Fluorine-doped cesium zinc chloride scintillator single crystal and preparation method and application thereof
By doping the Cs2ZnCl4 crystal with F-ion doping, its valence band-core band characteristics are regulated, and an efficient and fast attenuated fluorine-doped cesium chloride zinc scintillator single crystal is prepared, which solves the problems of low luminescence efficiency and long attenuation time of Cs2ZnCl4 crystal, and is suitable for a variety of radiation detection applications.
Patent Information
- Application Number
- CN202510579789.9
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-05-07
- Publication Date
- 2025-07-29
AI Technical Summary
The existing Cs2ZnCl4 crystals have low luminescence efficiency in the visible light region, and ion doping usually leads to a longer flicker attenuation time, making it difficult to achieve a balance between rapid attenuation and efficient luminescence.
By introducing F-ion-doped Cs2ZnCl4 crystals, the valence band-core band characteristics are regulated, and fluorine-doped cesium chloride zinc scintillator single crystals are prepared by hydrothermal method or other single crystal growth methods, and the doping amount is controlled at x≤50at%, preferably 1.5at%≤x≤25at%.
It significantly improves the scintillation light output efficiency, maintains ultra-fast time resolution, optimizes the cross-luminescence characteristics between the core belt and the valence band, and is suitable for radiation detection fields such as high-energy physics, nuclear physics experiments, space physics, oil well exploration, safety inspection and industrial detection.
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Abstract
Description
Technical Field
[0001] The present invention belongs to the technical field of scintillator materials for radiation detection, and in particular relates to a fluorine-doped cesium zinc chloride scintillator single crystal and a preparation method and application thereof. Background Art
[0002] Scintillators are functional materials that convert the energy of high-energy particles or radiation into visible light. They are widely used in high-energy physics, security inspections, nondestructive testing, nuclear medicine imaging, and other fields. The basic function of a scintillator is to absorb high-energy particles or radiation and then emit visible light. This light signal is then captured by photodetectors such as photomultiplier tubes or silicon photomultipliers (SiPMs), enabling the detection and measurement of radiation. Luminescent scintillators based on the valence-core band structure respond rapidly to high-energy radiation excitation and emit light, with a decay time typically less than 3 nanoseconds.
[0003] Scintillators with high count rates and ultrafast decay times are ideal materials for high-energy radiation detection and are highly favored in scientific research and application fields. This type of material can efficiently capture the trajectory and energy of high-energy particles. With its excellent performance, it has shown irreplaceable and important value in high-repetition-rate radiation imaging and high-energy physics experiments, providing key support for cutting-edge scientific research and technological applications. Cesium zinc chloride (Cs2ZnCl4) crystal is a metal halide scintillator crystal with unique structure and properties. It has a large effective atomic number (Z eff =86), showing good blocking ability and stability under high energy radiation. Cs2ZnCl4 belongs to the valence band-core band luminescent material, its unique 5p Cs + →3p Cl - The transition characteristics enable ultrafast luminescence in the ultraviolet-visible light band (250-390nm). At the same time, the material exhibits near-perfect decay time characteristics, with its fast component almost completely dominating the luminescence process, accounting for nearly 100%. This property gives it significant advantages in optoelectronic devices and scintillation detection applications requiring high temporal resolution.
[0004] At present, there are relatively few research reports on the new Cs2ZnCl4 scintillation crystal. In 2014, Natsuna Yahaba et al. first discovered that Cs2ZnCl4 single crystals grown by the Bridgman method have X-ray detection capabilities. Subsequently, Takahashi et al. reported that Cs2ZnCl4 crystals have ultrafast decay time characteristics (about 1.7 ns), with an emission peak at 310 nm, and the slow component accounts for less than 12% of the total scintillation. However, the light yield of the fast component of this material is slightly lower than that of the traditional barium fluoride (BaF2) crystal. To further improve the performance of Cs2ZnCl4 crystals, Rutstrom et al. grew Cs2ZnCl4 single crystals with better optical quality by the vertical Bridgman method. The light output (LY) of this crystal was increased to 1980 photons / MeV, and the single-component scintillation decay time was 1.66 nanoseconds. These research results indicate that Cs2ZnCl4 crystals have broad application prospects in fields such as high-energy physics and medical imaging.
[0005] Doping is an important means to regulate the performance of scintillators. By reasonably selecting the type of doping ions and regulating the doping concentration, the optical and scintillation properties of scintillation crystals can be improved, and the optimization of emission wavelength, energy transfer efficiency, light output, decay time, and energy resolution can be achieved. For example, research shows that yttrium doping can effectively suppress the slow luminescence component in barium fluoride scintillation crystals, and cerium-magnesium co-doping can accelerate the scintillation decay time of gadolinium gallium garnet single crystals.
[0006] Therefore, although the luminescence efficiency of Cs2ZnCl4 crystals in the visible light region is low, ion doping has been proven to be an effective modification strategy. Currently, various ion dopings are used to regulate the electronic structure of Cs2ZnCl4 crystals, which can usually significantly enhance the luminescence intensity, but often accompanied by slower time characteristics, which is not conducive to applications with high time resolution. Therefore, how to precisely regulate the valence band-core band characteristics of Cs2ZnCl4 crystals through doping to achieve a balance between fast decay and high-efficiency luminescence is the current research focus. Summary of the Invention
[0007] Aiming at the above technical problems, the purpose of the present invention is to provide a fluorine-doped cesium zinc chloride scintillator single crystal, its preparation method and application.
[0008] In the first aspect, the present invention provides a fluorine-doped cesium zinc chloride scintillator single crystal, and the chemical formula of the fluorine-doped cesium zinc chloride scintillator single crystal is Cs2ZnCl4:xF, and the F element is F - ; where: x represents the atomic ratio of the doping element, and x ≤ 50 at%.
[0009] Preferably, 1.5 at% ≤ x ≤ 50 at%, and preferably 6 at% ≤ x ≤ 25 at%.
[0010] In a second aspect, the present invention provides a method for preparing the above-mentioned fluorine-doped cesium zinc chloride scintillator single crystal, and the preparation method includes the following steps: (1) Weigh the raw material powders according to the elemental stoichiometric ratio in the chemical composition Cs2ZnCl4:xF of the fluorine-doped cesium zinc chloride scintillator single crystal; (2) Dissolve the raw material powders and grow crystals by the hydrothermal method, or directly load the raw material powders into a single crystal growth crucible after vacuum drying or after a melting reaction and grow crystals by the Bridgman method, the Czochralski method, the Kyropoulos method, the heat exchange method or the micro-pulling down method to obtain the fluorine-doped cesium zinc chloride scintillator single crystal.
[0011] Preferably, in step (1), the raw material powders include CsCl powder, ZnCl2 powder and CsF powder; preferably, the purity of the raw material powders can be ≥99.9 wt%.
[0012] Preferably, in step (2), the temperature for growing crystals by the hydrothermal method is 150-180 °C, the heat preservation time is 2-48 hours, and the cooling rate is 0.01-0.5 °C / min.
[0013] Preferably, in step (2), the temperature of the Bridgman method is 500-700 °C, the temperature gradient is 15-25 °C / cm, and the growth rate is 0.1-3 mm / h.
[0014] Preferably, in step (2), the temperature of the Czochralski method is 500-700 °C, the rotation speed is 3-20 r / min, the pulling rate is 0.2-3 mm / h, and the growth rate is 0.1-3 mm / h.
[0015] Preferably, in step (2), the temperature of the Kyropoulos method is 500-700 °C, and the growth rate is 0.1-3 mm / h.
[0016] Preferably, in step (2), the temperature of the heat exchange method is 500-700 °C, the constant temperature time is 1-24 hours, the cooling rate is 1-10 °C / h, and the growth rate is 0.1-3 mm / h.
[0017] Preferably, in step (2), the temperature of the micro-pulling down method is 500-700 °C, the crucible lowering speed is 1-10 mm / h, and the growth rate is 0.1-3 mm / h.
[0018] Beneficial effects (1) By using the fluorine doping technology, the present invention realizes the precise regulation of the valence band structure and the valence band-core band energy gap of the Cs2ZnCl4 crystal. On the basis of retaining its ultra-fast time resolution, the scintillation light output efficiency is significantly improved, and the cross-luminescence characteristics between the core band and the valence band are optimized; (2) The cesium zinc chloride (Cs2ZnCl4) single crystal scintillator prepared by fluorine doping in the present invention can be widely applied to radiation detection fields such as high-energy physics, nuclear physics experiments, space physics, oil well exploration, security inspection, and industrial detection due to its excellent performance. BRIEF DESCRIPTION OF THE DRAWINGS
[0019] Figure 1 Single crystal photos of pure Cs2ZnCl4 (left) and Cs2ZnCl4:F (right, F doping amount is 25 at%) grown by hydrothermal method; Figure 2 Fluorescence spectra of pure Cs2ZnCl4 single crystal and Cs2ZnCl4:F (F doping amount is 25 at%) single crystal; Figure 3 X-ray excitation emission spectra of pure Cs2ZnCl4 and Cs2ZnCl4:F (F doping amount is 25 at%) single crystals; Figure 4 Gamma-ray pulse height spectra of pure Cs2ZnCl4 and Cs2ZnCl4:F (F doping amount is 25 at%) single crystals; Figure 5 X-ray pulse time spectrum of pure Cs2ZnCl4 single crystal; Figure 6 X-ray pulse time spectrum of Cs2ZnCl4:F (F doping amount is 25 at%) single crystal; Figure 7 Graph of the change of X-ray excitation emission intensity of Cs2ZnCl4:xF (x = 0 - 50 at%) single crystal with doping concentration; Figure 8 XRD patterns of pure Cs2ZnCl4 and Cs2ZnCl4:F (F doping amount is 25 at%) single crystals. DETAILED DESCRIPTION OF THE INVENTION
[0020] The present invention will be further described below through the following embodiments. It should be understood that the following embodiments are only used to illustrate the present invention and do not limit the present invention.
[0021] First, the present invention provides a fluorine-doped cesium zinc chloride scintillator single crystal. The chemical formula of the fluorine-doped cesium zinc chloride scintillator single crystal can be Cs2ZnCl4:xF, and the F element can be F - ; wherein: x represents the atomic ratio of the doping element, x ≤ 50 at%, preferably 1.5 at% ≤ x ≤ 50 at%, more preferably 6 at% ≤ x ≤ 25 at%.
[0022] It should be noted that since ratio has a slightly larger ionic radius. According to the general principle that the doped ions must satisfy the similarity of ionic radius to the substituted ions in the matrix lattice, F - ions can effectively occupy the position of Cl - ions in the Cs2ZnCl4 lattice. In addition, the 2p electron orbitals of F - ions can enrich the valence band composition of Cs2ZnCl4, promote the effective recombination of valence electrons and holes, and thus improve its scintillation performance. If the F doping amount is too high, it will lead to poor crystallization quality of Cs2ZnCl4:F single crystal.
[0023] Single halogen substitution is easier to maintain structural stability, while double halogen doping may cause lattice distortion or phase separation, resulting in structural instability. The present invention only considers single F doping, and realizes the regulation of core-valence band luminescence (CVL) in Cs2ZnCl4 crystals (the emission position redshifts - which is more conducive to matching the response peak of the detector, thus improving the detection efficiency) and the optimization of scintillation performance (significantly improving the light output while maintaining the ultrafast scintillation characteristics) through fluorine (F) doping, rather than relying on the adjustment of growth parameters and conditions. In the existing Cs2ZnCl4 doping techniques, most introduce new luminescence centers or enhance the self-trapped exciton emission (STE) spectrum by introducing rare earth ions or ns 2 ions. Although these methods can significantly improve the scintillation light output, they often lead to a longer scintillation decay time, which obviously cannot meet the application requirements of ultrafast time resolution.
[0024] The present invention regulates the valence band-core band electron composition of the crystal by introducing F element and adjusting the F doping amount, and significantly improves the scintillation light output efficiency of the crystal on the basis of maintaining the ultrafast time resolution of the Cs2ZnCl4 scintillator.
[0025] Furthermore, in the present invention, the energy gap between the core band and the valence band of the fluorine-doped cesium zinc chloride scintillator is significantly reduced compared with that of the undoped cesium zinc chloride scintillator. This change effectively improves the efficiency of the transition between the core band and the valence band, accelerates the recombination process of electrons and holes, and thus significantly improves the light output while maintaining the ultrafast scintillation characteristics. Therefore, this material can meet the higher requirements of high time resolution optoelectronic devices and scintillation detection applications for scintillator materials in terms of light output and time response characteristics.
[0026] In most cases, doping significantly affects the scintillation performance (decay time, light output) by regulating the electronic band structure. By doping, the electronic band structure is changed, thus changing the radiative recombination process, and further affecting the light yield and decay time, while the unit cell parameters remain basically stable under the same growth method.
[0027] In some embodiments, the size of the fluorine-doped cesium zinc chloride scintillator single crystal is ≥1 mm in at least one dimension of length, width, and height.
[0028] In some embodiments, the decay time of the fluorine-doped cesium zinc chloride scintillator single crystal may be 1.12-2.09 ns.
[0029] In some embodiments, the light output channel address of the fluorine-doped cesium zinc chloride scintillator single crystal is 486-603 and is not equal to 486. The light output channel address refers to an indicator for measuring a scintillation light pulse at a certain pulse amplification voltage and gain, specifically the full energy peak position of the pulse height spectrum under fixed energy photon excitation.
[0030] In some embodiments, the light output of the fluorine-doped cesium zinc chloride scintillator single crystal is 776-963 and is not equal to 776 p.e. / MeV. Wherein, the light output is defined as the light output of the scintillating material at an energy of E γ The number of photons emitted by γ photons.
[0031] The following is an exemplary description of the preparation method of the fluorine-doped cesium zinc chloride scintillator single crystal provided by the present invention. The preparation method may include the following steps: (1) weighing raw material powder according to the stoichiometric ratio of elements in the chemical composition of the fluorine-doped cesium zinc chloride scintillator single crystal Cs2ZnCl4:xF; (2) dissolving the raw material powder and growing crystals by a hydrothermal method, or directly loading the raw material powder into a single crystal growth crucible after vacuum drying or loading it into a single crystal growth crucible after a melting reaction, and growing the crystals by a crucible descent method, a Czochralski method, a Kyropoulos method, a heat exchange method, or a micro-pull-down method to obtain the fluorine-doped cesium zinc chloride scintillator single crystal.
[0032] In some embodiments, in step (1), the raw material powder may include CsCl powder, ZnCl2 powder and CsF powder; preferably, the purity of the raw material powder may be ≥99.9 wt%.
[0033] In some embodiments, in step (2), concentrated hydrochloric acid is used to dissolve the raw material powder; preferably, the concentration of the concentrated hydrochloric acid can be 3-12M.
[0034] In some embodiments, in step (2), the temperature of the hydrothermal crystal growth method can be 150-180° C., the holding time can be 2-48 hours, and the cooling rate can be 0.01-0.5° C. / min.
[0035] In some embodiments, in step (2), the temperature of the crucible descent method can be 500-700° C., the temperature gradient can be 15-25° C. / cm, and the growth rate can be 0.1-3 mm / h.
[0036] In some embodiments, in step (2), the temperature of the Czochralski method can be 500 - 700 °C, the rotation speed can be 3 - 20 r / min, the pulling rate can be 0.2 - 3 mm / h, and the growth rate can be 0.1 - 3 mm / h.
[0037] In some embodiments, in step (2), the temperature of the Kyropoulos method can be 500 - 700 °C, and the growth rate can be 0.1 - 3 mm / h.
[0038] In some embodiments, in step (2), the temperature of the heat exchange method can be 500 - 700 °C, the constant temperature time can be 1 - 24 hours, the cooling rate can be 1 - 10 °C / h, and the growth rate can be 0.1 - 3 mm / h.
[0039] In some embodiments, in step (2), the temperature of the micro-pulling down method can be 500 - 700 °C, the descending speed of the crucible can be 1 - 10 mm / h, and the growth rate can be 0.1 - 3 mm / h.
[0040] The cesium zinc chloride fluoride doped scintillator single crystal obtained by the preparation method provided by the present invention can be applied in the field of radiation detection, and the field of radiation detection includes high energy physics, nuclear physics, medical imaging, industrial inspection, security inspection, resource exploration and other fields.
[0041] The following further gives examples to illustrate the present invention in detail. It should also be understood that the following examples are only used to further illustrate the present invention, and cannot be construed as limiting the protection scope of the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the above content of the present invention all fall within the protection scope of the present invention. The specific process parameters and the like in the following examples are also only an example within a suitable range, that is, those skilled in the art can make selections within a suitable range through the description herein, rather than being limited to the specific values in the following examples. If not specifically specified, the technical means used in the examples are conventional means well known to those skilled in the art.
[0042] Example 1
[0043] The preparation method of the cesium zinc chloride fluoride doped scintillator single crystal Cs2ZnCl4:3at%F (Cs2ZnCl 3.88 F 0.12 ) provided in this example includes the following steps: (1) Using CsCl and ZnCl2 as raw materials, and CsF as a dopant, weighing and mixing according to the elemental stoichiometric ratio in the cesium zinc chloride fluoride doped scintillator single crystal Cs2ZnCl4:3at%F to obtain a mixed powder; (2) Place the mixed powder in a reaction kettle, add concentrated hydrochloric acid and stir to dissolve it, then place it in an oven and heat it at a constant temperature of 160 °C for 24 hours, and cool it to room temperature at a cooling rate of 0.1 °C / min to obtain the fluorine-doped cesium zinc chloride scintillator single crystal.
[0044] Example 2
[0045] The preparation method of the fluorine-doped cesium zinc chloride scintillator single crystal Cs2ZnCl4: 6 at% F provided in this example refers to Example 1, and the main difference is that: In step (1), weigh the raw material powder according to the elemental stoichiometric ratio in the fluorine-doped cesium zinc chloride scintillator single crystal Cs2ZnCl4: 6 at% F.
[0046] Example 3
[0047] The preparation method of the fluorine-doped cesium zinc chloride scintillator single crystal Cs2ZnCl4: 12 at% F provided in this example refers to Example 1, and the main difference is that: In step (1), weigh the raw material powder according to the elemental stoichiometric ratio in the fluorine-doped cesium zinc chloride scintillator single crystal Cs2ZnCl4: 12 at% F.
[0048] Example 4
[0049] The preparation method of the fluorine-doped cesium zinc chloride scintillator single crystal Cs2ZnCl4: 25 at% F provided in this example refers to Example 1, and the main difference is that: In step (1), weigh the raw material powder according to the elemental stoichiometric ratio in the fluorine-doped cesium zinc chloride scintillator single crystal Cs2ZnCl4: 25 at% F.
[0050] Example 5
[0051] The preparation method of the fluorine-doped cesium zinc chloride scintillator single crystal Cs2ZnCl4: 50 at% F provided in this example refers to Example 1, and the main difference is that: In step (1), weigh the raw material powder according to the elemental stoichiometric ratio in the fluorine-doped cesium zinc chloride scintillator single crystal Cs2ZnCl4: 50 at% F.
[0052] Comparative Example 1
[0053] The preparation method of the cesium zinc chloride single crystal Cs2ZnCl4 provided in this comparative example refers to Example 1, and the main difference is that: in step (z), weigh the raw material powder according to the elemental stoichiometric ratio in the cesium zinc chloride single crystal Cs2ZnCl4.
[0054] The following Table 1 shows the relevant parameters of the preparation method of the fluorine-doped cesium zinc chloride scintillator single crystal Cs2ZnCl4: F single crystal prepared in the examples of the present invention: F doping amount / at% Dopant Growth method Main parameters of the growth method Example 1 3％ CsF Hydrothermal method Growth temperature 160 °C, cooling rate 0.1 °C / min Example 2 6％ CsF Hydrothermal method Growth temperature 160 °C, cooling rate 0.1 °C / min Example 3 12％ CsF Hydrothermal method Growth temperature 160 °C, cooling rate 0.1 °C / min Example 4 25％ CsF Hydrothermal method Growth temperature 160 °C, cooling rate 0.1 °C / min Example 5 50％ CsF Hydrothermal method Growth temperature 160 °C, cooling rate 0.1 °C / min
[0055] Table 2 below shows the performance parameters of some single crystal materials prepared in Examples 3 - 5 of the present invention and Comparative Example 1: Initial F doping amount / at% Optical output channel address Optical output Decay time Example 3 12％ 519 885 --- Example 4 25％ 603 963 1.83 ns Example 5 50％ 538 835 --- Comparative example 1 0 486 776 1.26 ns
[0056] Figure 1 Single crystal photos of pure Cs2ZnCl4 grown by hydrothermal method (left) and Cs2ZnCl4:F (right, F doping amount is 25 at%) Figure 2 are the fluorescence spectra of pure Cs2ZnCl4 single crystal and Cs2ZnCl4:F (F doping amount is 25 at%) single crystal. From Figure 1 and 2 it can be seen that after doping with F, the oriented growth of grains can be induced, making the crystal morphology more regular. An enhanced fluorescence emission appears near 268 nm, and there is an obvious red shift, indicating that the electronic energy band composition of the core valence band has changed after F doping.
[0057] Figure 3 are the X-ray excited emission spectra of pure Cs2ZnCl4 and Cs2ZnCl4:F (F doping amount is 25 at%) single crystals. From Figure 3 it can be seen that after doping with F, the crystal shows an obvious enhanced emission under X-rays, and the emission is located near 274 nm.
[0058] Figure 4 are the gamma-ray pulse height spectra of pure Cs2ZnCl4 and Cs2ZnCl4:F (F doping amount is 25 at%) single crystals. From Figure 4 it can be seen that after doping with F, the channel address representing the crystal light output of the crystal increases from 486 of the pure crystal to 603 of the F-doped crystal (F doping amount is 25 at%).
[0059] In order to determine the change in the time response characteristics of the Cs2ZnCl4 crystal after doping, the present invention used an X-ray fluorescence lifetime test system to obtain the scintillation decay curves of pure Cs2ZnCl4 and Cs2ZnCl4:F (F doping amount is 25 at%) single crystals under X excitation.
[0060] Figure 5 is the X-ray pulse time spectrum of pure Cs2ZnCl4 single crystal, Figure 6 is the X-ray pulse time spectrum of Cs2ZnCl4:F (F doping amount is 25 at%) single crystal. From Figure 5 and Figure 6 it can be seen that after doping with fluorine (F), the decay time of the Cs2ZnCl4 crystal does not deteriorate significantly, only slightly increasing from 1.26 ns (pure Cs2ZnCl4 crystal) to 1.83 ns (F doping amount is 25 at%).
[0061] Figure 7 This is a graph showing the X-ray excitation emission intensity of Cs2ZnCl4:xF (x=0-50at%) single crystal changing with doping concentration.
[0062] Figure 8 These are the XRD patterns of pure Cs2ZnCl4 and Cs2ZnCl4:F (F doping amount is 25at%) single crystals.
[0063] Although the content of the present invention has been described in detail through the above preferred embodiments, it should be appreciated that the above description should not be considered as a limitation of the present invention. After reading the above content, it will be apparent to those skilled in the art that various modifications and substitutions of the present invention will occur. Therefore, the protection scope of the present invention should be limited by the appended claims.
Claims
1. A cesium zinc chloride fluoride-doped single crystal scintillator, characterized in that, The chemical formula of the fluorine-doped cesium zinc chloride scintillator single crystal is Cs2ZnCl4:xF, and the F element is F - ; where: x represents the atomic ratio of the doped element, and x ≤ 50 at%.
2. The cesium zinc chloride fluoride-doped single crystal scintillator according to claim 1, characterized in that, 1.5 at% ≤ x ≤ 50 at%, preferably 6 at% ≤ x ≤ 25 at%.
3. A method for preparing a fluorine-doped cesium zinc chloride scintillator single crystal according to claim 1 or 2, characterized in that, The preparation method includes the following steps: (1) Weigh the raw material powders according to the elemental stoichiometric ratio in the chemical composition Cs2ZnCl4:xF of the fluorine-doped cesium zinc chloride scintillator single crystal; (2) Dissolve the raw material powders and grow crystals by the hydrothermal method, or directly load the raw material powders into a single crystal growth crucible after vacuum drying or after a melting reaction and grow crystals by the Bridgman method, the Czochralski method, the Kyropoulos method, the heat exchange method or the micro-pulling down method to obtain the fluorine-doped cesium zinc chloride scintillator single crystal.
4. The preparation method according to claim 3, characterized in that, In step (1), the raw material powders include CsCl powder, ZnCl2 powder and CsF powder; preferably, the purity of the raw material powders can be ≥ 99.9 wt%.
5. The preparation method according to claim 3 or 4, characterized in that, In step (2), the temperature for crystal growth by the hydrothermal method is 150 - 180 °C, the heat preservation time is 2 - 48 hours, and the cooling rate is 0.01 - 0.5 °C / min.
6. The preparation method according to any one of claims 3-5, characterized in that, In step (2), the temperature of the Bridgman method is 500 - 700 °C, the temperature gradient is 15 - 25 °C / cm, and the growth rate is 0.1 - 3 mm / h.
7. The preparation method according to any one of claims 3-6, characterized in that, In step (2), the temperature of the Czochralski method is 500 - 700 °C, the rotation speed is 3 - 20 r / min, the pulling rate is 0.2 - 3 mm / h, and the growth rate is 0.1 - 3 mm / h.
8. The preparation method according to any one of claims 3-7, characterized in that, In step (2), the temperature of the Kyropoulos method is 500 - 700 °C, and the growth rate is 0.1 - 3 mm / h.
9. The preparation method according to any one of claims 3-8, characterized in that, In step (2), the temperature of the heat exchange method is 500 - 700 °C, the constant temperature time is 1 - 24 hours, the cooling rate is 1 - 10 °C / h, and the growth rate is 0.1 - 3 mm / h.
10. The preparation method according to any one of claims 3-9, characterized in that, In step (2), the temperature of the micro-pulling down method is 500 - 700 °C, the crucible lowering speed is 1 - 10 mm / h, and the growth rate is 0.1 - 3 mm / h.