Birefringent crystal and preparation method and application thereof

Abstract

The invention belongs to the technical field of birefringent crystal material preparation, and particularly relates to a birefringent crystal and a preparation method and application thereof. The chemical formula of the crystal is Ba (SnF3) 2 (H2O), the crystal belongs to an orthorhombic crystal system, and the space group is Pnnm; the cell parameters are as follows: a = 9.9-10.0, b = 17.4-17.5, c = 4.3-4.4, and alpha = beta = gamma = 90 degrees; the birefringence of the Ba (SnF3) 2 (H2O) birefringence crystal reaches up to 0.33 (at546 nm), and the Ba (SnF3) 2 (H2O) birefringence crystal reflects remarkable optical anisotropy and is suitable for high-performance optical devices; secondly, the crystal has a broad band gap of 3.77 eV and an ultraviolet cut-off edge of 328 nm, so that the crystal can efficiently transmit ultraviolet light and is suitable for optical application of an ultraviolet band; besides, the spectrum transmission range of the crystal covers 2.5-13.3 microns, and the crystal shows excellent transmission performance from ultraviolet band to middle infrared band, meets the requirements of broadband optical devices, and has wide application potential in the optical field.

Description

Technical Field

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

[0002] The birefringence phenomenon refers to that when a light beam is incident on an anisotropic crystal, the incident light will be decomposed into two polarized lights with mutually perpendicular vibration directions, different propagation speeds and unequal refractive indexes. These two light rays are usually called the ordinary light (o-light) and the extraordinary light (e-light). The crystal that can produce the birefringence phenomenon is called a birefringent crystal, which is the core material for generating and modulating polarized light and is widely used in the fields of optical communication, polarization information processing, optical instrument manufacturing, etc.

[0003] So far, scientists have discovered thousands of crystals with birefringent properties, and some of them have been commercially applied on a large scale. For example, crystals such as magnesium fluoride, potassium dihydrogen phosphate, yttrium vanadate, calcium carbonate and α-BaB 2 O 4 etc. play important roles in optical communication and optical instrument manufacturing. Among them, metal fluorides have attracted much attention because they usually have a wide transmittance range and good thermal stability.

[0004] However, these commercially available metal fluoride birefringent crystals all have certain disadvantages and limitations. First, the metal fluoride crystals represented by magnesium fluoride have significant performance defects. Their birefringence values are relatively small, resulting in the need for a larger crystal size in actual applications to achieve the required polarization effect, which seriously restricts their application prospects in miniature portable optical communication devices. Second, in terms of thermal stability, magnesium fluoride has obvious deficiencies compared with birefringent materials with higher thermal stability such as calcium carbonate. Its stability at high temperatures is poor, resulting in easy damage of the device during long-term operation, reducing the service life and reliability of the equipment. In addition, the preparation process of high-purity magnesium fluoride is complex and the technical difficulty is large, which not only increases the production cost but also limits the feasibility of its large-scale commercial application.

[0005] Based on the above problems, developing high-performance birefringent crystals that are easy to grow, have low preparation costs, have large birefringence values, excellent physical and chemical stability, and a wide transmittance range has become an urgent problem to be solved in the current field of optical materials. Summary of the Invention

[0006] The purpose of the present invention is to provide a birefringent crystal, a preparation method thereof and an application thereof to solve the problems in the prior art. The birefringent crystal of the present invention has the characteristics of large birefringence, stable physical and chemical properties and a wide transmittance range.

[0007] To achieve the above object, the present invention is implemented by the following technical solutions: In a first aspect, the present invention provides a birefringent crystal, and the chemical formula of the crystal is Ba(SnF 3 ) 2 (H 2 O), which belongs to the orthorhombic crystal system, and the space group is Pnnm ; its unit cell parameters are a = 9.9~10.0 Å, b = 17.4~17.5 Å, c = 4.3~4.4 Å, ɑ = β = γ = 90°.

[0008] In a second aspect, the present invention provides a method for preparing a birefringent crystal, including the following steps: Mix a raw material containing barium element, substance A and water to obtain a mixture, crystallize the mixture and cool it to room temperature, and then obtain the birefringent crystal after filtration, washing and drying; Wherein, the substance A is stannous fluoride, stannous fluoroborate or a mixture of a raw material containing fluorine element and a raw material containing tin element.

[0009] Preferably, the molar ratio of barium element, fluorine element and tin element in the mixture is (1~10):(3~24):3.

[0010] Preferably, the raw material containing barium element is barium carbonate or barium metaphosphate.

[0011] Preferably, the raw material containing fluorine element is trifluoroacetic acid.

[0012] Preferably, the raw material containing tin element is stannous oxalate.

[0013] Preferably, the crystallization condition is to crystallize at 180 °C to 230 °C for not less than 120 h.

[0014] Preferably, the cooling rate for cooling to room temperature is 2 °C / h to 8 °C / h.

[0015] In a 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 an optical fiber polarizer.

[0017] Compared with the prior art, the present invention has the following beneficial effects: The Ba(SnF 3 ) 2 (H 2O) The birefringent crystal has a high birefringence of up to 0.33 (@546 nm), showing significant optical anisotropy, which is suitable for high-performance optical devices. Secondly, the crystal has a wide bandgap of 3.77 eV and an ultraviolet cut-off edge of 328 nm, enabling it to efficiently transmit ultraviolet light and be applicable to optical applications in the ultraviolet band. In addition, the spectral transmission range of the crystal covers 2.5 - 13.3 μm, showing excellent transmission performance from the ultraviolet to the mid-infrared band, meeting the requirements of wide-band optical devices and having broad application potential in the optical field.

[0018] Furthermore, the present invention can achieve high-yield preparation of colorless transparent crystals by using the hydrothermal synthesis method. This preparation method is simple and efficient, suitable for large-scale production. The prepared crystals show excellent physical and chemical stability in air and can maintain the integrity of their structure and performance for a long time. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required for use in the embodiments. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as limiting the scope. For those of ordinary skill in the art, without creative efforts, other related drawings can also be obtained based on these drawings.

[0020] Figure 1 Coordination environment of Ba 2+ ions in the birefringent crystal of the present invention; Figure 2 Coordination environment of Sn 2+ ions in the birefringent crystal of the present invention; Figure 3 Projection of the

[001] crystal plane of the birefringent crystal of the present invention; Figure 4 Along the c axis direction pore structure schematic diagram of the birefringent crystal of the present invention; Figure 5 Bandgap value schematic diagram of the birefringent crystal of the present invention; Figure 6 ATR schematic diagram of the birefringent crystal of the present invention; Figure 7 Thermogravimetric schematic diagram of the birefringent crystal of the present invention; Figure 8 Theoretically calculated birefringence of the birefringent crystal of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0021] To enable those skilled in the art to understand the features and effects of the present invention, the following provides 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 with respect to the present invention. In case of conflicts, the definitions in this specification shall prevail.

[0022] The theories or mechanisms described and disclosed herein, whether correct or incorrect, shall not in any way limit the scope of the present invention, that is, the content of the present invention can be implemented without being limited by any specific theory or mechanism.

[0023] In this text, 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 brevity and convenience. Accordingly, the description of numerical ranges or percentage ranges should be regarded as having covered and specifically disclosed all possible sub-ranges and individual numerical values within the range (including integers and fractions).

[0024] In this text, unless otherwise specified, the terms "comprising", "including", "containing", "having", or similar expressions 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 consists only of a".

[0025] In this text, for the sake of brevity of 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 being within the scope described in this specification.

[0026] The following further describes the present invention in detail with reference to the accompanying drawings: The first object of the present invention is to provide a birefringent crystal, and the chemical formula of the crystal is Ba(SnF 3 ) 2 (H 2 O), which belongs to the orthorhombic crystal system and the space group is Pnnm ; its unit cell parameters are a = 9.9~10.0 Å, b = 17.4~17.5 Å, c = 4.3~4.4 Å, ɑ = β = γ = 90°. Ba(SnF 3 ) 2 (H 2 O)'s asymmetric unit includes 1 Ba atom, 2 Sn atoms, 6 F atoms and 1 water molecule. The coordination situation of metal cations is as Figure 1and Figure 2 As shown, barium ions coordinate with 8 fluorine atoms and the oxygen atom in 1 water molecule. The Ba-F bond lengths are symmetric around. Stannous ions coordinate with 4 fluorine atoms to form a "seesaw" structure with two long bonds and two short bonds - [SnF4] 2- , Sn 2+ 's lone pair electron cloud faces the opposite direction of the short bond, and the two long bonds and two short bonds are symmetrically distributed. The three-dimensional crystal structure is as shown in Figure 3 and Figure 4 . Ba 2+ is bridged along the c axis direction by three F atoms to form an F-Ba-F… chain. [SnF4] 2- forms a one-dimensional chain connected by two long Sn-F bonds and is connected to the F-Ba-F… chain through two short Sn-F bonds to form two pore structures. The eight-membered small pore is composed of 2 diagonal Ba 2+ and 2 Sn 2+ , and one bridging F atom on each of the four sides. The twenty-four-membered large pore is composed of 6 Ba 2+ , 6 Sn 2+ , and 12 bridging F atoms. The water molecule coordinated with Ba 2+ is located in the large pore. The pores extend along the c axis direction, increasing the anisotropy of the structure, and the group bond lengths are symmetric on both left and right sides.

[0027] The second object of the present invention is to provide a preparation method of a birefringent crystal, including the following steps: Mix a raw material containing barium element, substance A and water to obtain a mixture. Crystallize the mixture at 180 °C to 230 °C for no less than 120 h and then cool it to room temperature. After filtration, washing and drying, a colorless transparent needle-shaped birefringent crystal with high purity and high crystallinity is obtained; wherein, the substance A is stannous fluoride, stannous fluoroborate or a mixture of a raw material containing fluorine element and a raw material containing tin element, and the cooling rate is 2 °C / h to 8 °C / h; the molar ratio of barium element, fluorine element and tin element in the mixture is (1 to 10):(3 to 24):3.

[0028] The present invention prepares colorless transparent crystals by the hydrothermal synthesis method. This method has the advantages of simple operation, mild conditions, high yield, etc., and is suitable for large-scale industrial production. Specifically, in the hydrothermal synthesis method, by carrying out the reaction in a hydrothermal environment of high temperature and high pressure, it can effectively promote the dissolution and recrystallization of raw materials, thereby obtaining high-purity crystals with high yield. This method does not require complex equipment or harsh reaction conditions, reduces production costs, and at the same time can precisely control the crystal growth process to ensure the uniformity and integrity of the crystal structure. In addition, the crystals prepared by the present invention exhibit excellent physical and chemical stability in air and can maintain the integrity of their structure and performance for a long time. The crystals are not easily affected by deliquescence, oxidation or thermal decomposition, and can maintain stable optical properties even under complex environmental conditions. This high stability gives it significant advantages in practical applications and can meet the requirements of optical devices for material durability and reliability.

[0029] Barium ions (Ba 2+ ) have a relatively large ionic radius and high polarizability. Its electron cloud distribution is asymmetric, and a significant polarization effect is generated under the action of the light wave electric field, enhancing the optical anisotropy of the crystal and thus increasing the birefringence. Secondly, the strong ionic bond and high electronegativity difference between barium and fluorine increase the band gap of the crystal, endowing it with wide-band gap characteristics. In addition, since the bonding structure formed by fluorine and metal elements has a relatively low phonon energy, reducing the absorption of photons by lattice vibrations, it can cover a wide wavelength range from ultraviolet to mid-infrared, broadening the spectral transmission range of the crystal and making the crystal have broad application potential in optical devices. Tin ions (Sn 2+ ) and fluoride ions (F - ) form a stable SnF 4 - coordination structure with a high degree of asymmetry, and the lone pair electrons of divalent tin ions also enhance the optical anisotropy of the crystal, thus significantly increasing the birefringence and making it suitable for use in polarization optical devices. Secondly, the strong bonding between tin and fluorine increases the band gap of the crystal, and at the same time, the electronic structure characteristics of tin reduce the energy level overlap between the conduction band and the valence band, further stabilizing the wide band gap, enabling the crystal to effectively transmit ultraviolet light and being suitable for optical applications in the ultraviolet band.

[0030] In the present invention, both stannous fluoride and stannous fluoroborate can simultaneously provide fluoride ions and tin ions, simplifying the raw material ratio and mixing process and improving the preparation efficiency. Secondly, they have high solubility in water, can quickly release ions, promote crystal growth and shorten the reaction time. In addition, these two raw materials have low costs and are easy to obtain, are suitable for large-scale industrial production, and their high-purity characteristics help to reduce the introduction of impurities and improve the optical properties of the crystals.

[0031] The raw material containing barium element is barium carbonate or barium metaphosphate. Barium carbonate has stable chemical properties, is easy to store and operate, and can slowly release barium ions in an acidic environment, facilitating the precise control of the crystal growth process and ensuring the uniformity of the crystal structure. Barium metaphosphate has the characteristic of high purity, which can reduce the introduction of impurities. At the same time, both are common barium salts with low costs and are suitable for large-scale industrial production.

[0032] The raw material containing fluorine element is trifluoroacetic acid, which has weak acidity and mild reaction conditions, and is suitable for the crystal growth process with high requirements for reaction conditions. In addition, trifluoroacetic acid has low volatility and high operation safety, is suitable for large-scale use in industrial production, and can reduce the metal impurities that may be introduced by inorganic fluorine sources, improving the purity of the crystal.

[0033] The raw material containing tin element is stannous oxalate, which has the characteristics of high solubility and strong reaction activity. It can quickly release tin ions in water, facilitating uniform mixing with other raw materials and promoting crystal growth. The oxalate ions in stannous oxalate decompose during the crystallization process, releasing carbon dioxide, which helps to regulate the crystal growth environment and form a porous structure.

[0034] The third object of the present invention is to provide an application of a birefringent crystal in the field of preparing optical polarization devices, wherein the optical polarization device is a polarization prism or a fiber polarizer. As important optical elements, polarization prisms and fiber polarizers are widely used in fields such as optical communication, optical information processing, optical measurement, and sensing. Due to its special pore structure (double pore structure), the birefringent crystal of the present invention greatly enhances the anisotropy, resulting in a very large birefringence of the crystal, and has the potential to be applied to optical elements of micro-devices.

[0035] The present invention will be further described below 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.

[0036] 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.

[0037] Example 1 Add 1 moL BaCO 3 、3 moL SnF2 Mix with water 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 120 h, then cool to room temperature at a rate of 8 °C / h, and then obtain colorless transparent birefringent crystals after filtration, washing, and drying.

[0038] Example 2 Mix 5 moL BaCO 3 , 2 moL Sn(BF 4 ) 2 with water to obtain a mixture. Seal the mixture in a hydrothermal reaction kettle and place it in a programmable box furnace. Crystallize at 220 °C for 130 h, then cool to room temperature at a rate of 7 °C / h, and then obtain colorless transparent birefringent crystals after filtration, washing, and drying.

[0039] Example 3 Mix 10 moL Ba(PO 3 ) 2 , 3 moL SnF 2 with water to obtain a mixture. Seal the mixture in a hydrothermal reaction kettle and place it in a programmable box furnace. Crystallize at 210 °C for 140 h, then cool to room temperature at a rate of 6 °C / h, and then obtain colorless transparent birefringent crystals after filtration, washing, and drying.

[0040] Example 4 Mix 6 moL Ba(PO 3 ) 2 , 3 moL Sn(BF 4 ) 2 with water to obtain a mixture. Seal the mixture in a hydrothermal reaction kettle and place it in a programmable box furnace. Crystallize at 200 °C for 150 h, then cool to room temperature at a rate of 5 °C / h, and then obtain colorless transparent birefringent crystals after filtration, washing, and drying.

[0041] Example 5 Mix 1 moL Ba(PO 3 ) 2 , 3 moL SnC 2 O 4 , 1 moL C F 3 COOH with water to obtain a mixture. Seal the mixture in a hydrothermal reaction kettle and place it in a programmable box furnace. Crystallize at 190 °C for 160 h, then cool to room temperature at a rate of 4 °C / h, and then obtain colorless transparent birefringent crystals after filtration, washing, and drying.

[0042] Example 6 Mix 10 moL Ba(PO 3 ) 2, 3 moL SnC 2 O 4 , 8 moL CF 3 COOH and water are mixed to obtain a mixture. The mixture is sealed in a hydrothermal reaction kettle and placed in a programmable box furnace. It is crystallized at 180 °C for 170 h, then cooled to room temperature at a rate of 2 °C / h, and then filtered, washed, and dried to obtain a colorless transparent birefringent crystal.

[0043] Example 7 4 moL Ba(PO 3 ) 2 , 3 moL SnC 2 O 4 , 3 moL CF 3 COOH and water are mixed to obtain a mixture. The mixture is sealed in a hydrothermal reaction kettle and placed in a programmable box furnace. It is crystallized at 180 °C for 170 h, then cooled to room temperature at a rate of 2 °C / h, and then filtered, washed, and dried to obtain a colorless transparent birefringent crystal.

[0044] Example 8 9 moL Ba(PO 3 ) 2 , 3 moL SnC 2 O 4 , 7 moL CF 3 COOH and water are mixed to obtain a mixture. The mixture is sealed in a hydrothermal reaction kettle and placed in a programmable box furnace. It is crystallized at 180 °C for 170 h, then cooled to room temperature at a rate of 2 °C / h, and then filtered, washed, and dried to obtain a colorless transparent birefringent crystal.

[0045] Crystal structure analysis of birefringent crystal Ba(SnF 3 ) 2 (H 2 O): Single crystal X-ray diffraction and powder X-ray diffraction methods are used to analyze the structures of the crystals prepared in Examples 1-8.

[0046] Among them, single crystal X-ray diffraction is carried out on an Agilent SuperNova Dual Wavelength CCD type X-ray single crystal diffractometer. The data collection temperature is 100 K, and the diffraction light source is monochromatized Mo-Kα ray (λ = 0.71073 Å); the data is processed by the Multi-Scan method for absorption correction. The structure analysis is completed using the SHELXTL-2014 program package; the positions of heavy atoms are determined by the direct method, and the coordinates of the remaining atoms are obtained by the difference Fourier synthesis method; the coordinates of all atoms and the anisotropic thermal parameters are refined by the full matrix least squares method based on F 2 .

[0047] Among them, the single-crystal X-ray diffraction results show that for the birefringent crystals prepared in Examples 1 to 8, the chemical formula of the crystal is Ba(SnF 3 ) 2 (H 2 O), which belongs to the orthorhombic crystal system and the space group is Pnnm ; its unit cell parameters are a = 9.9~10.0 Å, b = 17.4~17.5 Å, c = 4.3~4.4 Å, ɑ = 90°, β = 90°, γ = 90°, and its crystal structure is as shown in Figure 3 .

[0048] Diffuse reflectance absorption spectrum and infrared spectrum tests: The diffuse reflectance absorption spectrum test taking the birefringent crystal prepared in Example 1 as an example was carried out on a Lambda-950 ultraviolet-visible-near-infrared spectrophotometer of Perkin-Elmer Company in the United States. The birefringent crystal sample of the present invention was ground into powder, and BaSO 4 was used as the reference substrate. The test results are as shown in Figure 5 . The band gap of the birefringent crystal is about 3.77 eV, and the ultraviolet cut-off edge is about 328 nm. The infrared spectrum test was carried out on a Fisher Nicolet S50 FT-IR spectrometer, and the wave number range was 4000 - 400 cm -1 . The test results are as shown in Figure 6 . The crystal has good transmittance in the spectral range of 2.5 - 13.3 μm.

[0049] Thermogravimetric test: The thermogravimetric analysis taking the birefringent crystal prepared in Example 1 as an example was carried out on a METTLER TGA2. The crystal powder sample of the present invention was heated from 30 °C to 1000 °C at a heating rate of 10 °C / min in a nitrogen atmosphere and then held at a constant temperature for 10 min. As shown in the thermogravimetric analysis result in Figure 7 , the crystal begins to lose crystal water at 120 °C.

[0050] Birefringence theoretical calculation model: 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 of molecular dynamics (CP method), 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 band structure and birefringence properties of the crystal were calculated using the CASTEP software. The results are as Figure 8 shown. For the crystal of the present invention, in the wavelength range of 500 - 1200 nm, the birefringence value is 0.28 - 0.35.

[0051] The above are only the preferred embodiments of the present invention and are not used to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modifications, equivalent replacements, improvements, 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 crystal chemical formula is Ba(SnF3)2(H2O), belongs to the orthorhombic system, and the space group is Pnnm Its unit cell parameters are a =9.9~10.0 Å, b =17.4~17.5 Å, c =4.3~4.4 Å, 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 barium, substance A 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; Wherein, the substance A is stannous fluoride, stannous fluoroborate or a mixture of a raw material containing fluorine and a raw material containing tin.

3. The method for preparing a birefringent crystal according to claim 2, characterized in that: The molar ratio of barium element, fluorine element and tin element in the mixture is (1-10):(3-24):

3.

4. The method for preparing a birefringent crystal according to claim 2, characterized in that: The raw material containing barium element is barium carbonate or barium metaphosphate.

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

6. The method for preparing a birefringent crystal according to claim 2, characterized in that: The raw material containing tin element is stannous oxalate.

7. The method for preparing a birefringent crystal according to claim 2, characterized in that: The crystallization condition is crystallization at 180°C-230°C for no less than 120h.

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

9. Use of the birefringent crystal according to 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 fiber polarizer.

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