention is further described as follows in combination with the specific embodiments and the attached figures. The embodiments set out here are used to explain the invention only, but not all. The raw materials used in Embodiments 1-6 are of purity greater than 99.9%.

Embodiment 1. Preparation of Yttrium Barium Phosphate Single Crystal

[0117] a. BaCO.sub.3, Y.sub.2O.sub.3, and NH.sub.4H.sub.2PO.sub.4 are used as raw materials and blended following the stoichiometric proportions according to the chemical formula Ba.sub.3Y(PO.sub.4).sub.3 of the yttrium barium phosphate. Then, the NH.sub.4H.sub.2PO.sub.4 is further added to exceed the mass percent by 5%. [0118] b. The raw materials well prepared in step (1) are evenly mixed and placed in an aluminum oxide crucible and a muffle furnace for the first sintering; the sintering temperature is maintained at 850±50° C. and constant for 12 hours to remove the CO.sub.2, NH.sub.3, and H.sub.2O in the raw materials; after the first sintering, the raw materials are cooled down, ground, and evenly mixed; then, they are pressed into blocks with a cylindrical mold and placed in the aluminum oxide crucible for solid-phase reaction; the sintering temperature is maintained at 1350° C. and constant for 40 hours to obtain the yttrium barium phosphate polycrystalline material. [0119] c. The yttrium barium phosphate polycrystalline material synthesized in step (2) is placed in the iridium crucible of a single crystal growing furnace; before that, the furnace has been evacuated and filled with nitrogen as protective gas; the polycrystalline material is heated up with the medium-frequency induction heating method till melting; it is cooled down to coagulate after melting thoroughly and then heated up again to fully melt; this process is repeated three times to drain the bubbles from the melt; then, the melt is overheated 20° C. for 0.5 hours to obtain the homogeneously melted yttrium barium phosphate melt. [0120] d. An yttrium barium phosphate polycrystalline rod is used as the seed crystal and immersed into the polycrystalline melt in step (3) to have the top of the crystal perpendicular to and just in contact with the melt for single crystal growth.

[0121] The technological conditions of the single crystal growth are as follows: the growth temperature is 1850±50° C.; the pulling rate during necking of the seed crystal is 5 mm/h; after the diameter of the seed crystal is narrowed to around 1 mm, the single crystal begins to be cooled down slowly at 1-4° C./h for shouldering; during the shouldering, the pulling rate is reduced to 0.5-1 mm/h; when the shoulder diameter reaches around the desired size (22 mm), the crystal is heated up or cooled down at the rate of 1-4° C./h to perform constant diameter growth, during which the pulling rate is 0.5-0.7 mm/h; after the single crystal grows to the required size of about a 40 mm height, the crystal starts to be extracted as follows: first, the temperature is raised slowly at the rate of 15-20° C./h, and when the bottom of the crystal has a tendency of inward shrinkage, the pulling rate is increased to 10-15 mm/h to pull the crystal away from the melt.

[0122] After extracting, the crystal is maintained at a constant temperature in the thermal field for 45 min and then cooled down at a rate of 10° C./h to room temperature to obtain the yttrium barium phosphate crystal.

[0123] (5) After being taken out, the crystal is placed in a high-temperature resistance furnace for annealing treatment at 1300° C. The time of annealing is 24 hours so that the thermal stress generated during the growth of the Ba.sub.3Y(PO.sub.4).sub.3 crystal can be fully released.

[0124] The obtained Ba.sub.3Y(PO.sub.4).sub.3 single crystal is as shown in FIG. 2. It has good optical properties.

Embodiment 2. Preparation of Lanthanum Barium Phosphate Single Crystal

[0125] a. BaCO.sub.3, La.sub.2O.sub.3, and NH.sub.4H.sub.2PO.sub.4 are used as raw materials and blended following the stoichiometric proportions according to the chemical formula Ba.sub.3La(PO.sub.4).sub.3 of the lanthanum barium phosphate. Then, the NH.sub.4H.sub.2PO.sub.4 is further added to exceed the mass percent by 5%. [0126] b. The raw materials well prepared in step (1) are evenly mixed and placed in an aluminum oxide crucible for the first sintering; the sintering temperature is maintained at 900° C. and constant for 10 hours to decompose and remove the CO.sub.2, NH.sub.3, and H.sub.2O; after the first sintering, the raw materials are cooled down, ground, and evenly mixed; then, they are pressed into blocks and placed in the aluminum oxide crucible for solid-phase reaction; the sintering temperature is maintained at 1400° C. and constant for 30 hours to obtain the lanthanum barium phosphate polycrystalline material. [0127] c. The lanthanum barium phosphate polycrystalline material synthesized in step (2) is placed in the iridium crucible of a single crystal growing furnace; before that, the furnace has been evacuated and filled with nitrogen as protective gas to prevent oxidation of the iridium crucible; the polycrystalline material is heated up with the medium-frequency induction heating method till melting; it is cooled down to coagulate after melting thoroughly and then heated up again to fully melt; this process is repeated 2-4 times to drain the bubbles from the melt; then, the melt is overheated 15° C. for 1 hour to obtain the homogeneously melted lanthanum barium phosphate melt. [0128] d. An iridium rod is used as the seed crystal and immersed into the polycrystalline melt in step (3) to have the top of the crystal perpendicular to and just in contact with the melt for single crystal growth.

[0129] The technological conditions of the single crystal growth are as follows: the growth temperature is 1850±50° C.; the pulling rate during necking of the seed crystal is 6 mm/h; after the diameter of the seed crystal is narrowed to around 1.5 mm, the single crystal begins to be cooled down slowly at 5° C./h for shouldering; during the shouldering, the pulling rate is reduced to 0.3 mm/h; when the shoulder diameter reaches around the desired size (30 mm), the crystal is heated up or cooled down at the rate of 0-5° C./h to perform constant diameter growth, during which the pulling rate is 0.6 mm/h; after the crystal grows to the required size of about a 50 mm height, the crystal is extracted. The extracting process is as follows: first, the temperature is raised slowly at the rate of 20° C./h, and when the bottom of the crystal has a tendency of inward shrinkage, the pulling rate is increased to 15 mm/h to pull the crystal away from the melt.

[0130] (5) After extracting, the crystal is maintained at a constant temperature in the thermal field for 1 h and then cooled down at a rate of 10° C./h to room temperature to obtain the lanthanum barium phosphate single crystal. The obtained Ba.sub.3La(PO.sub.4).sub.3 single crystal is as shown in FIG. 3. It has good optical properties.

[0131] The annealing treatment of the crystal is the same as that in Embodiment 1.

Embodiment 3. Preparation of Gadolinium Calcium Phosphate Single Crystal

[0132] a. CaCO.sub.3, Gd.sub.2O.sub.3, and NH.sub.4H.sub.2PO.sub.4 are used as raw materials and blended following the stoichiometric proportions according to the chemical formula Ca.sub.3Gd(PO.sub.4).sub.3 of the gadolinium calcium phosphate. Then, the NH.sub.4H.sub.2PO.sub.4 is further added to exceed the mass percent by 3.5%.

[0133] b. The raw materials well prepared in step (1) are evenly mixed and placed in an aluminum oxide crucible and a muffle furnace for the first sintering; the sintering temperature is maintained at 900° C. and constant for 13 hours to remove the CO.sub.2, NH.sub.3, and H.sub.2O in the raw materials; after the first sintering, the raw materials are cooled down, ground, and evenly mixed; then, they are pressed into blocks with a cylindrical mold and placed in the aluminum oxide crucible for solid-phase reaction; the sintering temperature is maintained at 1350° C. and constant for 30 hours to obtain the gadolinium calcium phosphate polycrystalline material. [0134] c. The gadolinium calcium phosphate polycrystalline material synthesized in step (2) is placed in the iridium crucible of a single crystal growing furnace; before that, the furnace has been evacuated and filled with nitrogen as protective gas; the polycrystalline material is heated up with the medium-frequency induction heating method till melting; it is cooled down to coagulate after melting thoroughly and then heated up again to fully melt; this process is repeated three times to drain the bubbles from the melt; then, the melt is overheated 20° C. for 0.5 hours to obtain the homogeneously melted gadolinium calcium phosphate melt. [0135] d. An iridium rod is used as the seed crystal and immersed slowly into the polycrystalline melt in step (3) to have the top of the crystal perpendicular to and just in contact with the melt for single crystal growth.

[0136] The technological conditions of the single crystal growth are as follows: the growth temperature is 1800±50° C.; the pulling rate during necking of the seed crystal is 5 mm/h; after the diameter of the seed crystal is narrowed to around 1 mm, the single crystal begins to be cooled down slowly at 3° C./h for shouldering; during the shouldering, the pulling rate is reduced to 0.3-1 mm/h; when the shoulder diameter reaches around the desired size (15-25 mm), the crystal is heated up or cooled down at the rate of 1-4° C./h to perform constant diameter growth, during which the pulling rate is 0.5 mm/h; after the single crystal grows to the required size of about a 30-50 mm height, the crystal starts to be extracted as follows: first, the temperature is raised slowly at the rate of 25° C./h, and when the bottom of the crystal has a tendency of inward shrinkage, the pulling rate is increased to 15-20 mm/h to pull the crystal away from the melt.

[0137] After extracting, the crystal is maintained at a constant temperature in the thermal field for 45 min and then cooled down at a rate of 10° C./h to room temperature to obtain the gadolinium calcium phosphate single crystal. The obtained Ca.sub.3Gd(PO.sub.4).sub.3 single crystal is as shown in FIG. 4. It has good optical properties.

[0138] The annealing treatment of the crystal is the same as that in Embodiment 1.

Embodiment 4. Preparation of Strontium Lanthanum Phosphate Single Crystal

[0139] a. SrCO.sub.3, La.sub.2O.sub.3, and NH.sub.4H.sub.2PO.sub.4 are used as raw materials and blended following the stoichiometric proportions according to the chemical formula Sr.sub.3La(PO.sub.4).sub.3 of the lanthanum strontium phosphate. Then, the NH.sub.4H.sub.2PO.sub.4 is further added to exceed the mass percent by 3%. [0140] b. The raw materials well prepared in step (1) are evenly mixed and placed in an aluminum oxide crucible and a muffle furnace for the first sintering; the sintering temperature is maintained at 850 and constant for 15 hours to remove the CO.sub.2, NH.sub.3, and H.sub.2O in the raw materials; after the first sintering, the raw materials are cooled down, ground, and evenly mixed; then, they are pressed into blocks with a cylindrical mold and placed in the aluminum oxide crucible for solid-phase reaction; the sintering temperature is maintained at 1300-1400° C. and constant for 20-30 hours to obtain the strontium lanthanum phosphate polycrystalline material. [0141] c. The strontium lanthanum phosphate polycrystalline material synthesized in step (2) is placed in the iridium crucible of a single crystal growing furnace; before that, the furnace has been evacuated and filled with nitrogen as protective gas; the polycrystalline material is heated up with the medium-frequency induction heating method till melting; it is cooled down to coagulate after melting thoroughly and then heated up again to fully melt; this process is repeated three times to drain the bubbles from the melt; then, the melt is overheated 20° C. for 0.5 hours to obtain the homogeneously melted strontium lanthanum phosphate melt. [0142] d. An iridium rod is used as the seed crystal and immersed slowly into the polycrystalline melt in step (3) to have the top of the crystal perpendicular to and just in contact with the melt for single crystal growth.

[0143] The technological conditions of the single crystal growth are as follows: the growth temperature is 1800-1900° C.; the pulling rate during necking of the seed crystal is 6 mm/h; after the diameter of the seed crystal is narrowed to around 1 mm, the single crystal begins to be cooled down slowly at 1-3° C./h for shouldering; during the shouldering, the pulling rate is reduced to 0.4 mm/h; when the shoulder diameter reaches around the desired size (15-25 mm), the crystal is heated up or cooled down at the rate of 1-4° C./h to perform constant diameter growth, during which the pulling rate is 0.6 mm/h; after the single crystal grows to the required size of about a 20-35 mm height, the crystal is extracted as follows: first, the temperature is raised slowly at the rate of 20° C./h, and when the bottom of the crystal has a tendency of inward shrinkage, the pulling rate is increased to 10-15 mm/h to pull the crystal away from the melt.

[0144] After extracting, the crystal is maintained at a constant temperature in the thermal field for 45 min and then cooled down at a rate of 10° C./h to room temperature to obtain the strontium lanthanum phosphate single crystal. The annealing treatment of the crystal is the same as that in Embodiment 1.

Comparative Example 1: Preparation of the Yttrium Barium Phosphate Single Crystal According to the Stoichiometric Proportions

[0145] As described in Embodiment 1, provided however that, in step (1), the raw materials BaCO.sub.3, Y.sub.2O.sub.3, and NH.sub.4H.sub.2PO.sub.4 are mixed by stoichiometric proportions according to the chemical formula Ba.sub.3Y(PO.sub.4).sub.3 of the yttrium barium phosphate, and no excessive NH.sub.4H.sub.2PO.sub.4 is used. The results show that the Ba.sub.3Y(PO.sub.4).sub.3 single crystal cannot be grown as component deviation and stratification occurs after the yttrium barium phosphate raw materials are melted. The resulting product is Ba.sub.3Y(PO.sub.4).sub.3 polycrystal, as shown in FIG. 7.

Comparative Example 2: Preparation of the Gadolinium Calcium Phosphate Single Crystal with Excessive NH.SUB.4.H.SUB.2.PO.SUB.4 .(Exceeding its Mass Percent by 0.5%)

[0146] As described in Embodiment 3, provided however that, in step (1), the raw materials CaCO.sub.3, Gd.sub.2O.sub.3, and NH.sub.4H.sub.2PO.sub.4 are blended according to the stoichiometric proportions of the chemical formula Ca.sub.3Gd(PO.sub.4).sub.3, and NH.sub.4H.sub.2PO.sub.4 is further added to exceed the mass percent by 0.5%. The resulting single crystal has poor properties, as shown in FIG. 8. Studies have found that the 0.5% excessive phosphate is insufficient to compensate for the component deviation caused by phosphorus volatilization during the single crystal growth, resulting in poor crystallinity of the gadolinium calcium phosphate single crystal.

Embodiment 5. Preparation of Yttrium Strontium Phosphate Single Crystal

[0147] a. SrCO.sub.3, Y.sub.2O.sub.3, and NH.sub.4H.sub.2PO.sub.4 are used as raw materials and blended following the stoichiometric proportions according to the chemical formula Sr.sub.3Y(PO.sub.4).sub.3 of the yttrium strontium phosphate. Then, the NH.sub.4H.sub.2PO.sub.4 is further added to exceed the mass percent by 3%. [0148] b. The raw materials well prepared in step (1) are evenly mixed and placed in an aluminum oxide crucible for the first sintering; the sintering temperature is maintained at 900° C. and constant for 10 hours to decompose and remove the CO.sub.2, NH.sub.3, and H.sub.2O; after the first sintering, the raw materials are cooled down, ground, and evenly mixed; then, they are pressed into blocks and placed in the aluminum oxide crucible for solid-phase reaction; the sintering temperature is maintained at 1250° C. and constant for 48 hours to obtain the yttrium strontium phosphate polycrystalline material. [0149] c. The yttrium strontium phosphate polycrystalline material synthesized in step (2) is placed in the iridium crucible of a single crystal growing furnace; before that, the furnace has been evacuated and filled with nitrogen as protective gas to prevent the oxidation of the iridium crucible; the polycrystalline material is heated up with the medium-frequency induction heating method till melting; it is cooled down to coagulate after melting thoroughly and then heated up again to fully melt; this process is repeated three times to drain the bubbles from the melt; then, the melt is overheated 20° C. for 1 hour to obtain the homogeneously melted yttrium strontium phosphate melt. [0150] d. An iridium rod is used as the seed crystal and immersed slowly into the polycrystalline melt in step (3) to have the top of the crystal perpendicular to and just in contact with the melt for single crystal growth.

[0151] The technological conditions of the single crystal growth are as follows: the growth temperature is 1800° C.; the pulling rate during necking of the seed crystal is 3-3.5 mm/h, and the rotation rate is 8-15 r/min; after the diameter of the seed crystal is narrowed to 1 mm, the single crystal begins to be cooled down slowly at 0.5° C./h for shouldering; during the shouldering, the pulling rate is reduced to 0.3-0.4 mm/h, and the rotation rate is 6-8 r/min; when the shoulder diameter reaches the desired size (20 mm), the crystal is heated up or cooled down at the rate of 0.3° C./h to perform constant diameter growth, during which the pulling rate is 0.5 mm/h and the rotation rate is 8 r/min; after the single crystal grows to the required size of about a 36 mm height, the crystal starts to be extracted. The extracting process is as follows: first, the temperature is raised at the rate of 20° C./h, and when the bottom of the crystal has a tendency of inward shrinkage, the pulling rate is increased to 5 mm/h to pull the crystal away from the melt. After extracting, the crystal is maintained at a constant temperature in the thermal field for 1 h and then cooled down at a rate of 10° C./h to room temperature to obtain the yttrium strontium phosphate single crystal.

[0152] (5) After being taken out, the crystal is placed in a high-temperature resistance furnace for annealing treatment at 1300° C. The time of annealing is 24 hours so that the thermal stress generated during the growth of the Sr.sub.3Y(PO.sub.4).sub.3 single crystal can be fully released.

[0153] The picture of the obtained Sr.sub.3Y(PO.sub.4).sub.3 single crystal is shown in FIG. 9. The X-ray diffraction spectrum of the resulting crystal shows characteristic peaks at 20=27.88, 33.13°, and 45.74°, as shown in FIG. 10.

[0154] The Sr.sub.3Y(PO.sub.4).sub.3 single crystal obtained can effectively achieve the frequency doubling effect of the 2090 nm laser light, as shown in FIG. 12, and an AgGaS2 single crystal is used as a control, indicating that the Sr.sub.3Y(PO.sub.4).sub.3 single crystal can achieve effective frequency doubling in the infrared band.

[0155] The transmission spectrum of the Sr.sub.3Y(PO.sub.4).sub.3 single crystal is shown in FIG. 13. The crystal obtained has a transmittance of >80% in the 480 nm-4100 nm band, indicating that it has good optical uniformity. Its absorption cutoff edge is less than 180 nm, indicating that it has potential application in the deep ultraviolet band

Embodiment 6. Preparation of Gadolinium Strontium Phosphate Single Crystal

[0156] a. SrCO.sub.3, Gd.sub.2O.sub.3, and P.sub.2O.sub.5 are used as raw materials and blended following the stoichiometric proportions according to the chemical formula Sr.sub.3Gd(PO.sub.4).sub.3 of the gadolinium strontium phosphate. Then, the P.sub.2O.sub.5 is further added to exceed the mass percent by 5%. [0157] b. The raw materials well prepared in step (1) are evenly mixed and placed in an aluminum oxide crucible for the first sintering; the sintering temperature is maintained at 900° C. and constant for 15 hours to decompose and remove the CO.sub.2, NH.sub.3, and H.sub.2O; after the first sintering, the raw materials are cooled down, ground, and evenly mixed; then, they are pressed into blocks and placed in the aluminum oxide crucible for solid-phase reaction; the sintering temperature is maintained at 1300° C. and constant for 48 hours to obtain the gadolinium strontium phosphate polycrystalline material. [0158] c. The gadolinium strontium phosphate polycrystalline material synthesized in step (2) is placed in the iridium crucible of a single crystal growing furnace; before that, the furnace has been evacuated and filled with nitrogen as protective gas to prevent the oxidation of the iridium crucible; the polycrystalline material is heated up with the medium-frequency induction heating method till melting; it is cooled down to coagulate after melting thoroughly and then heated up again to fully melt; this process is repeated three times to drain the bubbles from the melt; then, the melt is overheated 20° C. for 1 hour to obtain the homogeneously melted gadolinium strontium phosphate melt. [0159] d. An yttrium strontium phosphate single crystal is used as the seed crystal and immersed slowly into the polycrystalline melt in step (3) to have the top of the crystal perpendicular to and just in contact with the melt for single crystal growth.

[0160] The technological conditions of the single crystal growth are as follows: the growth temperature is 1800° C.; the pulling rate during necking of the seed crystal is 4-4.5 mm/h, and the rotation rate is 8-15 r/min; after the diameter of the seed crystal is narrowed to 1 mm, the single crystal begins to be cooled down slowly at 0.8° C./h for shouldering; during the shouldering, the pulling rate is reduced to 0.4-0.5 mm/h, and the rotation rate is 6-8 r/min; when the shoulder diameter reaches the desired size (20 mm), the crystal is heated up or cooled down at the rate of 0.2° C./h to perform constant diameter growth, during which the pulling rate is 0.6 mm/h and the rotation rate is 8 r/min; after the single crystal grows to the required size of about a 30 mm height, the crystal starts to be extracted. The extracting process is as follows: first, the temperature is raised at the rate of 20° C./h, and when the bottom of the crystal has a tendency of inward shrinkage, the pulling rate is increased to 6 mm/h to pull the crystal away from the melt. After extracting, the crystal is maintained at a constant temperature in the thermal field for 1 h and then cooled down at a rate of 15° C./h to room temperature to obtain the yttrium strontium phosphate single crystal.

[0161] (5) After being taken out, the crystal is placed in a high-temperature resistance furnace for annealing treatment at 1300° C. The time of annealing is 24 hours so that the thermal stress generated during the growth of the Sr.sub.3Gd(PO.sub.4).sub.3 single crystal can be fully released.

[0162] The Sr.sub.3Gd(PO.sub.4).sub.3 single crystal obtained is 20 mm in diameter and 30 mm in height. A frequency doubling effect is observed when it is subjected to a 2090 nm laser light. Upon tests, the single crystal has a transmittance of >80% in the 480 nm-4100 nm band and has good optical uniformity.

Comparative Example 3

[0163] As described in Embodiment 5, provided however that, in step (1), the raw materials SrCO.sub.3, Y.sub.2O.sub.3, and NH.sub.4H.sub.2PO.sub.4 are mixed by stoichiometric proportions according to the chemical formula Sr.sub.3Y(PO.sub.4).sub.3, and no excessive NH.sub.4H.sub.2PO.sub.4 is used. The results show that the Sr.sub.3Y(PO.sub.4).sub.3 single crystal cannot be grown as component deviation and stratification occurs after the yttrium strontium phosphate raw materials are melted. The resulting product is Sr.sub.3Y(PO.sub.4).sub.3 polycrystal, as shown in FIG. 3.

Comparative Example 4

[0164] As described in Embodiment 5, provided however that, in step (1), the raw materials SrCO.sub.3, Y.sub.2O.sub.3, and NH.sub.4H.sub.2PO.sub.4 are blended following the stoichiometric proportions of the chemical formula Sr.sub.3Y(PO.sub.4).sub.3, and NH.sub.4H.sub.2PO.sub.4 is further added to exceed the mass percent by 0.5%.

[0165] The resulting single crystal has poor properties and is of an irregular shape, as shown in FIG. 8. Studies have found that the 0.5% excessive phosphate is insufficient to compensate for the component deviation caused by phosphorus volatilization during the single crystal growth, resulting in poor crystallinity of the yttrium strontium phosphate single crystal.

Embodiment 7

[0166] After orienting the Sr.sub.3Y(PO.sub.4).sub.3 single crystal prepared in Embodiment 5 with reference to the physical piezoelectric axis, a crystal plate is processed along the X and Y directions of the physical axis, with the thickness in the X direction, length in the Y direction, and width in the Z direction. The crystal plate size is: thickness×width×length=1.2 mm×3.5 mm×10.0 mm, and the crystal face in the thickness direction is plated with a conducting electrode. Upon testing of the crystal plate sample with an impedance analyzer, piezoelectric resonance and anti-resonance peaks are detected, indicating that the single crystal has a piezoelectric effect in the cutting orientation. In particular, when the temperature is raised to 1000° C., piezoelectric resonance and anti-resonance peaks are still observed, indicating that the single crystal can be applied as a high-temperature piezoelectric crystal. See FIG. 14.

Embodiment 8

[0167] After orienting the Sr.sub.3Y(PO.sub.4).sub.3 single crystal prepared in Embodiment 6 with reference to the physical piezoelectric axis, a crystal plate is processed along the X and Y directions of the physical axis, with the thickness in the X direction, length in the Y direction, and width in the Z direction. The crystal plate size is: thickness×width×length==(0.5-1.5)mm×(3.0-4.0)mm×(8.0-12.0)mm, and the crystal face in the thickness direction is plated with a conducting electrode.

[0168] Upon testing of the crystal plate sample with an impedance analyzer, piezoelectric resonance and anti-resonance peaks are detected, indicating that the single crystal has a piezoelectric effect in the cutting orientation. The same to Embodiment 5, when the temperature is raised to 1000° C., piezoelectric resonance and anti-resonance peaks are still observed, indicating that the single crystal can be applied as a high-temperature piezoelectric crystal. The resonant frequency and antiresonant frequency of the sample appear at 881.8 kHz and 887.8 kHz respectively.

Embodiment 9

[0169] After orienting the Sr.sub.3Y(PO.sub.4).sub.3 single crystal prepared in Embodiment 5 with reference to the physical piezoelectric axis, a crystal plate is processed along the X and Y directions of the physical axis, with the thickness in the Z direction, length in the X direction, and width in the Y direction. The crystal plate size is: thickness×width×length=1.0 mm×3.5 mm×10.0 mm, and the crystal face in the thickness direction is plated with a conducting electrode.

[0170] Upon testing of the crystal plate sample with an impedance analyzer, piezoelectric resonance and anti-resonance peaks are detected, indicating that the single crystal has a piezoelectric effect in the orientation. When the temperature is raised to 1000° C., piezoelectric resonance and anti-resonance peaks are still observed, so the single crystal can be applied as a high-temperature piezoelectric single crystal. The piezoelectric activity of the single crystal, despite the cutting orientations, does not change when the temperature is raised from room temperature to 1000° C. The impedance analysis diagram is shown in FIG. 15.

Embodiment 10

[0171] After orienting the Sr.sub.3Y(PO.sub.4).sub.3 single crystal prepared in Embodiment 6 with reference to the physical piezoelectric axes X, Y, and Z, the crystal is rotated 0 degrees)(0=0-180° along the X, Y, and Z axes to prepare Sr.sub.3Y(PO.sub.4).sub.3 single crystal of different cutting orientations. Then, crystal plates are processed with reference to Embodiment 5. The crystal plate size is: thickness×width×length=(0.5-1.5)mmx(3.0-4.0)mmx(8.0−12.0)mm, and the crystal face in the thickness direction is plated with a conducting electrode.

[0172] Upon testing of the above series of Sr.sub.3Y(PO.sub.4).sub.3 single crystal plate samples with an impedance analyzer, piezoelectric resonance and anti-resonance peaks are detected, despite the different cutting orientations, indicating that the single crystal has a piezoelectric effect in all orientations of the space. The piezoelectric activity of the single crystal, despite the cutting orientations, does not change even at 1000° C., so it can be applied as a high-temperature piezoelectric crystal.

Embodiment 11. Preparation of Ytterbium Barium Phosphate Single Crystal

[0173] a. BaCO.sub.3, Yb.sub.2O.sub.3, and NH.sub.4H.sub.2PO.sub.4 are used as raw materials and blended following the stoichiometric proportions according to the chemical formula Ba.sub.3Yb(PO.sub.4).sub.3 of the ytterbium barium phosphate. Then, the NH.sub.4H.sub.2PO.sub.4 is further added to exceed the mass percent by 5%. [0174] b. The raw materials well prepared in step (1) are evenly mixed and placed in an aluminum oxide crucible for the first sintering; the sintering temperature is maintained at 900° C. and constant for 10 hours to decompose and remove the CO.sub.2, NH.sub.3, and H.sub.2O; after the first sintering, the raw materials are cooled down, ground, and evenly mixed; then, they are pressed into blocks and placed in the aluminum oxide crucible for solid-phase reaction; the sintering temperature is maintained at 1250° C. and constant for 48 hours to obtain the ytterbium barium phosphate polycrystalline material. [0175] c. The ytterbium barium phosphate polycrystalline material synthesized in step (2) is placed in the iridium crucible of a single crystal growing furnace; before that, the furnace has been evacuated and filled with nitrogen as protective gas to prevent the oxidation of the iridium crucible; the polycrystalline material is heated up with the medium-frequency induction heating method till melting; it is cooled down to coagulate after melting thoroughly and then heated up again to fully melt; this process is repeated three times to drain the bubbles from the melt; then, the melt is overheated 20° C. for 1 hour to obtain the homogeneously melted ytterbium barium phosphate melt. [0176] d. An iridium rod is used as the seed crystal and immersed slowly into the polycrystalline melt in step (3) to have the top of the crystal perpendicular to and just in contact with the melt for single crystal growth.

[0177] The technological conditions of the single crystal growth are as follows: the growth temperature is 1800° C.; the pulling rate during necking of the seed crystal is 1-3.5 mm/h, and the rotation rate is 6-12 r/min; after the diameter of the seed crystal is narrowed to 1 mm, the single crystal begins to be cooled down slowly at 0.5° C./h for shouldering; during the shouldering, the pulling rate is reduced to 0.3-0.5 mm/h, and the rotation rate is 6-8 r/min; when the shoulder diameter reaches the desired size (15 mm), the crystal is heated up or cooled down at the rate of 0.3° C./h to perform constant diameter growth, during which the pulling rate is 0.5 mm/h and the rotation rate is 5 r/min; after the single crystal grows to the required size of about a 30 mm height, the crystal starts to be extracted. The extracting process is as follows: first, the temperature is raised at the rate of 15° C./h, and when the bottom of the crystal has a tendency of inward shrinkage, the pulling rate is increased to 5 mm/h to pull the crystal away from the melt. After extracting, the crystal is maintained at a constant temperature in the thermal field for 1 h and then cooled down at a rate of 10-30° C./h to room temperature to obtain the ytterbium barium phosphate single crystal.

[0178] (5) After being taken out, the crystal is placed in a high-temperature resistance furnace for annealing treatment at 1300° C. The time of annealing is 48 hours so that the thermal stress generated during the growth of the Ba.sub.3Yb(PO.sub.4).sub.3 single crystal can be fully released.

[0179] The picture of the Ba.sub.3Yb(PO.sub.4).sub.3 single crystal obtained is shown in FIG. 17. The X-ray diffraction spectrum of the crystal obtained shows characteristic peaks at 20=26.93°, 31.99° and 44.10°, as shown in FIG. 18.

[0180] The dielectric spectrum and resistivity characteristics of the Ba.sub.3Yb(PO.sub.4).sub.3 single crystal are shown in FIG. 19 and FIG. 20. It has both low dielectric losses (<1.1) and high resistivity (ρ>10.sup.7 (Ohm.Math.cm)) at 900° C., indicating that it has potential applications in the high-temperature piezoelectric field.