Birefringent crystal and preparation method and application thereof

Abstract

The invention belongs to the technical field of birefringent crystal material preparation, and relates to a birefringent crystal and a preparation method and application thereof. The chemical formula of the birefringent crystal is MgGaF5 (H2O) 2, the birefringent crystal belongs to an orthorhombic system, and the space group is Ima; and the cell parameter is # imgabs0 # alpha = beta = gamma = 90 degrees. The birefringent crystal provided by the invention is colorless and transparent granular and has a wide band gap of 4.5 eV and an ultraviolet cut-off edge of 275nm; meanwhile, the crystal has large birefringence within the wave band of 500-1000 nm, the birefringence value is about 0.2 and is 16 times that of MgF2, and the crystal is an excellent ultraviolet birefringence crystal material capable of being applied to preparation of optical polarization devices.

Description

Technical Field

[0001] The present invention belongs to the technical field of the preparation of birefringent crystal materials, and relates to a birefringent crystal, a preparation method thereof and an application thereof. Background Art

[0002] Birefringent crystal materials are one of the most important components in optical polarization devices. They have the function of modulating polarized light and play a key role in the fields of fiber optic sensing and nonlinear optics. When a light wave is projected onto the crystal interface, generally two refracted light beams will be generated. One of the light beams follows the law of refraction and is called the ordinary light, and its refractive index is a fixed value; the other light beam does not follow the law of refraction and is called the extraordinary light, and its refractive index varies with the propagation direction of the light. This phenomenon is called birefringence. Due to the anisotropy of the crystal material, the included angle between these two refracted light rays is related to the propagation direction and polarization state of the light wave. The crystal that produces the birefringence phenomenon is called a birefringent crystal.

[0003] Birefringent crystals have many uses in our actual life. For example, they can be used in various polarization devices, such as optical polarizers, beam splitters, wave plates, circulators and phase matching devices. In the past few decades, many commercial birefringent materials have been synthesized and developed, including YVO 4 , TiO 2 , LiNbO 3 , CaCO 3 , MgF 2 and a-BaB 2 O 4 (a-BBO) crystals. Among these materials, CaCO 3 is the most common birefringent material in current commerce and can be used in the visible or ultraviolet region. However, since it is a natural mineral containing impurities, it is difficult to fabricate a polarization prism with high optical quality. The YVO 4 crystal has a large birefringence, but this crystal is opaque at wavelengths below 400 nm and is only suitable for the near-infrared range. The binary halide MgF 2 has the widest transparent range (130 nm - 8 μm) among them and is currently the most commonly used deep ultraviolet birefringent material. However, its too small birefringence (0.012 @ 546 nm) limits the practical application of MgF 2 .

[0004] At present, the development of new birefringent crystals with strong optical anisotropy is an important research direction. Metal fluorides have become a class of highly potential materials due to their wide transmission range, high laser damage threshold, and easy synthesis. However, there are still relatively few metal fluoride compounds with remarkable properties and practical applications developed so far. Therefore, it is urgent to explore a new type of birefringent crystal that is easy to grow and has excellent properties to meet the growing demand for optical applications. Summary of the Invention

[0005] The purpose of the present invention is to provide a birefringent crystal, its preparation method and application, so as to solve the problems in the prior art, having a wide bandgap and a large birefringence in the wavelength range of 500 - 1000 nm.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] In the first aspect, the present invention provides a birefringent crystal, the chemical formula of the birefringent crystal is MgGaF 5 (H 2 O) 2 , belonging to the orthorhombic crystal system, and the space group is Imma; its unit cell parameters are ɑ = β = γ = 90°.

[0008] In the second aspect, the present invention provides a preparation method of a birefringent crystal, including the following steps: mixing a raw material containing magnesium element, a raw material containing gallium element, a raw material containing fluorine element and water to obtain a mixture, crystallizing the mixture and cooling it to room temperature, and then obtaining the birefringent crystal after filtration, washing and drying.

[0009] Preferably, the molar ratio of magnesium element, gallium element and fluorine element in the mixture is (1 - 15):1:(1 - 5).

[0010] Preferably, the raw material containing magnesium element is magnesium acetate tetrahydrate or magnesium nitrate hexahydrate.

[0011] Preferably, the raw material containing gallium element is gallium oxide, gallium fluoride or gallium metal.

[0012] Preferably, the raw material containing fluorine element is hydrofluoric acid or trifluoroacetic acid.

[0013] Preferably, the crystallization conditions are: crystallizing at 180°C - 230°C for not less than 72 hours.

[0014] Preferably, the cooling rate for cooling to room temperature is 3°C / h - 8°C / h.

[0015] In the third aspect, the present invention provides an application of the birefringent crystal in the field of preparing optical polarization devices.

[0016] Preferably, the optical polarization device is a polarization prism or a polarizing plate.

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] The birefringent crystal of the present invention is colorless and transparent granular, has a wide band gap of 4.5 eV, and the ultraviolet cut-off edge is 275 nm; at the same time, the crystal of the present invention has a large birefringence in the wavelength range of 500-1000 nm, and the birefringence values are all about 0.2, which is 16 times that of MgF 2 and is an excellent ultraviolet birefringent crystal material that can be used to prepare optical polarization devices.

[0019] Furthermore, the preparation method of the present invention adopts the methods of solution mixing and crystallization, which is simple to operate and suitable for large-scale preparation; at the same time, the raw materials used in the present invention are all chemicals that are easy to obtain and relatively inexpensive, reducing the production cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required in the embodiments. It should be understood that the following drawings only show some embodiments of the present invention, and therefore should not be regarded as limiting the scope. For those of ordinary skill in the art, other related drawings can be obtained based on these drawings without creative efforts.

[0021] Figure 1 is the coordination environment of Mg 2+ ions in the birefringent crystal of the present invention;

[0022] Figure 2 is the coordination environment of Ga 3+ ions in the birefringent crystal of the present invention;

[0023] Figure 3 is the ab-plane projection of the birefringent crystal of the present invention;

[0024] Figure 4 is the three-dimensional crystal structure of the birefringent crystal of the present invention along the b-axis direction;

[0025] Figure 5 is the schematic diagram of the band gap value of the birefringent crystal of the present invention;

[0026] Figure 6 is the schematic diagram of the theoretical birefringence value of the birefringent crystal of the present invention;

[0027] Figure 7 (a) is the schematic diagram before extinction of the birefringent crystal of the present invention;

[0028] Figure 7(b) is the schematic diagram after extinction of the birefringent crystal of the present invention. Detailed implementation mode

[0029] To enable those skilled in the art to understand the features and effects of the present invention, the following is a general description and definition of the terms and expressions mentioned in the specification and claims. Unless otherwise specified, all technical and scientific terms used herein shall have the ordinary meanings understood by those skilled in the art for the present invention. In case of conflicts, the definitions in this specification shall prevail.

[0030] The theories or mechanisms described and disclosed herein, whether correct or incorrect, shall not limit the scope of the present invention in any way, that is, the content of the present invention can be implemented without being limited by any specific theory or mechanism.

[0031] In this article, all features defined in the form of numerical ranges or percentage ranges, such as numerical values, quantities, contents, and concentrations, are only for the sake of simplicity and convenience. Accordingly, the description of numerical ranges or percentage ranges shall be regarded as having covered and specifically disclosed all possible sub-ranges and individual numerical values within the range (including integers and fractions).

[0032] In this article, unless otherwise specified, "comprising", "including", "containing", "having" or similar terms cover the meanings of "consisting of" and "consisting essentially of". For example, "A comprises a" covers the meanings of "A comprises a and others" and "A only comprises a".

[0033] In this article, for the sake of concise description, all possible combinations of all technical features in each embodiment or example are not described. Therefore, as long as there is no contradiction in the combination of these technical features, the technical features in each embodiment or example can be combined arbitrarily, and all possible combinations should be considered as the scope described in this specification.

[0034] The following further describes the present invention in detail with reference to the accompanying drawings:

[0035] The first object of the present invention is to provide a birefringent crystal, the chemical formula of which is MgGaF 5 (H 2 O) 2 , belonging to the orthorhombic crystal system, with the space group Imma; its unit cell parameters are ɑ = β = γ = 90°. As Figure 1 and Figure 2 shown, the magnesium atom coordinates with four fluorine atoms in the equatorial plane (the lengths of the four Mg-F bonds are equal, about ), and coordinates with the oxygen atoms in two water molecules in the vertical direction (the Mg-O bond length is ), forming a C4-type distorted octahedron - MgF 4 O 2 . The gallium atom forms a six-coordinate GaF 6 octahedron with six fluorine atoms (the equatorial Ga-F bond length is and the axial Ga-F bond length is ). As Figure 3 and Figure 4 show, the GaF 6 octahedra are bridged by axial F atoms to form a one-dimensional chain along the b-axis direction. The MgF 4 O 2 octahedron is connected to the Ga-F-Ga-F one-dimensional chain through sharing of equatorial angular F atoms and forms a three-dimensional pore crystal structure along the b-axis direction.

[0036] The birefringent crystal of the present invention has excellent physical and optical properties. Specifically, the birefringent crystal of the present invention has a colorless and transparent granular morphology, exhibits excellent stability in air, is not easily broken or deliquescent, and is suitable for the preparation of optical polarization devices. In terms of optical properties, as Figure 5 shows, the birefringent crystal has a wide bandgap of 4.5 eV, enabling it to exhibit excellent light transmission performance in a wide wavelength range. In addition, as Figure 6 shows, in the wavelength range of 500 - 1000 nm, the birefringent crystal exhibits significant birefringence characteristics, and the birefringence values are all around 0.2, which is 16 times that of the traditional MgF 2 crystal. Its large birefringence characteristics make it have important application potential in optical polarization devices.

[0037] The second object of the present invention is to provide a method for preparing a birefringent crystal, comprising the following steps:

[0038] Mix a raw material containing magnesium element, a raw material containing gallium element, a raw material containing fluorine element and water to obtain a mixture, crystallize the mixture at 180 °C to 230 °C for not less than 72 hours, and after cooling to room temperature, filter, wash and dry to obtain the birefringent crystal. Among them, the molar ratio of magnesium element, gallium element and fluorine element in the mixture is (1 - 15):1:(1 - 5); the cooling rate for cooling to room temperature is 3 °C / h to 8 °C / h.

[0039] In the present invention, the magnesium element provides the stability of the crystal framework. The magnesium ion (Mg 2+ ) in the crystal combines with fluoride ions (F - ) and gallium ions (Ga 3+)A stable coordination structure is formed, which helps in the formation of the orthorhombic system of the birefringent crystal. Secondly, the presence of magnesium is beneficial to the bandgap width (4.5 eV) of the birefringent crystal, endowing the birefringent crystal with broadband gap characteristics and making it suitable for optical devices in the ultraviolet light band. Gallium in the crystal forms a complex coordination structure with fluoride ions and magnesium ions, further enhancing the stability of the crystal. At the same time, the introduction of gallium also significantly improves the birefringence of the birefringent crystal. This is due to the synergistic effect of its electronic structure and ionic radius with those of magnesium and fluorine, resulting in excellent large birefringence characteristics of the birefringent crystal in terms of optical properties. Fluorine is the main source of anions in the birefringent crystal, forming stable ionic bonds with magnesium and gallium to construct the basic framework of the crystal; meanwhile, the high electronegativity and small ionic radius of fluorine contribute to the formation of a dense crystal structure, thereby improving the mechanical strength and optical properties of the crystal.

[0040] The preparation method of the present invention adopts the methods of solution mixing and crystallization, which is simple to operate and suitable for large-scale preparation; through the steps of filtration, washing and drying, impurities can be effectively removed to obtain high-purity birefringent crystals; in addition, by adjusting the raw material ratio and crystallization conditions, the unit cell parameters and optical properties of the crystal can be precisely controlled.

[0041] Exemplarily, the raw material containing magnesium element is magnesium acetate tetrahydrate (Mg(CH 3 COO) 2 ·4H 2 O) or magnesium nitrate hexahydrate (Mg(NO 3 ) 2 ·6H 2 O). Both of them have high solubility in water, which is convenient for uniform mixing, and can stably release magnesium ions during the crystallization process to ensure the uniformity of the crystal structure. In addition, magnesium acetate tetrahydrate and magnesium nitrate hexahydrate are common magnesium salts with low prices and are suitable for industrial production.

[0042] Exemplarily, the raw material containing gallium element is gallium oxide (Ga 2 O 3 ) or gallium fluoride (GaF 3) or gallium element (Ga). Gallium oxide dissolves slowly in solution, can stably release gallium ions, is convenient for controlling the crystal growth process, ensures the uniformity of the crystal structure, and gallium oxide is a common gallium compound with a relatively low price, suitable for large-scale industrial production. Gallium fluoride has a high solubility in water, can quickly release gallium ions, and is convenient for uniform mixing with other raw materials; it can quickly participate in the reaction during the crystallization process, helping to accelerate the crystal growth rate. In addition, gallium fluoride not only provides gallium element but also can be used as a source of fluorine element, simplifying the raw material ratio. The gallium element itself has a high purity, can reduce the introduction of impurities, and helps to prepare high-purity birefringent crystals. The gallium element can slowly release gallium ions under specific conditions (such as acidic environment), facilitating the precise control of the crystal growth process.

[0043] Exemplarily, the raw material containing fluorine element is hydrofluoric acid (HF) or trifluoroacetic acid (CF 3 COOH). Hydrofluoric acid is a strong acid, can effectively dissolve the raw materials containing gallium or magnesium, promote the reaction, and hydrofluoric acid can provide a high concentration of fluoride ions, helping to form a fluorine-containing crystal structure and adjust the optical properties of the crystal; it also has a high reaction activity, can accelerate the crystallization process, and shorten the preparation time. Compared with hydrofluoric acid, trifluoroacetic acid has a weaker acidity and milder reaction conditions, which is suitable for the crystal growth process with higher requirements for reaction conditions. Trifluoroacetic acid is an organic fluorine compound, can provide fluoride ions, and at the same time its organic part plays a template role during the crystallization process, helping to regulate the crystal structure; and its volatility is low, the operation is relatively safe, and it is suitable for large-scale use in industrial production.

[0044] The preparation method of the present invention adopts the methods of solution mixing and crystallization, with simple operation and suitable for large-scale preparation; secondly, the raw materials used in the present invention, such as magnesium acetate tetrahydrate, magnesium nitrate hexahydrate, gallium oxide, and gallium fluoride, etc., are all chemicals that are easy to obtain and relatively inexpensive, reducing the production cost and improving the market competitiveness of the product.

[0045] The third object of the present invention is to provide an application of the birefringent crystal in the field of preparing optical polarization devices; wherein, the optical polarization device is a polarization prism or a polarizing sheet. The crystal of the present invention has excellent optical transparency in the ultraviolet band and can be effectively applied to ultraviolet optical devices. Compared with the limitations of traditional birefringent materials (such as MgF 2 ) in the ultraviolet band, the crystal of the present invention shows stronger applicability in the preparation of ultraviolet polarization devices. The crystal of the present invention has a large birefringence in the 500-1000 nm band, and the birefringence values are all about 0.2, which is MgF 216 times that, enabling it to efficiently separate and regulate the polarization state of light waves, thereby improving the performance of polarization prisms and polarizers. The crystal of the present invention is colorless, transparent, granular, with excellent optical uniformity and transparency, enabling it to minimize light scattering and energy loss when preparing polarization prisms and polarizers, thereby improving the light transmission efficiency and imaging quality of the device. The crystal of the present invention is not prone to deliquescence and fragmentation in air, showing good physical and chemical stability. This characteristic enables it to work stably for a long time under complex environmental conditions, extend the service life of polarization devices, and reduce maintenance costs.

[0046] The following further elaborates the present invention in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by the appended claims of this application.

[0047] Conventional instrument equipment in the art is used in the following embodiments. For the experimental methods without specific conditions noted in the following embodiments, they are usually carried out under conventional conditions or according to the conditions recommended by the manufacturer. Various raw materials are used in the following embodiments. Unless otherwise stated, commercially available products are used, and their specifications are the conventional specifications in the art. In the specification of the present invention and the following embodiments, unless otherwise specified, "%" represents weight percentage, "parts" represents weight parts, and the ratio represents weight ratio.

[0048] Unless otherwise defined, all professional and scientific terms used herein have the same meaning as those familiar to those skilled in the art. In addition, any methods and materials similar or equivalent to the described content can be applied to the methods of this application. The preferred implementation methods and materials described herein are for illustrative purposes only.

[0049] Example 1

[0050] Mix 2 mol of Mg(CH 3 COO) 2 ·4(H 2 O), 1 mol of Ga 2 O 3 , 2 mol of HF and 5 mL of H 2 O to obtain a mixture. Seal the mixture in a hydrothermal reaction kettle and place it in a programmable box furnace. Crystallize at 230 °C for 72 h, then cool to room temperature at a rate of 3 °C / h, and then obtain a colorless and transparent birefringent crystal after filtration, washing, and drying.

[0051] Example 2

[0052] Mix 2 mol of Mg(CH 3 COO) 2·4(H 2 O), 1 mol GaF 3 , 3 mol HF and 5 mL H 2 O were mixed to obtain a mixture. The mixture was sealed in a hydrothermal reaction kettle and placed in a programmable box furnace. It was crystallized at 220 °C for 80 h, then cooled to room temperature at a rate of 5 °C / h, and then obtained a colorless and transparent birefringent crystal after filtration, washing, and drying.

[0053] Example 3

[0054] 10 mol Mg(CH 3 COO) 2 ·4(H 2 O), 1 mol Ga 2 O 3 , 4 mol CF 3 (CH 3 COOH) and 10 mL H 2 O were mixed to obtain a mixture. The mixture was sealed in a hydrothermal reaction kettle and placed in a programmable box furnace. It was crystallized at 200 °C for 96 h, then cooled to room temperature at a rate of 6 °C / h, and then obtained a colorless and transparent birefringent crystal after filtration, washing, and drying.

[0055] Example 4

[0056] 12 mol Mg(NO 3 ) 2 ·6(H 2 O), 1 mol GaF 3 , 12 mol HF and 10 mL H 2 O were mixed to obtain a mixture. The mixture was sealed in a hydrothermal reaction kettle and placed in a programmable box furnace. It was crystallized at 190 °C for 108 h, then cooled to room temperature at a rate of 6 °C / h, and then obtained a colorless and transparent birefringent crystal after filtration, washing, and drying.

[0057] Example 5

[0058] 15 mol Mg(NO 3 ) 2 ·6(H 2 O), 1 mol Ga, 15 mol HF and 15 mL H 2 O were mixed to obtain a mixture. The mixture was sealed in a hydrothermal reaction kettle and placed in a programmable box furnace. It was crystallized at 180 °C for 120 h, then cooled to room temperature at a rate of 8 °C / h, and then obtained a colorless and transparent birefringent crystal after filtration, washing, and drying.

[0059] Birefringent crystal MgGaF 5 (H 2 O)2 Crystal structure analysis of:

[0060] Single crystal X-ray diffraction and powder X-ray diffraction methods were used to analyze the structures of the crystals prepared in Examples 1 to 5.

[0061] Among them, single crystal X-ray diffraction was carried out on an Agilent SuperNova Dual Wavelength CCD X-ray single crystal diffractometer; the data collection temperature was 100 K, and the diffraction light source was monochromatized Mo-Kα rays. The data was corrected for absorption using the Multi-Scan method. The structure analysis was completed using the SHELXTL-2014 program package; the positions of the heavy atoms were determined by the direct method, and the coordinates of the remaining atoms were obtained by the difference Fourier synthesis method; all atomic coordinates and anisotropic thermal parameters were refined using the full-matrix least squares method based on F 2 of all atoms.

[0062] Among them, the results of single crystal X-ray diffraction showed that the crystal chemical formula of the crystals prepared in Examples 1 to 5 was MgGaF 5 (H 2 O) 2 , belonging to the orthorhombic system, with the space group Imma; its unit cell parameters were ɑ = β = γ = 90°; its crystal structure was as shown in Figure 3 and Figure 4 .

[0063] Diffuse reflectance absorption spectrum test:

[0064] The diffuse reflectance absorption spectrum test taking the crystal prepared in Example 1 as an example was carried out on a Perkin-Elmer Lambda-950 ultraviolet-visible-near-infrared spectrophotometer. The crystal sample was ground into powder, and BaSO 4 was used as a reference substrate. The test results were as shown in Figure 5 , and the band gap of this birefringent crystal was about 4.5 eV, and the ultraviolet cut-off edge was about above 275 nm.

[0065] Birefringence theoretical calculation:

[0066] The CASTEP (Cambridge Serial Total Energy Package) module of Materials Studio software was adopted. The CASTEP software is an ab initio quantum mechanics program based on methods, which can predict various properties through the number and type of atoms, including lattice parameters, band structures, structural properties, molecular symmetries, solid-state densities, wave functions, charge densities, and optical properties. The CASTEP software provides an ab initio method for molecular dynamics, which can be used to simulate the solid, interface, and surface properties of various material systems such as metals, semiconductors, and ceramics. Using the cif file obtained by X-ray diffraction analysis, the birefringence properties of the patented crystal were calculated using the CASTEP software. The results are as Figure 6 shown. In the wavelength range of 500 - 1000 nm, this birefringent crystal exhibits significant birefringence characteristics, and the birefringence values are all around 0.2.

[0067] Figure 7 (a) is a schematic diagram of the birefringent crystal obtained in Example 1 before extinction; Figure 7 (b) is a schematic diagram of the birefringent crystal obtained in Example 1 after extinction.

[0068] It is only a preferred embodiment of the present invention and is not used to limit the present invention. For those skilled in the art, various changes and modifications can be made to the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A birefringent crystal, characterized in that: The chemical formula of the birefringent crystal is MgGaF5(H2O)2, which belongs to the orthorhombic system and has a space group of Imma. Its unit cell parameters are ɑ=β=γ=90°.

2. The method for preparing a birefringent crystal according to claim 1, characterized in that: The following steps are involved: A raw material containing magnesium, a raw material containing gallium, a raw material containing fluorine and water are mixed to obtain a mixture, the mixture is crystallized and cooled to room temperature, and then filtered, washed and dried to obtain the birefringent crystal.

3. The method for preparing a birefringent crystal according to claim 2, characterized in that: The molar ratio of magnesium element, gallium element and fluorine element in the mixture is (1-15):1:(1-5).

4. The method for preparing a birefringent crystal according to claim 2, characterized in that: The raw material containing magnesium element is magnesium acetate tetrahydrate or magnesium nitrate hexahydrate.

5. The method for preparing a birefringent crystal according to claim 2, characterized in that: The raw material containing gallium element is gallium oxide, gallium fluoride or single gallium.

6. The method for preparing a birefringent crystal according to claim 2, characterized in that: The raw material containing fluorine element is hydrofluoric acid or trifluoroacetic acid.

7. The method for preparing a birefringent crystal according to claim 2, characterized in that: The crystallization conditions are: crystallization at 180° C. to 230° C. for no less than 72 hours.

8. The method for preparing a birefringent crystal according to claim 2, characterized in that: The cooling rate to room temperature is 3°C / h to 8°C / h.

9. Use of the birefringent crystal as claimed in claim 1 in the field of preparing optical polarization devices.

10. The use according to claim 9, characterized in that: The optical polarization device is a polarization prism or a polarizer.

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