Sr2Ga2GeS7 compound, Sr2Ga2GeS7 nonlinear optical crystal and preparation method and application of Sr2Ga2GeS7 nonlinear optical crystal

Abstract

The invention discloses a Sr2Ga2GeS7 compound and a Sr2Ga2GeS7 nonlinear optical crystal as well as a preparation method and application of the Sr2Ga2GeS7 nonlinear optical crystal, and belongs to the technical field of optical materials. According to the preparation method, the Sr2Ga2GeS7 compound is obtained by adopting a high-temperature solid-phase reaction; the Sr2Ga2GeS7 nonlinear optical crystal is grown by adopting a high-temperature melt spontaneous crystallization method or a Bridgman-Stockbarger method, and the obtained Sr2Ga2GeS7 crystal takes [GaS4] < 5-> and [GeS4] < 4-> as main structural elements, has a wider infrared transmission range, a larger band gap value (3.02 eV), a higher laser damage resistance threshold value, higher hardness, good mechanical properties, difficulty in fragmentation and deliquescence, and easiness in processing and storage; the method has a good application prospect in the aspect of preparing infrared nonlinear optical devices.

Description

Technical Field

[0001] The present invention relates to the technical field of optical materials, and more specifically to a Sr2Ga2GeS7 compound and a Sr2Ga2GeS7 nonlinear optical crystal, and a preparation method and use thereof. Background Art

[0002] Nonlinear optical crystals are crystals that show more than secondary nonlinear optical effects for strong laser electric fields. The so-called nonlinear optical effects refer to frequency doubling, sum frequency, difference frequency, parametric amplification and other effects. Only crystals without a symmetry center can have nonlinear optical effects. Using the nonlinear optical effects of crystals, nonlinear optical devices such as second harmonic generators, up and down frequency converters, and optical parametric oscillators can be made. The laser generated by the laser can be converted in frequency through nonlinear optical devices to obtain lasers with more useful wavelengths, making the laser more widely used.

[0003] According to the different application bands of materials, they can be divided into three categories: ultraviolet region, visible and near-infrared region, and mid-infrared region nonlinear optical materials. Nonlinear optical crystal materials in the visible and ultraviolet regions can already meet the requirements of practical applications; for example, in the double frequency (532nm) crystals, the main practical ones are KTP (KTiOPO4), BBO (β-BaB2O4), and LBO (LiB3O5) crystals; in the triple frequency (355nm) crystals, BBO, LBO, and CBO (CsB3O5) are available. The development of nonlinear crystals in the infrared band is relatively slow. Most infrared nonlinear optical crystals are ternary or quaternary chalcogenides, among which AgGaS2 (AGS) and AgGaSe2 (AGSe) have become relatively mature commercial crystals. However, they still have defects such as small band gap, low laser damage threshold, and two-photon absorption problems for near-infrared lasers (such as Nd:YAG 1064nm). In addition, AGS and AGSe have large anisotropic thermal expansion, making it difficult to grow high-quality large-size crystals.

[0004] Since the infrared band contains two important atmospheric windows (3-5μm and 8-12μm), it has important applications in environmental monitoring, laser guidance, radar, communications, medical treatment and other fields. Therefore, finding excellent new infrared nonlinear optical crystal materials with balanced performance has become one of the difficulties and frontier directions in the current research field of nonlinear optical materials. Summary of the invention

[0005] In view of the above problems, the present invention provides a Sr2Ga2GeS7 compound and a Sr2Ga2GeS7 nonlinear optical crystal as well as a preparation method and use thereof.

[0006] In order to achieve the above object, the present invention adopts the following technical solution:

[0007] A chemical compound with the formula Sr2Ga2GeS7.

[0008] Another object of the present invention is to provide a method for preparing a Sr2Ga2GeS7 compound, the steps of which are as follows:

[0009] The Sr-containing substance, Ga-containing substance, Ge-containing substance and S single substance are prepared and mixed uniformly in a molar ratio of Sr:Ga:Ge:S=2:2:1:7, and then heated to 900-1000°C for solid phase reaction to obtain a compound with a chemical formula of Sr2Ga2GeS7; the Sr-containing substance is strontium single substance or strontium sulfide; the Ga-containing substance is gallium single substance or gallium trisulfide; the Ge-containing substance is germanium single substance or germanium disulfide.

[0010] Preferably, the step of heating for solid phase reaction is: grinding the above ingredients, putting them into a graphite crucible for mixing, and then putting them into a quartz tube, evacuating the quartz tube to 10 -3 Pa and melt-package it, put it into a muffle furnace, heat it to 600°C at a rate of 20-25°C / h, and keep it at this temperature for 10 hours; continue to heat it to 900-1000°C at a rate of 20-25°C / h, and keep it at this temperature for 80 hours; after cooling it to room temperature at a rate of 4-5°C / h, take out the sample, crush it and grind it to obtain a powdered Sr2Ga2GeS7 compound.

[0011] The Sr2Ga2GeS7 compound can be prepared according to the following chemical reaction formula:

[0012] (1)2SrS+Ga2S3+GeS2=Sr2Ga2GeS7;

[0013] (2)2SrS+Ga2S3+Ge+2S=Sr2Ga2GeS7;

[0014] (3)2SrS+2Ga+GeS2+3S=Sr2Ga2GeS7;

[0015] (4)2SrS+2Ga+Ge+5S=Sr2Ga2GeS7;

[0016] (5)2Sr+2Ga+Ge+7S=Sr2Ga2GeS7.

[0017] Another object of the present invention is to provide a Sr2Ga2GeS7 nonlinear optical crystal, which has no symmetry center, belongs to the tetragonal system, has a space group of P-421m, and has a unit cell parameter of c = 6.3406(2), α = β = γ = 90°, Z = 2, unit cell volume

[0018] Another object of the present invention is to provide a method for preparing Sr2Ga2GeS7 nonlinear optical crystals, which adopts a high-temperature melt spontaneous crystallization method to grow Sr2Ga2GeS7 nonlinear optical crystals. The specific steps are: heating the powdered Sr2Ga2GeS7 compound to melt to obtain a high-temperature melt and maintaining it for 96-100 hours, then cooling it to room temperature at a cooling rate of 2-3°C / h to obtain a light yellow Sr2Ga2GeS7 crystal.

[0019] Another object of the present invention is to provide a method for preparing Sr2Ga2GeS7 nonlinear optical crystals, which adopts a crucible descent method to grow Sr2Ga2GeS7 nonlinear optical crystals. The specific steps are: placing a powdered Sr2Ga2GeS7 compound into a crucible descending furnace growth device, slowly heating up until the powder melts, and after the powder is completely melted, the crystal growth device vertically descends at a speed of 0.1-2.5mm / h, and the Sr2Ga2GeS7 nonlinear optical crystal grows during the descent of the crystal growth device, and the growth period is 20-30 days.

[0020] Preferably, the crucible descent method for growing Sr2Ga2GeS7 nonlinear optical crystals also includes post-processing of the Sr2Ga2GeS7 nonlinear optical crystals: after the crystal growth is completed, the crystal is still left in the growth furnace for annealing and cooled to room temperature at a rate not greater than 5°C / h.

[0021] The powdered Sr2Ga2GeS7 compound is prepared by the above method.

[0022] Another object of the present invention is to provide the use of the above-mentioned Sr2Ga2GeS7 nonlinear optical crystal in the preparation of mid- and far-infrared laser frequency doubling devices, infrared communication devices or infrared laser guidance devices.

[0023] Through the above technical scheme, it can be known that the present invention discloses a Sr2Ga2GeS7 compound and a Sr2Ga2GeS7 nonlinear optical crystal and a preparation method and use. The method adopts a high temperature solid phase reaction to obtain a Sr2Ga2GeS7 compound; the Sr2Ga2GeS7 nonlinear optical crystal is grown by a high temperature melt spontaneous crystallization method or a crucible descent method, and the obtained Sr2Ga2GeS7 crystal is [GaS4] 5- and [GeS4] 4- It is the main structural element, has a wide infrared transmittance range, a large band gap value (3.02eV), a high laser damage threshold, a large hardness, good mechanical properties, is not easy to break, is not easy to deliquesce, is easy to process and preserve, and has good application prospects in the preparation of infrared nonlinear optical devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 : A comparison of the X-ray diffraction spectrum of the powdered Sr2Ga2GeS7 compound and the X-ray spectrum of the theoretical Sr2Ga2GeS7 obtained by single crystal structure analysis;

[0025] Figure 2 :Schematic diagram of the structure of Sr2Ga2GeS7 nonlinear optical crystal;

[0026] Figure 3 :Band gap diagram of Sr2Ga2GeS7 nonlinear optical crystal;

[0027] Figure 4 :Raman spectrum characterization of Sr2Ga2GeS7 nonlinear optical crystal;

[0028] Figure 5 : is a working principle diagram of a typical nonlinear optical device made of the Sr2Ga2GeS7 nonlinear optical crystal of the present invention, wherein 1: laser, 2: emitted light beam, 3: Sr2Ga2GeS7 nonlinear optical crystal after crystal post-processing and optical processing, 4: generated output light beam, 5: filter;

[0029] Figure 6 : Comparison of the relationship between the double-frequency intensity and sample particle size of Sr2Ga2GeS7 nonlinear optical crystal and AgGaS2 crystal. DETAILED DESCRIPTION

[0030] The technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0031] The reagents involved in the embodiments of the present invention are all purchased from commercial channels, and the methods not mentioned are conventional experimental methods and will not be described in detail here.

[0032] Example 1 Preparation of powdered Sr2Ga2GeS7 compound:

[0033] The Sr2Ga2GeS7 compound is prepared by a high temperature solid phase method using the reaction formula 2SrS+Ga2S3+GeS2=Sr2Ga2GeS7, wherein the SrS is 0.2393 g, the Ga2S3 is 0.2356 g, and the GeS2 is 0.1367 g, that is, SrS:Ga2S3:GeS2=2mmol:1mmol:1mmol;

[0034] The specific operation steps are as follows: weigh the reagents separately in a glove box filled with inert gas argon and with a water and oxygen content of 0.01 ppm, put them into a mortar, mix and grind them carefully, move them into a graphite crucible, and then put them into a quartz tube (Φ10 mm×20 cm), and evacuate the quartz tube to 10 -3 After Pa, the quartz tube was melted and packaged with a hydrogen-oxygen flame, placed in a muffle furnace, heated to 600°C at 20°C / h and kept warm for 10h, then heated to 1000°C at 20°C / h and kept warm for 80h, and then slowly cooled to room temperature at 4°C / h; after taking out the sample from the graphite crucible, it was placed in a mortar and crushed to obtain a powdered Sr2Ga2GeS7 compound product.

[0035] Example 2 Preparation of powdered Sr2Ga2GeS7 compound:

[0036] The Sr2Ga2GeS7 compound is prepared by a high temperature solid phase method using the reaction formula 2SrS+Ga2S3+Ge+2S=Sr2Ga2GeS7, wherein the SrS is 0.2393 g, the Ga2S3 is 0.2356 g, the Ge is 0.0726 g, and the S is 0.0641 g, that is, SrS:Ga2S3:Ge:S=2mmol:1mmol:1mmol:2mmol;

[0037] The specific operation steps are as follows: weigh the reagents separately in a glove box filled with inert gas argon and with a water and oxygen content of 0.01 ppm, put them into a mortar, mix and grind them carefully, move them into a graphite crucible, and then put them into a quartz tube (Φ10 mm×20 cm), and evacuate the quartz tube to 10 -3 After Pa, the quartz tube was melted and packaged with a hydrogen-oxygen flame, placed in a muffle furnace, heated to 600°C at 22°C / h and kept warm for 10h, then heated to 950°C at 25°C / h and kept warm for 80h, and then slowly cooled to room temperature at 5°C / h; after taking out the sample from the graphite crucible, it was placed in a mortar and crushed to obtain a powdered Sr2Ga2GeS7 compound product.

[0038] Example 3 Preparation of powdered Sr2Ga2GeS7 compound:

[0039] The Sr2Ga2GeS7 compound is prepared by a high temperature solid phase method using the reaction formula 2SrS+2Ga+GeS2+3S=Sr2Ga2GeS7, wherein the SrS is 0.2393g, the Ga is 0.0697g, the GeS2 is 0.1367g, and the S is 0.096g, that is, SrS:Ga:GeS2:S=2mmol:2mmol:1mmol:3mmol;

[0040] The specific operation steps are as follows: weigh the reagents separately in a glove box filled with inert gas argon and with a water and oxygen content of 0.01 ppm, put them into a mortar, mix and grind them carefully, move them into a graphite crucible, and then put them into a quartz tube (Φ10 mm×20 cm), and evacuate the quartz tube to 10 -3 After Pa, the quartz tube was melted and packaged with a hydrogen-oxygen flame, placed in a muffle furnace, heated to 600°C at 20°C / h and kept warm for 10 hours, then heated to 950°C at 25°C / h and kept warm for 80 hours, and then slowly cooled to room temperature at 5°C / h; after taking out the sample from the graphite crucible, it was placed in a mortar and crushed to obtain a powdered Sr2Ga2GeS7 compound product.

[0041] Example 4 Preparation of powdered Sr2Ga2GeS7 compound:

[0042] The Sr2Ga2GeS7 compound is prepared by a high temperature solid phase method using the reaction formula 2SrS+2Ga+Ge+5S=Sr2Ga2GeS7, wherein the SrS is 0.2393 g, the Ga is 0.1394 g, the Ge is 0.0726 g, and the S is 0.1603 g, that is, SrS:Ga:Ge:S=2mmol:2mmol:1mmol:5mmol;

[0043] The specific operation steps are as follows: weigh the reagents separately in a glove box filled with inert gas argon and with a water and oxygen content of 0.01 ppm, put them into a mortar, mix and grind them carefully, move them into a graphite crucible, and then put them into a quartz tube (Φ10 mm×20 cm), and evacuate the quartz tube to 10 -3 After Pa, the quartz tube was melted and packaged with a hydrogen-oxygen flame, placed in a muffle furnace, heated to 600°C at 25°C / h and kept warm for 10 hours, then heated to 900°C at 25°C / h and kept warm for 80 hours, and then slowly cooled to room temperature at 4°C / h; after taking out the sample from the graphite crucible, it was placed in a mortar and crushed to obtain a powdered Sr2Ga2GeS7 compound product.

[0044] Example 5 Preparation of powdered Sr2Ga2GeS7 compound:

[0045] The Sr2Ga2GeS7 compound is prepared by a high temperature solid phase method using the reaction formula 2Sr+2Ga+Ge+7S=Sr2Ga2GeS7, wherein the Sr is 0.1752 g, the Ga is 0.1394 g, the Ge is 0.0726 g, and the S is 0.2244 g, that is, Sr:Ga:Ge:S=2mmol:2mmol:1mmol:7mmol;

[0046] The specific operation steps are as follows: weigh the reagents separately in a glove box filled with inert gas argon and with a water and oxygen content of 0.01 ppm, put them into a mortar, mix and grind them carefully, move them into a graphite crucible, and then put them into a quartz tube (Φ10 mm×20 cm), and evacuate the quartz tube to 10 -3 After Pa, the quartz tube was melted and packaged with a hydrogen-oxygen flame, placed in a muffle furnace, heated to 600°C at 20°C / h and kept warm for 10 hours, then heated to 900°C at 25°C / h and kept warm for 80 hours, and then slowly cooled to room temperature at 4°C / h; after taking out the sample from the graphite crucible, it was placed in a mortar and crushed to obtain a powdered Sr2Ga2GeS7 compound product.

[0047] The powdered Sr2Ga2GeS7 compound obtained in the above examples 1-5 was subjected to powder X-ray diffraction analysis, and the obtained X-ray diffraction spectrum was compared with the X-ray spectrum of the theoretical Sr2Ga2GeS7 obtained by single crystal structure analysis. Figure 1 As shown. Figure 1 It can be seen that the experimental values ​​are consistent with the theoretical values ​​of the theoretical X-ray spectrum of Sr2Ga2GeS7, indicating that the powder sample is a pure phase of strontium gallium germanium sulfur.

[0048] Example 6: Preparation of Sr2Ga2GeS7 crystals by high temperature melt spontaneous crystallization method:

[0049] The Sr2Ga2GeS7 powder obtained in Example 1-5 was placed in a Φ10mm×25mm quartz glass tube and evacuated to 10 -3 Pa, encapsulated with oxyhydrogen flame, placed in a tubular growth furnace, slowly raised to 1000°C, kept constant for 96-100 hours, slowly cooled to room temperature at a rate of 2-3°C / h, and closed the tubular growth furnace; after the quartz tube is cooled, cut it open to obtain a light yellow transparent Sr2Ga2GeS7 crystal.

[0050] Example 7: Preparation of Sr2Ga2GeS7 crystals by the crucible descent method:

[0051] The Sr2Ga2GeS7 powder obtained in Example 1-5 was placed in a Φ10mm×25mm quartz glass tube and evacuated to 10 -3 Pa, encapsulated with oxyhydrogen flame, placed in a tubular growth furnace, slowly heated to 1000℃ and kept warm for 2-3 days to completely melt the raw materials, then the growth device was vertically lowered at a speed of 0.1-2.5mm / h. After the crystal growth was completed, the crucible was lowered into the furnace and cooled to room temperature for 50h to obtain a light yellow transparent Sr2Ga2GeS7 crystal.

[0052] Example 8 Performance Test of Sr2Ga2GeS7 Crystal

[0053] 1. The Sr2Ga2GeS7 crystals prepared in the above-mentioned Examples 6-7 were tested and found to be tetragonal in structure, with a space group of P-421m and unit cell parameters of c = 6.3406(2), α = β = γ = 90°, Z = 2, unit cell volume It has frequency doubling effect. See the schematic diagram for the structure. Figure 2 .

[0054] Depend on Figure 2 It can be seen that the valences of Sr, Ga, Ge and S atoms in the Sr2Ga2GeS7 nonlinear optical crystal structure are +2, +3, +4 and -2 respectively; Ga and Ge atoms are both four-coordinated, and coordinate with four S atoms to form [GaS4] 5- and [GeS4] 4- Tetrahedron, [GaS4] 5- and [GeS4] 4- The tetrahedrons form a Cairo pentagonal layered structure by sharing S atoms, and the eight-coordinated Sr atoms fill the space between the layers.

[0055] 2. The band gap of the obtained Sr2Ga2GeS7 crystal was measured by UV-Vis-NIR diffuse reflectance spectrometer. The results are as follows: Figure 3 As shown. Figure 3 It can be seen that the band gap of Sr2Ga2GeS7 nonlinear optical crystal is 3.02eV. Compared with the band gap of silver gallium sulfide (AgGaS2) crystal of 2.64eV, the crystal obtained by the present invention has a wider band gap, indicating that its ability to resist laser damage is relatively strong and can be applied to high-energy laser systems.

[0056] 3. Raman spectroscopy was performed on the obtained Sr2Ga2GeS7 crystal, and the results were as follows Figure 4 As shown. Figure 4 It can be seen that in the range of 500-4000cm -1 There is no obvious absorption peak in the wavenumber range, indicating that the crystal has a wide infrared transmittance range (2.5-20μm), covering the two important atmospheric windows of 3-5μm and 8-12μm, and is a potential infrared nonlinear optical material with excellent performance.

[0057] Example 9

[0058] Figure 5This is a working principle diagram of a typical nonlinear optical device made of the Sr2Ga2GeS7 nonlinear optical crystal of the present invention, wherein 1 is a laser, 2 is an emitted light beam, 3 is the Sr2Ga2GeS7 nonlinear optical crystal after crystal post-processing and optical processing, 4 is the generated output laser beam, and 5 is a filter.

[0059] The device made of the Sr2Ga2GeS7 nonlinear optical crystal of the present invention can be a frequency doubling generator, an up / down frequency converter, an optical parametric oscillator, etc. The laser 1 can be a holmium-doped yttrium aluminum garnet (Ho:YAG) laser or other lasers.

[0060] Taking the frequency doubling device using the Q-switched Ho:Tm:Cr:YAG laser as the light source as an example, according to Figure 5 In the optical system shown, a Sr2Ga2GeS7 nonlinear optical crystal is placed at the position shown in 3. At room temperature, a Q-switched (Ho:Tm:Cr:YAG) laser 1 is used to emit an infrared beam with a wavelength of 2090nm as a light source, which is injected into the SrGaGeS nonlinear optical crystal through a fully converging lens 2 to generate a doubled frequency light with a wavelength of 1045nm. The output beam 4 contains a fundamental frequency light with a wavelength of 2090nm and a doubled frequency light of 1045nm. After passing through a filter 5 (function: filtering out the fundamental frequency light component and only allowing the doubled frequency light to pass), the doubled frequency light with a wavelength of 1045nm is obtained.

[0061] The Kurtz-Perry method was used to measure the relationship between the frequency doubling intensity of Sr2Ga2GeS7 nonlinear optical crystal and the sample particle size, with AgGaS2 crystals of the same particle size (particle size range 200-250μm) as a control. The results are as follows: Figure 6 As shown. Figure 6 It can be seen that under the same particle size conditions, the intensity of the nonlinear optical crystal SrGaGeS output is 1.3 times that of the AgGaS2 crystal.

[0062] The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referenced to each other.

[0063] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A compound with the chemical formula Sr2Ga2GeS7.

2. A method for preparing the Sr2Ga2GeS7 compound according to claim 1, characterized in that: Here are the steps: The Sr-containing substance, Ga-containing substance, Ge-containing substance and S single substance are prepared and mixed uniformly in a molar ratio of Sr:Ga:Ge:S=2:2:1:7, and then heated to 900-1000°C for solid phase reaction to obtain a compound with a chemical formula of Sr2Ga2GeS7; the Sr-containing substance is strontium single substance or strontium sulfide; the Ga-containing substance is gallium single substance or gallium trisulfide; the Ge-containing substance is germanium single substance or germanium disulfide.

3. The method for preparing the Sr2Ga2GeS7 compound according to claim 2, characterized in that: The step of heating for solid phase reaction is: grinding the above ingredients, putting them into a graphite crucible for mixing, and then putting them into a quartz tube, evacuating the quartz tube to 10 -3 Pa and melt-package it, put it into a muffle furnace, heat it to 600°C at a rate of 20-25°C / h, and keep it at this temperature for 10 hours; continue to heat it to 900-1000°C at a rate of 20-25°C / h, and keep it at this temperature for 80 hours; after cooling it to room temperature at a rate of 4-5°C / h, take out the sample, crush it and grind it to obtain a powdered Sr2Ga2GeS7 compound.

4. A Sr2Ga2GeS7 nonlinear optical crystal, characterized in that: The Sr2Ga2GeS7 nonlinear optical crystal has no symmetry center, belongs to the tetragonal system, has a space group of P-421m, and has a unit cell parameter of c = 6.3406(2), α = β = γ = 90°, Z = 2, unit cell volume 5. A method for preparing the Sr2Ga2GeS7 nonlinear optical crystal according to claim 4, characterized in that: The Sr2Ga2GeS7 nonlinear optical crystal is grown by a high-temperature melt spontaneous crystallization method. The specific steps are as follows: the powdered Sr2Ga2GeS7 compound is heated to melt to obtain a high-temperature melt and maintained for 96-100 hours, and then cooled to room temperature at a cooling rate of 2-3°C / h to obtain a light yellow Sr2Ga2GeS7 crystal.

6. A method for preparing the Sr2Ga2GeS7 nonlinear optical crystal according to claim 4, characterized in that: The crucible descent method is used to grow Sr2Ga2GeS7 nonlinear optical crystals. The specific steps are: putting powdered Sr2Ga2GeS7 compound into a crucible descending furnace growth device, slowly heating up until the powder melts, and after the powder is completely melted, the crystal growth device vertically descends at a speed of 0.1-2.5mm / h. During the descent of the crystal growth device, Sr2Ga2GeS7 nonlinear optical crystals are grown, and the growth cycle is 20-30 days.

7. The method for preparing the Sr2Ga2GeS7 nonlinear optical crystal according to claim 6, characterized in that: The method also includes post-processing of the Sr2Ga2GeS7 nonlinear optical crystal: after the crystal growth is completed, the crystal is still kept in the growth furnace for annealing, and is cooled to room temperature at a rate of no more than 5°C / h.

8. The method for preparing the Sr2Ga2GeS7 nonlinear optical crystal according to claim 5 or 6, characterized in that: The powdered Sr2Ga2GeS7 compound is prepared by the method described in 2 or 3.

9. Use of the Sr2Ga2GeS7 nonlinear optical crystal according to claim 4 or the Sr2Ga2GeS7 nonlinear optical crystal obtained by the method according to any one of claims 5 or 6 in the preparation of mid- and far-infrared laser frequency doubling devices, infrared communication devices or infrared laser guidance devices.

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