Abstract:
The present invention provides a barium fluoroborate compound, a nonlinear optical crystal and the preparation method and use thereof. Both of the barium fluoroborate compound and the nonlinear optical crystal have a chemical formula of Ba 4 B 11 O 20 F. The crystal belongs to orthorhombic crystal system, has a space group Cmc2 1  with unit cell parameters of a=18.802(3) Å, b=10.7143(19) Å, c=8.6113(14) Å, V=1734.7(5) Å 3 . The crystal has a powder second harmonic generation efficiency of 10 times that of KDP (KH 2 PO 4 ). The ultraviolet cutoff wavelength is about 170 nm. The barium fluoroborate compound is prepared by a solid-state reaction. The barium fluoroborate nonlinear optical crystal prepared by a high temperature melting liquid method has large hardness and is easy to be cut, polished and stored. This crystal can be used widely in preparing the nonlinear optical devices of the second harmonic generator, the up frequency converter, the down frequency converter or the optical parametric oscillator or the like.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a compound and a nonlinear optical crystal having a chemical formula of Ba 4 B 11 O 20 F, the preparation method thereof and its uses in manufacturing nonlinear optical devices. 
       BACKGROUND OF THE INVENTION 
       [0002]    Nonlinear optical devices of the second harmonic generator, the up/down frequency converter, or the optical parametric oscillator or the like can be manufactured by using the nonlinear optical effect of crystals. A laser produced from laser generator can be converted in its frequency by passing through a nonlinear optical device, whereby obtaining more lasers with useful wavelengths, allowing the laser generator finding wider applications. Among the nonlinear optical crystal materials, the halogen-containing nonlinear optical crystals are a kind of relatively special crystal materials. Because halogen ions have many similarities in coordination structures, their physical-chemical properties allow for avoiding a sudden change in the crystalline structure when the halogen ions replace with each other (partly or totally), and thus it is easy to achieve the modification of the materials. 
         [0003]    At the same time, the focus topic has been put forth for the study and exploring of new nonlinear optical materials that exhibit high effect of second harmonic, broad transparent region, high damage threshold and stable physical-chemical properties. Currently, the main nonlinear optical materials include: BBO (β-BaB 2 O 4 ), LBO (LiB 3 O 5 ) crystal, CBO (CsB 3 O 5 ) crystal, CLBO (CsLiB 6 O 10 ) crystal and KBBF (KBe 2 BO 3 F 2 ) crystal. Although the crystal growing techniques for these materials have been developed increasingly, there are still apparent disadvantages, such as, deliquescence tendence of the crystals, long period of growing, serious behavior of growing in layers as well as high price. Accordingly, it is still an important and hard work to seek new nonlinear optical crystal materials. 
         [0004]    In order to make up the shortages of the nonlinear optical crystals mentioned above, scientists all over the world are still focusing highly on the study and research of various novel nonlinear crystals. They pay attention to not only the optical properties and mechanical properties of the crystals, but also the preparing properties of the crystals more and more. In the case of finding a useful nonlinear optical crystal, a laser with limited wavelength can be converted to a laser with a new wavelength range by using the nonlinear optical effects of second harmonic, frequency mixing and optical parametric oscillating or the like. By using this technique, it is possible to fill up the gap in the spectrum range of the laser wavelengths irradiated by various laser devices, and allow for the laser generator finding a wider application. 
       SUMMARY OF THE INVENTION 
       [0005]    One object of the present invention is to provide a barium fluoroborate compound having a chemical formula of Ba 4 B 11 O 20 F, and a process for preparing the same. 
         [0006]    Another object of the present invention is to provide a barium fluoroborate nonlinear optical crystal having a chemical formula of Ba 4 B 11 O 20 F, and a process for preparing the same. 
         [0007]    Yet another object of the present invention is to provide a use of the barium fluoroborate nonlinear optical device, and a use thereof for preparing the second harmonic generator, the up frequency converter, the down frequency converter or the optical parametric oscillator. 
         [0008]    The technical solution of the present invention is as follows. 
         [0009]    The barium fluoroborate compound provided by the present invention has a chemical formula of Ba 4 B 11 O 20 F and it is a barium fluoroborate compound prepared by a solid-state reaction according to the following chemical reactions: 
         [0000]      7BaCO 3 +BaF 2 +22H 3 BO 3 →2Ba 4 B 11 O 20 F+7CO 2 ↑+33H 2 O↑;   (1)
 
         [0000]      7Ba(NO 3 ) 2 +BaF 2 +22H 3 BO 3 →2Ba 4 B 11 O 20 F+13NO 2 ↑+NO↑+33H 2 O↑+4O 2 ↑;   (2)
 
         [0000]      7BaO+BaF 2 +22H 3 BO 3 →2Ba 4 B 11 O 20 F+33H 2 O↑;   (3)
 
         [0000]      7Ba(OH) 2 +BaF 2 +22H 3 BO 3 →2Ba 4 B 11 O 20 F+40H 2 O↑;   (4)
 
         [0000]      7Ba(HCO 3 ) 2 +BaF 2 +22H 3 BO 3 →2Ba 4 B 11 O 20 F+14CO 2 ↑+40H 2 O↑;   (5)
 
         [0000]      7BaCO 3 +BaF 2 +11B 2 O 3 →2Ba 4 B 11 O 20 F+7CO 2 ↑;   (6)
 
         [0000]      7Ba(NO 3 ) 2 +BaF 2 +11B 2 O 3 →2Ba 4 B 11 O 20 F+13NO 2 ↑+NO↑+4O 2 ↑;   (7)
 
         [0000]      7BaO+BaF 2 +11B 2 O 3 →2Ba 4 B 11 O 20 F;   (8)
 
         [0000]      7Ba(OH) 2 +BaF 2 +11B 2 O 3 →2Ba 4 B 11 O 20 F+7H 2 O↑;   (9)
 
         [0000]      7Ba(HCO 3 ) 2 +BaF 2 +11B 2 O 3 →2Ba 4 B 11 O 20 F+14CO 2 ↑+7H 2 O↑;   (10)
 
         [0000]      7BaC 2 O 4 +BaF 2 +11B 2 O 3 →2Ba 4 B 11 O 20 F+7CO 2 ↑+7CO.   (11)
 
         [0010]    The barium fluoroborate nonlinear optical crystal provided by the present invention has a chemical formula of Ba 4   13   11 O 20 F and a molecular weight of 1007.27, wherein the crystal has no symmetric center, belongs to orthorhombic crystal system, has a space group Cmc2 1  with unit cell parameters of a=18.802(3) Å, b=10.7143(19) Å, c=8.6113(14) Å, Z=4, V=1734.7(5) Å 3 . The crystal has a powder second harmonic generation efficiency of 10 times that of KDP (KH 2 PO 4 ). The ultraviolet cutoff wavelength is about 170 nm. 
         [0011]    The method for manufacturing the barium fluoroborate nonlinear optical crystal grows the barium fluoroborate nonlinear optical crystal by a high temperature melting liquid method through the steps of: 
         [0012]    a. mixing a single-phase polycrystalline powder of a barium fluoroborate compound with a flux uniformly, heating the obtained mixture to 720˜1050° C. at a temperature-increasing rate of 1˜30° C./h, keeping the mixture at this temperature for 5˜80 h to obtain a mixed melting liquid, and then reducing the temperature of the mixed melting liquid to 705˜760° C., or 
         [0013]    weighing a starting material for preparing barium fluoroborate according to a molar ratio, mixing the starting material for preparing barium fluoroborate with a flux uniformly, heating the obtained mixture to 720˜1050° C. at a temperature-increasing rate of 1˜30° C./h, keeping the mixture at this temperature for 5˜80 h to obtain a mixed melting liquid, and then reducing the temperature of the mixed melting liquid to 705˜760° C.; 
         [0014]    the flux is NaF, NaF—H 3 BO 3 , LiF—PbO, H 3 BO 3 —PbO, NaF—PbO or KF—PbO; 
         [0015]    the starting material for preparing barium fluoroborate comprises a barium-containing compound, barium fluoride, boric acid in a molar ratio of 7:1:22; or comprises a barium-containing compound, barium fluoride, boron oxide in a molar ratio of 7:1:11; 
         [0016]    the barium-containing compound is barium carbonate, barium nitrate, barium oxide, barium hydroxide, barium bicarbonate or barium oxalate; 
         [0017]    the single-phase polycrystalline powder of the barium fluoroborate compound is prepared by a solid-state reaction with the steps of 
         [0018]    subjecting the above starting material to: uniformly mixing, grinding, heating at 550° C. for 24 hours in a muffle furnace, cooling to room temperature; the second grinding, heating at 650° C. for 24 hours in a muffle furnace, cooling to room temperature; the third grinding, heating at 750° C. for 48 hours in a muffle furnace, removing and grinding so as to obtain a completely sintered single-phase polycrystalline powder of the barium fluoroborate compound; 
         [0019]    the molar ratio of the barium fluoroborate to the flux is 1:3˜10; 
         [0020]    b. preparing a barium fluoroborate seed crystal: a part of the mixed melting liquid as obtained in step a is slowly cooled to room temperature at a rate of 0.5˜10° C./h to obtain a barium fluoroborate seed crystal by spontaneous crystallization; 
         [0021]    c. placing a crucible containing the mixed melting liquid as obtained in step a into a crystal growing furnace, attaching the seed crystal as obtained in step b on a seed crystal rod for introducing the seed crystal into the crystal growing furnace from the top thereof, preheating the seed crystal for 5-60 minutes, introducing the seed crystal to contact the liquid surface of the mixed melting liquid or into the mixed melting liquid for remelting, keeping the temperature of the mixture for 5˜60 minutes, and reducing the temperature to reach a saturation temperature at a ratio of 1˜10° C./h; 
         [0022]    d. reducing the temperature of the mixture at a rate of 0.1˜5° C./day while rotating the seed crystal rod at a rate of 0˜60 rpm for growing crystal, drawing the crystal out of the surface of the mixed melting liquid when a single crystal has obtained the desired size, reducing the temperature of the crystal to room temperature at a rate of 1˜80° C./h, and removing the crystal from the furnace to obtain a barium fluoroborate nonlinear optical crystal. 
         [0023]    In the flux of NaF—H 3 BO 3 , the molar ratio of NaF to H 3 BO 3  is 1˜8:1˜5; and in the flux of H 3 BO 3 —PbO, the molar ratio of H 3 BO 3  to PbO is 1˜6:1˜5. 
         [0024]    In the fluxes of LiF—PbO, NaF—PbO or KF—PbO, the molar ratio of the fluoride to lead oxide is 2˜5:1˜3. 
         [0025]    The barium fluoroborate nonlinear optical crystal prepared in the present invention has a chemical formula of Ba 4 B 11 O 20 F and a molecular weight of 1007.27, belongs to orthorhombic crystal system, has a space group Cmc2 1  with unit cell parameters of a=18.802(3) Å, b=10.7143(19) Å, c=8.6113(14) Å, V=1734.7(5) Å 3 . The crystal has a powder second harmonic generation efficiency of about 10 times that of KDP (KH 2 PO 4 ). The ultraviolet cutoff wavelength is about 170 nm. 
         [0026]    Because the use of fluxes NaF, NaF—H 3 BO 3 , LiF—PbO, NaF—PbO, H 3 BO 3 -PbO or KF—PbO in the growing of the barium fluoroborate nonlinear optical crystal, the product has a high purity, is easy to grow up and transparent without wrapping, it possess the advantages of relatively high growing speed, low cost and easy to obtain larger size crystal, or the like. The obtained crystal has the advantages of relatively broad transparent region, large hardness, good mechanical properties, hard to be broken and deliquesced, easy to be processed and stored. At room temperature, when a Q-switched Nd:YAG laser generator is used as the light source and the incident infrared light has a wavelength of 1064 nm, a nonlinear optical device manufactured by using the present barium fluoroborate nonlinear optical crystal will generate a green laser with a wavelength of 532 nm which has an intensity of 10 times that of KDP (KH 2 PO 4 ). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]      FIG. 1  shows the X-ray diffraction pattern of Ba 4 B 11 O 20 F powders of the invention. 
           [0028]      FIG. 2  is a schematic illustration of the processing principle of a nonlinear optical device made from the present barium fluoroborate nonlinear optical crystal, wherein,  1  is a laser generator,  2  is an emitted laser beam,  3  is a Ba 4 B 11 O 20 F crystal,  4  is an output beam, and  5  is a filter. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0029]    The present invention will be described in detail with reference to the drawings and examples below: 
       Example 1 
       [0030]    Preparation of a Ba 4 B 11 O 20 F compound according to the following reaction equation: 
         [0000]      7BaO+BaF 2 +22H 3 BO 3 →2Ba 4 B 11 O 20 F+33H 2 ↑
 
         [0031]    BaO, BaF 2  and H 3 BO 3  were put in a mortar at a molar ratio of 7/2:1/2:11, mixed and ground carefully, and then charged into a Φ100 mm×100 mm lidless corundum crucible. The crucible was placed in a muffle furnace, heated slowly to 550° C. and held at this temperature for 24 hours. After cooling to room temperature, the crucible was taken out and at which time the sample was relatively loose. Subsequently, the sample was removed and ground thoroughly again. Then the sample was charged into a crucible again and heated to 750° C. in the muffle furnace and held for 48 h. The sample was then removed and ground in a mortar to yield a Ba 4 B 11 O 20 F compound. The product was subjected to X-ray analysis, and the resultant X-ray pattern was consistent with the X-ray pattern obtained from barium fluoroborate Ba 4 B 11 O 20 F single crystal structure. 
         [0032]    The obtained barium fluoroborate Ba 4 B 11 O 20 F compound single-phase polycrystalline powder and a flux NaF were mixed at a molar ratio of Ba 4 B 11 O 20 F:NaF=1:3, and then charged into a Φ80 mm×80 mm lidless platinum crucible. The obtain mixture was heated to 780° C. at a temperature-increasing rate of 30° C./h and kept at this temperature for 15 h to obtain a mixed melting liquid. Subsequently, the temperature was reduced to 735° C. 
         [0033]    The temperature was slowly reduced to room temperature at a rate of 0.5° C./h to obtain a barium fluoroborate seed crystal by spontaneous crystallization. 
         [0034]    Growing crystal in the compound melting liquid: the obtained Ba 4 B 11 O 20 F seed crystal was attached on a seed crystal rod for introducing the seed crystal into the crystal growing furnace from the top thereof The seed crystal was preheated above the melting liquid surface for 10 minutes, and then dipped into the liquid surface so that the seed crystal remelted in the mixed melting liquid. The temperature was kept for 30 minutes, and then reduced to a saturation temperature of 730° C. at a rate of 10° C./h. 
         [0035]    Temperature was further reduced at a rate of 2° C./day while rotating the seed crystal rod at a rate of 10 rpm. The crystal was drawn out of the surface of the mixed melting liquid when the crystal had completed its growing. The temperature of the crystal was reduced to room temperature at a rate of 10° C./h to obtain a Ba 4   13   11 O 20 F crystal with a size of 56 mm×40 mm×30 mm. 
         [0036]    In the above reaction equation, the starting material barium oxide can be replaced by barium carbonate, barium nitrate, barium oxalate, barium hydroxide or barium bicarbonate; the boronic acid can be replaced by boron oxide. 
       Example 2 
       [0037]    Preparation of a Ba 4 B 11 O 20 F compound according to the following reaction equation: 
         [0000]      7Ba(NO 3 ) 2 +BaF 2 +22H 3 BO 3 →2Ba 4 B 11 O 20   F+13 NO 2 ↑+NO↑+33H 2 O↑+ 4 O 2 ↑
 
         [0038]    Ba(NO 3 ) 2 , BaF 2  and H 3 BO 3  were weighed at a molar ratio of 7/2:1/2:11 directly as starting materials. The weighed starting materials and a flux KF—PbO in which the molar ratio of KF to PbO is 2:1 were mixed at a molar ratio of 1:4, and then charged into a Φ80 mm×80 mm lidless platinum crucible. The obtain mixture was heated to 745° C. and kept at this temperature for 5 h to obtain a mixed melting liquid. Subsequently, the temperature was reduced to 715° C. 
         [0039]    The temperature was slowly reduced to room temperature at a rate of 1.5° C./h to obtain a barium fluoroborate seed crystal by spontaneous crystallization. 
         [0040]    The obtained Ba 4 B 11 O 20 F seed crystal was attached on a seed crystal rod for introducing the seed crystal into the crystal growing furnace from the top thereof. The seed crystal was preheated above the melting liquid surface for 10 minutes, and then dipped into the liquid surface so that the seed crystal remelted in the mixed melting liquid. The temperature was kept for 30 minutes, and reduced quickly to a saturation temperature of 710° C. Temperature was further reduced at a rate of 1° C./day without rotating the seed crystal rod. The crystal was drawn out of the surface of the mixed melting liquid when the crystal had grown to the desired size. The temperature of the crystal was reduced to room temperature at a rate of 20° C. /h followed by removing the crystal from the furnace to obtain a Ba 4 B 11 O 20 F crystal with a size of 36 mm×20 mm×18 mm. 
         [0041]    In the above reaction equation, the starting material barium nitrate can be replaced by barium carbonate, barium oxide, barium oxalate, barium hydroxide or barium bicarbonate; the boronic acid can be replaced by boron oxide. 
       Example 3 
       [0042]    Preparation of a Ba 4 B 11 O 20 F compound according to the following reaction equation: 
         [0000]      7BaCO 3 +BaF 2 +22H 3 BO 3 →2Ba 4 B 11 O 20 F+7CO 2 ↑+33H 2 O↑
 
         [0043]    BaCO 3 , BaF 2  and H 3 BO 3  were put in a mortar at a molar ratio of 7/2:1/2:11, mixed and ground carefully, and then charged into a Φ100 mm×100 mm lidless corundum crucible and compacted. The crucible was placed in a muffle furnace, heated slowly to 550° C. and held at this temperature for 24 hours. After cooling to room temperature, the crucible was taken out and at which time the sample was relatively loose. Subsequently, the sample was removed and ground thoroughly again. Then the sample was charged into a crucible again and heated to 750° C. in the muffle furnace and held for 48 h. The sample was then removed and ground in a mortar to yield a Ba 4 B 11 O 20 F compound. The product was subjected to X-ray analysis, and the resultant X-ray pattern was consistent with the X-ray pattern obtained from barium fluoroborate Ba 4 B 11 O 20 F single crystal structure. 
         [0044]    The obtained Ba 4 B 11 O 20 F compound and a flux NaF were mixed at a molar ratio of Ba 4 B 11 O 20 F:NaF=1:5, and then charged into a Φ80 mm×80 mm lidless platinum crucible. 
         [0045]    The obtain mixture was heated to 760° C. and kept at this temperature for 10 h to obtain a mixed melting liquid. Subsequently, the temperature was reduced to 730° C. 
         [0046]    The temperature was slowly reduced to room temperature at a rate of 2.5° C./h to obtain a barium fluoroborate seed crystal by spontaneous crystallization. 
         [0047]    The obtained seed crystal was attached on a seed crystal rod for introducing seed crystal into the crystal growing furnace from the top thereof. The seed crystal was preheated above the melting liquid surface for 10 minutes, and then partly dipped into the liquid surface so that the seed crystal remelted in the mixed melting liquid. The temperature was kept for 20 minutes, and reduced quickly to a saturation temperature of 720° C. 
         [0048]    Temperature was further reduced at a rate of 2° C./day while rotating the seed crystal rod at a rate of 30 rpm. The crystal was drawn out of the surface of the mixed melting liquid when the crystal had grown to the desired size. The temperature of the crystal was reduced to room temperature at a rate of 30° C./h followed by removing the crystal from the furnace to obtain a Ba 4 B 11 O 20 F crystal with a size of 36 mm×40 mm×30 mm. 
         [0049]    In the above reaction equation, the starting material barium carbonate can be replaced by barium oxide, barium nitrate, barium oxalate, barium hydroxide or barium bicarbonate; the boronic acid can be replaced by boron oxide. 
       Example 4 
       [0050]    Preparation of a Ba 4 B 11 O 20 F compound according to the following reaction equation: 
         [0000]      7Ba(OH) 2 +BaF 2 +22H 3 BO 3 →2Ba 4 B 11 O 20 F+40H 2 O↑
 
         [0051]    Ba(OH) 2 , BaF 2  and H 3 BO 3  were put in a mortar at a molar ratio of 7/2:1/2:11, mixed and ground carefully, and then charged into a Φ100 mm×100 mm lidless corundum crucible and compacted. The crucible was placed in a muffle furnace, heated slowly to 550° C. and held at this temperature for 24 hours. After cooling to room temperature, the crucible was taken out and at which time the sample was relatively loose. Subsequently, the sample was removed and ground thoroughly again. Then the sample was charged into a crucible again and heated to 750° C. in the muffle furnace and held for 48 h. The sample was then removed and ground in a mortar to yield a Ba 4 B 11 O 20 F compound. The product was subjected to X-ray analysis, and the resultant X-ray pattern was consistent with the X-ray pattern obtained from barium fluoroborate Ba 4 B 11 O 20 F single crystal structure. 
         [0052]    The obtained Ba 4 B 11 O 20 F compound and a flux KF—PbO in which the molar ratio of KF to PbO is 1:1 were mixed at a molar ratio of 1:7, and then charged into a Φ80 mm×80 mm lidless platinum crucible. The obtain mixture was heated to 720° C. and kept at this temperature for 30 h to obtain a mixed melting liquid. Subsequently, the temperature was reduced to 705° C. 
         [0053]    The temperature was slowly reduced to room temperature at a rate of 2.5° C./h to obtain a barium fluoroborate seed crystal by spontaneous crystallization. 
         [0054]    The obtained Ba 4 B 11 O 20 F seed crystal was attached on a seed crystal rod for introducing the seed crystal into the crystal growing furnace from the top thereof. The seed crystal was preheated above the melting liquid surface for  5  minutes, and then dipped into the liquid surface so that the seed crystal remelted in the mixed melting liquid. The temperature was kept for 5 minutes, and reduced quickly to a saturation temperature of 700° C. 
         [0055]    Temperature was further reduced at a rate of 2° C./day while rotating the seed crystal rod at a rate of 50 rpm. The crystal was drawn out of the surface of the mixed melting liquid when the crystal had grown to the desired size. The temperature of the crystal was reduced to room temperature at a rate of 60° C./h followed by removing the crystal from the furnace to obtain a Ba 4 B 11 O 20 F crystal with a size of 32 mm×42 mm×16 mm. 
         [0056]    In the above reaction equation, the starting material barium hydroxide can be replaced by barium carbonate, barium nitrate, barium oxalate, barium oxide or barium bicarbonate; the boronic acid can be replaced by boron oxide. 
       Example 5 
       [0057]    Preparation of a Ba 4 B 11 O 20 F compound according to the following reaction equation: 
         [0000]      7Ba(HCO 3 ) 2 +BaF 2 +22H 3 BO 3 →2Ba 4 B 11 O 20 F+14CO 2 ↑+40H 2 O↑
 
         [0058]    Ba(HCO 3 ) 2 , BaF 2  and H 3 BO 3  were weighed at a molar ratio of 7/2:1/2:11 directly as starting materials. The weighed starting materials and a flux LiF—PbO in which the molar ratio of LiF to PbO is 3:1 were mixed at a molar ratio of 1:8, and then charged into a Φ80 mm×80 mm lidless platinum crucible. The obtain mixture was heated to 800° C. and kept at this temperature for 60 h to obtain a mixed melting liquid. Subsequently, the temperature was reduced to 760° C. 
         [0059]    The temperature was slowly reduced to room temperature at a rate of 3.5° C./h to obtain a barium fluoroborate seed crystal by spontaneous crystallization. 
         [0060]    The obtained Ba 4 B 11 O 20 F seed crystal was attached on a seed crystal rod for introducing the seed crystal into the crystal growing furnace from the top thereof. The seed crystal was preheated above the melting liquid surface for 15 minutes, and then dipped into the liquid surface so that the seed crystal remelted in the mixed melting liquid. The temperature was kept for 30 minutes, and reduced quickly to a saturation temperature of 755° C. 
         [0061]    Temperature was further reduced at a rate of 3□/day while rotating the seed crystal or the crucible at a rate of 5 rpm. The crystal was drawn out of the surface of the mixed melting liquid when the crystal had grown to the desired size. The temperature of the crystal was reduced to room temperature at a rate of 1° C./h followed by removing the crystal from the furnace to obtain a Ba 4 B 11 O 20 F crystal with a size of 35 mm×25 mm×10 mm. 
         [0062]    In the above reaction equation, the starting material barium bicarbonate can be replaced by barium carbonate, barium nitrate, barium oxalate, barium hydroxide or barium oxide; the boronic acid can be replaced by boron oxide. 
       Example 6 
       [0063]    Preparation of a Ba 4 B 11 O 20 F compound according to the following reaction equation: 
         [0000]      7BaO+BaF 2 +11B 2 O 3 →2Ba 4 B 11 O 20 F
 
         [0064]    BaO, BaF 2  and B 2 O 3  were weighed at a molar ratio of 7:1:11 directly as starting materials. The weighed starting materials and a flux NaF—PbO in which the molar ratio of NaF to PbO is 3:2 were mixed at a molar ratio of 1:6, and then charged into a Φ80 mm×80 mm lidless platinum crucible. The obtain mixture was heated to 760° C. and kept at this temperature for 60 h to obtain a mixed melting liquid. Subsequently, the temperature was reduced to 725° C. 
         [0065]    The temperature was slowly reduced to room temperature at a rate of 5° C./h to obtain a barium fluoroborate seed crystal by spontaneous crystallization. 
         [0066]    The obtained Ba 4 B 11 O 20 F seed crystal was attached on a seed crystal rod for introducing the seed crystal into the crystal growing furnace from the top thereof. The seed crystal was preheated above the melting liquid surface for 20 minutes, and then dipped into the liquid surface so that the seed crystal remelted in the mixed melting liquid. The temperature was kept for 5 minutes, and reduced quickly to a saturation temperature of 715° C. 
         [0067]    Temperature was further reduced at a rate of 3° C./day while rotating the seed crystal rod at a rate of 15 rpm. The crystal was drawn out of the surface of the mixed melting liquid when the crystal had grown to the desired size. The temperature of the crystal was reduced to room temperature at a rate of 15° C./h followed by removing the crystal from the furnace to obtain a Ba 4 B 11 O 20 F crystal with a size of 35 mm×25 mm×20 mm. 
         [0068]    In the above reaction equation, the starting material barium oxide can be replaced by barium carbonate, barium nitrate, barium oxalate, barium hydroxide or barium bicarbonate; the boron oxide can be replaced by boronic acid. 
       Example 7 
       [0069]    Preparation of a Ba 4 B  11 O 20 F compound according to the following reaction equation: 
         [0000]      7BaCO 3 +BaF 2 +11B 2 O 3 →2Ba 4 B 11 O 20   F+7 CO 2 ↑
 
         [0070]    BaCO 3 , BaF 2  and B 2 O 3  were weighed at a molar ratio of 7:1:11 directly as starting materials. The weighed starting materials and a flux NaF—H 3 BO 3  in which the molar ratio of NaF to H 3 BO 3  is 5:3 were mixed at a molar ratio of 1:5, and then charged into a Φ80 mm×80 mm lidless platinum crucible. The obtain mixture was heated to 760° C. and kept at this temperature for 80 h to obtain a mixed melting liquid. Subsequently, the temperature was reduced to 730° C. 
         [0071]    The temperature was slowly reduced to room temperature at a rate of 10° C./h to obtain a barium fluoroborate seed crystal by spontaneous crystallization. 
         [0072]    The obtained seed crystal was attached on a seed crystal rod for introducing the seed crystal into the crystal growing furnace from the top thereof. The seed crystal was preheated above the melting liquid surface for 25 minutes, and then partly dipped into the liquid surface so that the seed crystal remelted in the mixed melting liquid. The temperature was kept for 25 minutes, and reduced quickly to a saturation temperature of 720° C. 
         [0073]    Temperature was further reduced at a rate of 5° C./day while rotating the seed crystal rod at a rate of 30 rpm. The crystal was drawn out of the surface of the mixed melting liquid when the crystal had grown to the desired size. The temperature of the crystal was reduced to room temperature at a rate of 35° C./h followed by removing the crystal from the furnace to obtain a Ba 4 B 11 O 20 F crystal with a size of 26 mm×35 mm×20 mm. 
         [0074]    In the above reaction equation, the starting material barium carbonate can be replaced by barium bicarbonate, barium nitrate, barium oxalate, barium hydroxide or barium oxide; the boron oxide can be replaced by boronic acid. 
       Example 8 
       [0075]    Preparation of a Ba 4 B 11 O 20 F compound according to the following reaction equation: 
         [0000]      7Ba(NO 3 ) 2 +BaF 2 +11B 2 O 3 →2Ba 4 B 11 O 20   F+13 NO 2 ↑+NO↑+4O 2 ↑
 
         [0076]    Ba(NO 3 ) 2 , BaF 2  and B 2 O 3  were put in a mortar at a molar ratio of  7 : 1 : 11 , mixed and ground carefully, and then charged into a Φ100 mm×100 mm lidless corundum crucible and compacted. The crucible was placed in a muffle furnace, heated slowly to 550° C. and held at this temperature for 24 hours. After cooling, the crucible was taken out and at which time the sample was relatively loose. Subsequently, the sample was removed and ground thoroughly again. Then the sample was charged into a crucible again and heated to 750° C. in the muffle furnace and held for 48 h. The sample was then removed and ground in a mortar to yield a Ba 4   13   11 O 20 F compound. The product was subjected to X-ray analysis, and the resultant X-ray pattern was consistent with the X-ray pattern obtained from barium fluoroborate Ba 4 B 11 O 20 F single crystal structure. 
         [0077]    The obtained Ba 4 B 11 O 20 F compound and a flux KF—PbO in which the molar ratio of KF to PbO is 2:1 were mixed at a molar ratio of 1:4, and then charged into a Φ80 mm×80 mm lidless platinum crucible. The obtain mixture was heated to 850° C. and kept at this temperature for 70 h to obtain a mixed melting liquid. Subsequently, the temperature was reduced to 725° C. 
         [0078]    The temperature was slowly reduced to room temperature at a rate of 4.0° C./h to obtain a barium fluoroborate seed crystal by spontaneous crystallization. 
         [0079]    The obtained Ba 4 B 11 O 20 F seed crystal was attached on a seed crystal rod for introducing the seed crystal into the crystal growing furnace from the top thereof. The seed crystal was preheated above the melting liquid surface for 8 minutes, and then dipped into the liquid surface so that the seed crystal remelted in the mixed melting liquid. The temperature was kept for 8 minutes, and reduced quickly to a saturation temperature of 720° C. 
         [0080]    Temperature was further reduced at a rate of 0.8° C./day while rotating the seed crystal rod at a rate of 10 rpm. The crystal was drawn out of the surface of the mixed melting liquid when the crystal had grown to the desired size. The temperature of the crystal was reduced to room temperature at a rate of 5° C./h followed by removing the sample from the furnace to obtain a Ba 4 B 11 O 20 F crystal with a size of 16 mm×25 mm×18 mm. 
         [0081]    In the above reaction equation, the starting material barium nitrate can be replaced by barium carbonate, barium bicarbonate, barium oxalate, barium hydroxide or barium oxide; the boron oxide can be replaced by boronic acid. 
       Example 9 
       [0082]    Preparation of a Ba 4 B 11 O 20 F compound according to the following reaction equation: 
         [0000]      7Ba(OH) 2 +BaF 2 +11B 2 O 3 →2Ba 4 B 11 O 20 F+7H 2 O↑
 
         [0083]    Ba(OH) 2 , BaF 2  and B 2 O 3  were put in a mortar at a molar ratio of 7:1:11, mixed and ground carefully, and then charged into a Φ100 mm×100 mm lidless corundum crucible and compacted. The crucible was placed in a muffle furnace, heated slowly to 550° Cand held at this temperature for 24 hours. After cooling, the crucible was taken out and at which time the sample was relatively loose. Subsequently, the sample was removed and ground thoroughly again. Then the sample was charged into a crucible again and heated to 750° C. in the muffle furnace and held for 48 h. The sample was then removed and ground in a mortar to yield a Ba 4 B 11 O 20 F compound. The product was subjected to X-ray analysis, and the resultant X-ray pattern was consistent with the X-ray pattern obtained from barium fluoroborate Ba 4 B 11 O 20 F single crystal structure. 
         [0084]    The obtained Ba 4 B 11 O 20 F compound and a flux LiF—PbO in which the molar ratio of LiF to PbO is 5:1 were mixed at a molar ratio of 1:10, and then charged into a Φ80 mm×80 mm lidless platinum crucible. The obtain mixture was heated to 850° C. and kept at this temperature for 45 h to obtain a mixed melting liquid. Subsequently, the temperature was reduced to 750° C. 
         [0085]    The temperature was slowly reduced to room temperature at a rate of 6.5° C./h to obtain a barium fluoroborate seed crystal by spontaneous crystallization. 
         [0086]    The obtained Ba 4 B 11 O 20 F seed crystal was attached on a seed crystal rod for introducing the seed crystal into the crystal growing furnace from the top thereof. The seed crystal was preheated above the melting liquid surface for 15 minutes, and then dipped into the liquid surface so that the seed crystal remelted in the mixed melting liquid. The temperature was kept for 15 minutes, and reduced quickly to a saturation temperature of 740° C. 
         [0087]    Temperature was further reduced at a rate of 2° C./day while rotating the seed crystal rod at a rate of 60 rpm. The crystal was drawn out of the surface of the mixed melting liquid when the crystal had grown to the desired size. The temperature of the crystal was reduced to room temperature at a rate of 80° C./h followed by removing the sample from the furnace to obtain a Ba 4 B 11 O 20 F crystal with a size of 22 mm×32 mm×16 mm. 
         [0088]    In the above reaction equation, the starting material barium hydroxide can be replaced by barium carbonate, barium bicarbonate, barium oxalate, barium nitrate or barium oxide; the boron oxide can be replaced by boronic acid. 
       Example 10 
       [0089]    Preparation of a Ba 4 B 11 O 20 F compound according to the following reaction equation: 
         [0000]      7Ba(HCO 3 ) 2 +BaF 2 +11B 2 O 3 →2Ba 4 B 11 O 20 F+14CO 2 ↑+7H 2 O↑
 
         [0090]    Ba(HCO 3 ) 2 , BaF 2  and B 2 O 3  were put in a mortar at a molar ratio of  7 : 1 : 11 , mixed and ground carefully, and then charged into a Φ100 mm×100 mm lidless corundum crucible and compacted. The crucible was placed in a muffle furnace, heated slowly to 550° C. and held at this temperature for 24 hours. After cooling, the crucible was taken out and at which time the sample was relatively loose. Subsequently, the sample was removed and ground thoroughly again. Then the sample was charged into a crucible again and heated to 750° C. in the muffle furnace and held for 48 h. The sample was then removed and ground in a mortar to yield a Ba 4 B 11 O 20 F compound. The product was subjected to X-ray analysis, and the resultant X-ray pattern was consistent with the X-ray pattern obtained from barium fluoroborate Ba 4 B 11 O 20 F single crystal structure. The obtained Ba 4 B 11  O 20 F compound and a flux NaF—PbO in which the molar ratio of NaF to PbO is 2:3 were mixed at a molar ratio of 1:6, and then charged into a Φ80 mm×80 mm lidless platinum crucible. The obtain mixture was heated to 950° C. and kept at this temperature for 70 h to obtain a mixed melting liquid. Subsequently, the temperature was reduced to 750° C. 
         [0091]    The temperature was slowly reduced to room temperature at a rate of 4.0° C./h to obtain a barium fluoroborate seed crystal by spontaneous crystallization. 
         [0092]    The obtained Ba 4 B 11 O 20 F seed crystal was attached on a seed crystal rod for introducing the seed crystal into the crystal growing furnace from the top thereof. The seed crystal was preheated above the melting liquid surface for 20 minutes, and then dipped into the liquid surface so that the seed crystal remelted in the mixed melting liquid. The temperature was kept for 25 minutes, and reduced quickly to a saturation temperature of 745° C. 
         [0093]    Temperature was further reduced at a rate of 3° C./day while rotating the seed crystal rod at a rate of 50 rpm. The crystal was drawn out of the surface of the mixed melting liquid when the crystal had grown to the desired size. The temperature of the crystal was reduced to room temperature at a rate of 70° C./h followed by removing the sample from the furnace to obtain a Ba 4 B 11 O 20 F crystal with a size of 25 mm×25 mm×18 mm. 
         [0094]    In the above reaction equation, the starting material barium bicarbonate can be replaced by barium carbonate, barium hydroxide, barium oxalate, barium nitrate or barium oxide; the boron oxide can be replaced by boronic acid. 
       Example 11 
       [0095]    Preparation of a Ba 4 B 11 O 20 F compound according to the following reaction equation: 
         [0000]      7Ba(HCO 3 ) 2 +BaF 2 +11B 2 O 3 →2Ba 4 B 11 O 20 F+14CO 2 ↑+7H 2 O↑
 
         [0096]    Ba(HCO 3 ) 2 , BaF 2  and B 2 O 3  were put in a mortar at a molar ratio of  7 : 1 : 11 , mixed and ground carefully, and then charged into a Φ100 mm×100 mm lidless corundum crucible and compacted. The crucible was placed in a muffle furnace, heated slowly to 550° C. and held at this temperature for 24 hours. After cooling, the crucible was taken out and at which time the sample was relatively loose. Subsequently, the sample was removed and ground thoroughly again. Then the sample was charged into a crucible again and heated to 750° C. in the muffle furnace and held for 48 h. The sample was then removed and ground in a mortar to yield a Ba 4 B 11 O 20 F compound. The product was subjected to X-ray analysis, and the resultant X-ray pattern was consistent with the X-ray pattern obtained from barium fluoroborate Ba 4 B 11 O 20 F single crystal structure. 
         [0097]    The obtained Ba 4 B 11 O 20 F compound and a flux H 3 BO 3 —PbO in which the molar ratio of H 3 BO 3  to PbO is 5:4 were mixed at a molar ratio of 1:3, and then charged into a Φ180 mm×80 mm lidless platinum crucible. The obtain mixture was heated to 800° C. and kept at this temperature for 50 h to obtain a mixed melting liquid. Subsequently, the temperature was reduced to 720° C. 
         [0098]    The temperature was slowly reduced to room temperature at a rate of 4.0° C./h to obtain a barium fluoroborate seed crystal by spontaneous crystallization. 
         [0099]    The obtained Ba 4 B 11 O 20 F seed crystal was attached on a seed crystal rod for introducing the seed crystal into the crystal growing furnace from the top thereof. The seed crystal was preheated above the melting liquid surface for 20 minutes, and then dipped into the liquid surface so that the seed crystal remelted in the mixed melting liquid. The temperature was kept for 25 minutes, and reduced quickly to a saturation temperature of 715° C. 
         [0100]    Temperature was further reduced at a rate of 2° C./day while rotating the seed crystal rod at a rate of 28 rpm. The crystal was drawn out of the surface of the mixed melting liquid when the crystal had grown to the desired size. The temperature of the crystal was reduced to room temperature at a rate of 25° C./h followed by removing the sample from the furnace to obtain a Ba 4 B 11 O 20 F crystal with a size of 31 mm×26 mm×18 mm. 
         [0101]    In the above reaction equation, the starting material barium bicarbonate can be replaced by barium carbonate, barium hydroxide, barium oxalate, barium nitrate or barium oxide; the boron oxide can be replaced by boronic acid. 
       Example 12 
       [0102]    Preparation of a Ba 4 B 11 O 20 F compound according to the following reaction equation: 
         [0000]      7BaC 2 O 4 +BaF 2 +11B 2 O 3 →2Ba 4 B 11 O 20 F+7CO 2 ↑+7CO
 
         [0103]    BaC 2 O 4 , BaF 2  and B 2 O 3  were weighed at a molar ratio of 7:1:11 directly as starting materials. The weighed starting materials and a flux PbO—H 3 BO 3  in which the molar ratio of PbO to H 3 BO 3  is 5:3 were mixed at a molar ratio of 1:5, and then charged into a Φ80 mm×80 mm lidless platinum crucible. The obtain mixture was heated to 720° C. and kept at this temperature for 80 h to obtain a mixed melting liquid. Subsequently, the temperature was reduced to 690° C. 
         [0104]    The temperature was slowly reduced to room temperature at a rate of 2° C./h to obtain a barium fluoroborate seed crystal by spontaneous crystallization. 
         [0105]    The obtained seed crystal was attached on a seed crystal rod for introducing the seed crystal into the crystal growing furnace from the top thereof. The seed crystal was preheated above the melting liquid surface for 25 minutes, and then partly dipped into the liquid surface so that the seed crystal remelted in the mixed melting liquid. The temperature was kept for 25 minutes, and reduced quickly to a saturation temperature 680° C. 
         [0106]    Temperature was further reduced at a rate of 5° C./day while rotating the seed crystal rod at a rate of 30 rpm. The crystal was drawn out of the surface of the mixed melting liquid when the crystal had grown to the desired size. The temperature of the crystal was reduced to room temperature at a rate of 20° C./h followed by removing the crystal from the furnace to obtain a Ba 4 B 11 O 20 F crystal with a size of 25 mm×22 mm×20 mm. 
         [0107]    In the above reaction equation, the starting material barium oxalate can be replaced by barium bicarbonate, barium nitrate, barium carbonate, barium hydroxide or barium oxide; the boron oxide can be replaced by boronic acid. 
       Example 13 
       [0108]    A Ba 4 B 11 O 20 F crystal obtained in any one of Examples 1-12 was processed into a second harmonic device with a size of 5 mm×5 mm×6 mm according to the phase-matching orientation, and disposed at position  3  as shown in  FIG. 2 . At room temperature, when Q-switched Nd:YAG laser generator was used as the light source and the incident light had a wavelength of 1064 nm, an infrared light beam  2  with a wavelength of 1064 nm generated by the Q-switched Nd:YAG laser generator  1  irradiated into the Ba 4 B 11 O 20 F single crystal  3 , resulting in a green second harmonic light with a wavelength of 532 nm which has an output intensity of 10 times that of KDP under the same condition. The output light beam  4  contained an infrared light with a wavelength of 1064 nm and a green light with a wavelength of 532 nm, and it was filtered by filter  5  to obtain a green laser with a wavelength of 532 nm.