An alkaline earth metal borate birefringent crystal, its growth method and use

Abstract

This invention relates to an alkaline earth metal borate birefringent crystal, its growth method, and its applications. The crystal has the chemical formula Ba₂Mg₂Tb₂(BO₃)₄O, belongs to the trigonal crystal system, space group P63CM, and has the following cell parameters. This birefringent crystal, Ba₂Mg₂Tb₂(BO₃)₄O, possesses several significant advantages: low-cost growth, stable physical and mechanical properties, high transmittance in the ultraviolet and deep ultraviolet regions, excellent birefringence, and low deliquescence. Furthermore, the water insolubility of Ba₂Mg₂Tb₂(BO₃)₄O makes it easy to process and cut, further enhancing its practical value. These characteristics make Ba₂Mg₂Tb₂(BO₃)₄O-based birefringent crystals have broad industrial application prospects in the field of optical communication devices, meeting the needs of ultraviolet and deep ultraviolet birefringence technology and promoting the development of related industries.

Description

Technical Field

[0001] This invention relates to the field of birefringent crystal technology, specifically to an alkaline earth metal borate birefringent crystal, its growth method, and its applications. Background Technology

[0002] Birefringence refers to the phenomenon where two refracted beams of light are produced when a light wave is projected onto an interface due to the anisotropy of the crystal material. Because of this anisotropy, birefringent crystals have wide applications in optics, electronics, biology, and other fields. Especially in optics, they are crucial for manufacturing polarization devices, optical components, and materials with special optical properties, playing a vital role in improving the performance of optical systems and driving technological advancement.

[0003] Currently available birefringent crystals include CaCO3, YVO4, TiO2, LiNbO3, and α-BBO. Each commonly used birefringent crystal has its unique advantages and disadvantages. For example, CaCO3 (Icelandic ore) exhibits significant birefringence and good light transmission, making it suitable for fabricating optical devices such as polarizing prisms. However, its application in the deep ultraviolet spectral region is limited, and it is a non-renewable resource. YVO4 has high birefringence and a wide transmission range, making it widely used in fiber optic communication. However, its manufacturing cost is high, and its stability may be affected by certain environments. TiO2 (rutile) has excellent birefringence and good wear resistance, but its single crystal hardness makes it difficult to process and it is susceptible to chemical corrosion. MgF2 has a wide transmission spectral range and a stable refractive index, but the device size is relatively large, and its stability may be affected by high temperature and humidity environments. α-BaB2O4 (α-BBO) performs excellently in the visible and ultraviolet regions, making it suitable for fabricating high-performance ultraviolet optical devices. However, it is difficult to obtain high-quality, large-size single crystals, and its manufacturing cost is high. LiNbO3 exhibits excellent nonlinear optical and electro-optic effects and stable physicochemical properties, but its birefringence may be inferior to other crystals in some applications, and its preparation cost is also relatively high. Therefore, birefringent crystals still have a wide range of areas for exploration in terms of application requirements, cost budgets, and preparation processes.

[0004] Borates are candidate materials for the ultraviolet and deep ultraviolet fields due to their short ultraviolet cutoff edges and structural diversity. The BO3 or BO4 structural units in borates can combine to form diverse structures, endowing the materials with multifunctionality. Their wide transparency range, high laser damage threshold, and excellent stability make them highly promising in the optical field, especially in the increasing applications of the deep ultraviolet band. In recent years, those skilled in the art have modified the structure of borates to improve their birefringence and ultraviolet transmittance. Related research results have shown broad application prospects in optical devices, laser technology, and optical communication, especially in optical devices such as polarizing prisms, optical isolators, and prism polarizers. However, the preparation of borate ultraviolet birefringent crystals in existing technologies remains challenging, requiring stringent growth conditions, and the crystals may exhibit stability issues under specific environmental conditions. Furthermore, performance optimization and industrialization have not yet achieved satisfactory results.

[0005] In conclusion, the development of novel birefringent crystal materials suitable for ultraviolet, especially deep ultraviolet, wavelengths is not only of great scientific significance, but also of urgent practical importance. Summary of the Invention

[0006] In view of the problems of existing technology, the purpose of this invention is to develop a new type of birefringent crystal material. This crystal not only has a simple growth method, but also has excellent crystal performance, which can meet the growing scientific research and industrial needs of optical devices.

[0007] To achieve the above-mentioned objectives, the technical solution provided by this invention is as follows:

[0008] A birefringent alkaline earth metal borate crystal with the chemical formula Ba₂Mg₂Tb₂(BO₃)₄O, belonging to the trigonal crystal system, space group P63CM, and cell parameters of [missing information].

[0009] A method for growing alkaline earth metal borate birefringent crystals includes the following steps:

[0010] (1) Weigh BaCO3, Mg2(OH)2CO3, Tb(NO3)3·6H2O and H3BO3 according to the molar ratio 2:1.02-1.07:2:4.2-4.5, put the weighed raw materials into the grinding equipment, and grind them thoroughly until a uniform powder mixture is formed;

[0011] (2) Place the powder mixture in a 100mL platinum crucible, put it in a programmable temperature controlled furnace, and preheat it at 500℃ for more than 24 hours until the components are completely decomposed.

[0012] (3) Place the platinum crucible in a programmable temperature controlled furnace and heat it to 940-960℃ and keep it for 6 hours to promote further reaction of the raw materials and melt them to obtain molten liquid;

[0013] (4) Then the molten liquid is cooled to 700℃ at a rate of 1.5-1.6℃ / h to carry out preliminary crystal nucleation and growth. After the crystal grows to the predetermined size, it is lifted out of the melt and finally cooled to room temperature at a rate of 15-20℃ / h to obtain Ba2Mg2Tb2(BO3)4O birefringent crystal.

[0014] In this preparation method, the proportions of each component must be strictly controlled. Fine adjustments to the proportions of each component will affect the purity and composition of the crystal. Increasing the proportion of BaCO3 will increase the barium content in the crystal, thereby affecting its optical properties. Changes in the proportion of Tb(NO3)3·6H2O will affect the luminescence properties and birefringence effect of the crystal.

[0015] It is also necessary to control the preheating time of the raw materials, as well as the temperature and cooling rate during the melting stage.

[0016] Extending the preheating time helps ensure complete decomposition and uniform mixing of the raw materials, thereby improving crystal quality. However, excessively long preheating times increase energy consumption and production costs. Insufficient preheating time leads to incomplete decomposition of the raw materials, affecting crystal growth and performance.

[0017] Increasing the melting temperature helps accelerate the reaction and melting process of raw materials, but excessively high temperatures lead to the volatilization and loss of raw materials, as well as changes in crystal structure. Decreasing the melting temperature prolongs the melting time, affecting production efficiency.

[0018] The cooling rate during the initial nucleation and growth stages has a significant impact on the nucleation rate and growth morphology of crystals. A faster cooling rate promotes the formation of small crystal nuclei, while a slower cooling rate favors the growth of large crystals. Ultimately, variations in the cooling rate affect the thermal stress and internal defects of the crystal. An excessively fast cooling rate leads to internal cracks and stress, while an excessively slow cooling rate prolongs the production cycle.

[0019] Preferably, oxygen is introduced into the temperature-controlled furnace during the raw material synthesis and crystal growth process. In steps (2), (3), and (4) of the above preparation method, oxygen is introduced into the temperature-controlled furnace at a flow rate of 0.6 to 0.7 mL / min. High-flow-rate oxygen (O2) is a key medium in preheating raw materials and cultivating crystals, and the oxygen flow rate significantly affects the crystal growth mechanism. When the oxygen flow rate is below 0.5 mL / min, the crystal growth process encounters significant obstacles, and the oxygen concentration in the reaction medium is too low to provide the necessary support for stable crystal growth. As the oxygen flow rate gradually increases, i.e., from below 0.5 mL / min to close to 0.5 mL / min and continues to rise, the crystal growth conditions improve, and crystal growth becomes smoother. This strongly demonstrates that a suitable oxygen concentration plays an indispensable role in the formation and stable growth of crystals. When the oxygen flow rate further increases to 0.8 mL / min, the crystallization quality begins to deteriorate, specifically manifested in increased difficulty in controlling crystal size. Excessive oxygen concentration leads to overly vigorous chemical reactions in the growth environment, thereby interfering with the normal crystal growth mechanism. Excessive oxygen concentration not only does not help crystal growth, but also leads to a decline in its performance, specifically manifested as reduced crystallinity and uneven size distribution.

[0020] To obtain crystals with optimal performance, the oxygen flow rate should be precisely controlled within the range of 0.6 to 0.7 mL / min. Within this flow rate range, crystals can achieve stable and high-quality growth, thereby producing single-crystal materials with excellent performance.

[0021] Preferably, in the above preparation method, the purity of BaCO3, Mg2(OH)2CO3, Tb(NO3)3·6H2O and H3BO3 in step (1) is not less than 99%.

[0022] The present invention also provides the application of alkaline earth metal borate birefringent crystals for the ultraviolet band, wherein the crystal Ba2Mg2Tb2(BO3)4O is used to fabricate ultraviolet / deep ultraviolet optical devices.

[0023] The beneficial effects of this invention are as follows: This invention proposes a birefringent crystal, Ba2Mg2Tb2(BO3)4O, along with its unique Czochralski growth method and application areas, aiming to fill the current shortage of birefringent crystal materials in the ultraviolet and deep ultraviolet bands. This crystal exhibits several significant advantages: low-cost growth, stable physical and mechanical properties, high transmittance in the ultraviolet and deep ultraviolet regions, excellent birefringence, and low deliquescence. Furthermore, the water insolubility of Ba2Mg2Tb2(BO3)4O makes it easy to process and cut, further enhancing its practical value. These characteristics make Ba2Mg2Tb2(BO3)4O-based birefringent crystals have broad industrial application prospects in the field of optical communication devices, meeting the needs of ultraviolet and deep ultraviolet birefringence technology and promoting the development of related industries. Attached Figure Description

[0024] To make the objectives, technical solutions, and advantages of the present invention clearer, the preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, wherein:

[0025] Figure 1 Image of Ba2Mg2Tb2(BO3)4O crystal;

[0026] Figure 2 The transmission spectrum of Ba2Mg2Tb2(BO3)4O crystal;

[0027] Figure 3 The birefringence pattern of Ba2Mg2Tb2(BO3)4O crystal is shown. Detailed Implementation

[0028] The present application will be further described in detail below with reference to comparative examples and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0029] The following raw materials are all high-purity (99%) BaCO3, Mg2(OH)2CO3, Tb(NO3)3·6H2O and H3BO3 purchased from Sinopharm Chemical Reagent Co., Ltd.

[0030] Example 1

[0031] An alkaline earth metal borate birefringent crystal is prepared according to the method described below:

[0032] (1) Weigh out BaCO3, Mg2(OH)2CO3, Tb(NO3)3·6H2O and H3BO3 precisely according to the molar ratio of 2:1.02:2:4.2, and then put these raw materials into a grinder until they are ground into a fine and uniform powder mixture;

[0033] (2) Place the ground powder mixture in a 100mL platinum crucible and then put it into a programmable temperature controlled furnace. Preheat it at 500℃ for 24 hours to ensure that all raw materials can be completely decomposed.

[0034] (3) After preheating, adjust the furnace temperature to 940℃ and maintain this temperature for 6 hours to allow the raw materials to react further and melt completely. At the same time, introduce oxygen into the furnace at a flow rate of 0.6 mL / min.

[0035] (4) After the melting reaction is complete, the temperature is slowly cooled to 700°C at a rate of 1.2°C / h to provide favorable conditions for crystal nucleation and growth. During this process, oxygen is continuously introduced at the same flow rate.

[0036] (5) When the crystals have grown to the desired size, stop cooling. Remove the crystals from the liquid surface and cool the remaining melt to room temperature at a rate of 15℃ / h. Finally, Ba2Mg2Tb2(BO3)4O crystals are collected.

[0037] Example 2

[0038] An alkaline earth metal borate birefringent crystal is prepared according to the method described below:

[0039] (1) Weigh out BaCO3, Mg2(OH)2CO3, Tb(NO3)3·6H2O and H3BO3 precisely according to the molar ratio of 2:1.05:2:4.35, and grind them thoroughly in a grinding equipment until a fine and uniform powder mixture is formed;

[0040] (2) Place the ground powder in a platinum crucible and put it into a programmable temperature controlled furnace. Preheat it at 500°C for about 24.5 hours, while introducing oxygen at a flow rate of 0.65 mL / min to ensure complete decomposition of the raw materials.

[0041] (3) After preheating, raise the furnace temperature to 950°C and maintain it for 6 hours to promote full reaction and melting of the raw materials. During this process, continue to introduce oxygen at a constant flow rate;

[0042] (4) Cool slowly to 700℃ at a rate of 1.4℃ / h to provide a good environment for stable crystal growth;

[0043] (5) When the crystal grows to the predetermined size, the crystal is lifted off the liquid surface and the remaining melt is cooled to room temperature at a rate of 17.5℃ / h, and finally Ba2Mg2Tb2(BO3)4O crystal is obtained.

[0044] Example 3

[0045] An alkaline earth metal borate birefringent crystal is prepared according to the method described below:

[0046] (1) Weigh out BaCO3, Mg2(OH)2CO3, Tb(NO3)3·6H2O and H3BO3 accurately according to the molar ratio of 2:1.07:2:4.5, and grind them thoroughly in a grinding equipment until they are fine and uniform;

[0047] (2) Place the ground powder in a platinum crucible and put it into a programmable temperature controlled furnace. Preheat it at 500°C for 25 hours while introducing oxygen at a flow rate of 0.7 mL / min to ensure complete decomposition of the raw materials.

[0048] (3) After preheating, raise the furnace temperature to 960°C and maintain it for 6 hours to allow the raw materials to react further and melt completely;

[0049] (4) After the molten liquid stabilizes, it is slowly cooled to 700℃ at a rate of 1.6℃ / h to allow the crystal to grow smoothly;

[0050] (5) After the crystal grows to the required size, the crystal is lifted off the liquid surface. The remaining melt is cooled to room temperature at a rate of 20℃ / h, and finally removed from the platinum crucible and cleaned to obtain Ba2Mg2Tb2(BO3)4O crystal.

[0051] Using the methods of Examples 1-3, Example 2 yielded the alkaline earth metal borate birefringent crystal with a moderate growth rate and excellent crystal quality, representing the best result. Example 1 successfully prepared alkaline earth metal borate birefringent crystals. Due to the selection of lower raw material ratios, melting temperatures, cooling rates, and oxygen flow rates, the crystal growth rate was slower, resulting in higher crystal quality and fewer defects, but the crystal preparation cycle was longer. Example 3, using the method of this example, achieved a faster crystal growth rate, obtaining larger crystal sizes or higher yields, but the crystal quality was slightly inferior to that of Example 2, and the resulting crystals were prone to localized cracking.

[0052] Analysis of the reasons for the optimal crystal quality in Example 2:

[0053] Raw material ratio: Fine-tuning the molar ratio, such as increasing the proportion of Mg and B, helps to form a more stable crystal structure.

[0054] Preheating time: A slightly longer preheating time (24.5 hours) ensures complete decomposition of the raw materials and reduces impurities during the melting process.

[0055] Melting temperature: A higher melting temperature (950℃) promotes a full reaction between raw materials, which is beneficial for forming a uniform molten liquid.

[0056] Cooling rate: A moderate cooling rate (1.4℃ / h) provides a stable growth environment for the crystals, which is conducive to the orderly arrangement and growth of the crystals.

[0057] Oxygen flow rate: A slightly higher oxygen flow rate (0.65 mL / min) promoted the oxidative decomposition of the raw materials and the release of gases during the melting process, reducing defects in the crystals. The combined effect of these factors resulted in the best crystal quality obtained in Example 2.

[0058] Comparative Example 1

[0059] An alkaline earth metal borate birefringent crystal is prepared according to the method described below:

[0060] (1) First, accurately weigh BaCO3, Mg2(OH)2CO3, Tb(NO3)3·6H2O and H3BO3 as raw materials according to a molar ratio of 2:1.1:2:4.3. Then, put these weighed raw materials into a grinding device and grind them thoroughly until a uniform and fine powder mixture is formed.

[0061] (2) Carefully place the uniformly ground powder mixture into a 100 mL platinum crucible, and then place it into a temperature-controlled furnace. Preheat the furnace at 500 °C for 24.5 hours to ensure complete decomposition of carbonates, hydrated nitrates, and boric acid in the raw materials. Simultaneously, introduce oxygen into the furnace at a flow rate precisely controlled at 0.65 mL / min to maintain a stable atmosphere inside the furnace.

[0062] (3) After preheating, the platinum crucible is placed back in the programmable temperature furnace and heated to 950°C, which is maintained for 6 hours. This step aims to promote further reaction and complete melting of the raw materials. During this process, oxygen is continuously introduced at a constant flow rate.

[0063] (4) Once the molten liquid reaches a stable state, it is slowly cooled to 700°C at a rate of 1.4°C / h. This stage is crucial for crystal nucleation and initial growth. By precisely controlling the cooling rate, the crystal growth conditions can be optimized. Oxygen is continuously introduced into the furnace at a constant flow rate.

[0064] (5) Once the crystal has grown to the predetermined size, the growth process is stopped, and the crystal is lifted off the liquid surface. Finally, the remaining melt is cooled to room temperature at a rate of 17.5℃ / h. Throughout the process, oxygen is continuously introduced at a constant flow rate. Finally, the crystal is removed from the platinum crucible and cleaned to obtain a Ba2Mg2Tb2(BO3)4O birefringent crystal.

[0065] This embodiment reveals that the Ba2Mg2Tb2(BO3)4O birefringent crystal prepared in this embodiment exhibits characteristics of cracking and poor crystallinity. In this embodiment, the proportion of Mg2(OH)2CO3 is too high (1.1), exceeding the limited range of 1.02-1.07. This leads to an imbalance in the composition ratio in the molten liquid, thereby affecting the crystallization process and stability of the crystal, making the crystal prone to cracking and exhibiting poor crystallinity.

[0066] Comparative Example 2

[0067] An alkaline earth metal borate birefringent crystal is prepared according to the method described below:

[0068] (1) Accurately weigh BaCO3, Mg2(OH)2CO3, Tb(NO3)3·6H2O and H3BO3 as raw materials according to the molar ratio 2:1.05:2:4.35, and put them into the grinding equipment for thorough grinding until a uniform and fine powder mixture is formed;

[0069] (2) Carefully place the evenly ground powder mixture into a 100mL platinum crucible and then place it into a programmable temperature controlled furnace. Here, the preheating temperature is adjusted to 550℃ and the preheating time is maintained for more than 24.5 hours to ensure that the carbonates, hydrated nitrates and boric acid in the raw materials can be completely decomposed. During the preheating process, oxygen is introduced into the furnace and the flow rate is precisely controlled at 0.65mL / min to maintain the stability of the atmosphere in the furnace.

[0070] (3) After preheating, the platinum crucible is placed in the programmable temperature controlled furnace and heated to 950°C. This temperature is maintained for 6 hours to promote further reaction and complete melting between the raw materials, forming a uniform molten liquid. During this process, oxygen is continuously introduced at a constant flow rate.

[0071] (4) Once the molten liquid reaches a stable state, it is slowly cooled to 700℃ at a rate of 1.4℃ / h to promote crystal growth;

[0072] (5) Once the crystal has grown to the predetermined size, the cooling process is stopped, and the crystal is lifted off the liquid surface. Finally, the remaining melt is cooled to room temperature at a rate of 17.5 °C / h. Throughout the process, oxygen is continuously introduced at a constant flow rate. Finally, the Ba2Mg2Tb2(BO3)4O birefringent crystal is obtained by removing it from the platinum crucible and cleaning it.

[0073] As observed in this embodiment, the Ba2Mg2Tb2(BO3)4O birefringent crystal prepared in this embodiment is characterized by cracking and poor crystallinity. In this embodiment, the increase in preheating temperature (adjusted from 500℃ to 550℃) will accelerate the decomposition and melting process of the raw materials. However, the excessively high preheating temperature will cause the raw materials to decompose too quickly or unevenly, generating gases or impurities. These gases or impurities will form bubbles or defects during crystal growth, resulting in the crystal being prone to cracking and having poor crystallinity.

[0074] Comparative Example 3

[0075] An alkaline earth metal borate birefringent crystal is prepared according to the method described below:

[0076] (1) Accurately weigh BaCO3, Mg2(OH)2CO3, Tb(NO3)3·6H2O and H3BO3 as raw materials according to the molar ratio 2:1.05:2:4.35, and grind them thoroughly until a uniform and fine powder mixture is formed;

[0077] (2) Place the uniformly ground powder mixture in a 100mL platinum crucible, put it into a programmable temperature controlled furnace, preheat it at 500℃ for 24.5 hours, and at the same time introduce oxygen into the furnace at a flow rate of 0.65mL / min.

[0078] (3) The platinum crucible was placed in the programmable temperature controlled furnace and heated to 950°C and maintained at that temperature for 6 hours to promote further reaction and complete melting between the raw materials. During this process, oxygen was continuously introduced at a flow rate of 0.65 mL / min.

[0079] (4) In this step, the cooling rate of the molten liquid is adjusted to 2℃ / h, and it is slowly cooled to 700℃. Oxygen is then introduced into the furnace at a flow rate of 0.65mL / min.

[0080] (5) Once the crystal has grown to the predetermined size, the cooling process is stopped, and the crystal is lifted off the liquid surface. Finally, the remaining melt is cooled to room temperature at a rate of 17.5℃ / h. Throughout the process, oxygen is continuously introduced at a constant flow rate. Finally, the Ba2Mg2Tb2(BO3)4O birefringent crystal is obtained by removing it from the platinum crucible and cleaning it.

[0081] This embodiment reveals that the Ba2Mg2Tb2(BO3)4O birefringent crystal prepared in this embodiment exhibits cracking characteristics because adjusting the cooling rate affects the crystal growth rate and morphology. The initial cooling rate in this embodiment (2℃ / h) exceeded the given range (1.2-1.6℃ / h). An excessively rapid cooling rate leads to uneven thermal stress distribution during crystal growth, increasing the risk of crystal cracking.

[0082] Comparative Example 4

[0083] An alkaline earth metal borate birefringent crystal is prepared according to the method described below:

[0084] (1) Accurately weigh BaCO3, Mg2(OH)2CO3, Tb(NO3)3·6H2O and H3BO3 as raw materials according to the molar ratio 2:1.05:2:4.35, and grind them thoroughly until a uniform and fine powder mixture is formed;

[0085] (2) Place the uniformly ground powder mixture into a 100 mL platinum crucible and put it into a programmable temperature controlled furnace. Preheat the furnace at 500 °C for 24.5 hours. During this process, the oxygen flow rate into the furnace is adjusted to 0.4 mL / min.

[0086] (3) After preheating, the platinum crucible is placed in the programmable temperature control furnace and heated to 950°C. The temperature is maintained for 6 hours. During this step, oxygen is continued to be introduced at an oxygen flow rate of 0.4 mL / min to promote further reaction and complete melting between the raw materials to form a uniform molten liquid.

[0087] (4) Once the molten liquid reaches a stable state, it is slowly cooled to 700°C at a rate of 1.4°C / h. This stage is crucial for crystal nucleation and initial growth. During this process, oxygen is continuously introduced at a flow rate of 0.4 mL / min.

[0088] (5) Once the crystal has grown to the predetermined size, the cooling process is stopped, and the crystal is lifted off the liquid surface. Finally, the remaining melt is cooled to room temperature at a rate of 17.5 °C / h. Throughout the process, oxygen is continuously introduced at a flow rate of 0.8 mL / min. Finally, the crystal is removed from the platinum crucible and cleaned to obtain a Ba2Mg2Tb2(BO3)4O birefringent crystal.

[0089] Comparative Example 5

[0090] An alkaline earth metal borate birefringent crystal is prepared according to the method described below:

[0091] (1) Accurately weigh BaCO3, Mg2(OH)2CO3, Tb(NO3)3·6H2O and H3BO3 as raw materials according to the molar ratio 2:1.05:2:4.35, and grind them thoroughly until a uniform and fine powder mixture is formed;

[0092] (2) Place the uniformly ground powder mixture into a 100 mL platinum crucible and put it into a programmable temperature controlled furnace. Preheat the furnace at 500 °C for 24.5 hours. During this process, the oxygen flow rate into the furnace is adjusted to 0.8 mL / min.

[0093] (3) After preheating, the platinum crucible is placed in the programmable temperature control furnace and heated to 950°C. The temperature is maintained for 6 hours. During this step, oxygen is continued to be introduced at an oxygen flow rate of 0.8 mL / min to promote further reaction and complete melting between the raw materials to form a uniform molten liquid.

[0094] (4) Once the molten liquid reaches a stable state, it is slowly cooled to 700°C at a rate of 1.4°C / h. This stage is crucial for crystal nucleation and initial growth. During this process, oxygen is continuously introduced at a flow rate of 0.8 mL / min.

[0095] (5) Once the crystal has grown to the predetermined size, the cooling process is stopped, and the crystal is lifted off the liquid surface. Finally, the remaining melt is cooled to room temperature at a rate of 17.5 °C / h. Throughout the process, oxygen is continuously introduced at a flow rate of 0.8 mL / min. Finally, the crystal is removed from the platinum crucible and cleaned to obtain a Ba2Mg2Tb2(BO3)4O birefringent crystal.

[0096] Comparative Examples 4 and 5 revealed that the prepared Ba2Mg2Tb2(BO3)4O birefringent crystals exhibited reduced crystallinity and uneven size distribution because the oxygen flow rates (0.4 mL / min and 0.8 mL / min) exceeded the given range (0.6-0.7 mL / min). High-flow-rate oxygen (O2) is a key medium in preheating raw materials and crystal growth, significantly affecting the crystal growth mechanism. When the oxygen flow rate was below 0.5 mL / min, crystal growth was significantly hindered; the oxygen concentration in the reaction medium was too low to provide the necessary support for stable crystal growth. As the oxygen flow rate gradually increased—from below 0.5 mL / min to near 0.5 mL / min and continuing to rise—the crystal growth conditions improved, and crystal growth became smoother. This strongly demonstrates that an appropriate oxygen concentration plays an indispensable role in crystal formation and stable growth. When the oxygen flow rate further increased to 0.8 mL / min, the crystal quality began to deteriorate, specifically manifested as increased difficulty in controlling crystal size and poor crystallinity, leading to cracking. Excessive oxygen concentration leads to overly vigorous chemical reactions in the growth environment, thereby interfering with the normal crystal growth mechanism. Instead of aiding crystal growth, excessive oxygen concentration can degrade its properties, manifesting as reduced crystallinity and uneven size distribution.

[0097] Based on the comprehensive analysis of the above experimental results, in order to obtain crystals with optimal performance, the oxygen flow rate should be precisely controlled within the range of 0.6 to 0.7 mL / min. Within this flow rate range, the crystals can achieve stable and high-quality growth, thereby producing single-crystal materials with excellent performance.

[0098] In summary, changes in parameters such as raw material ratio, preheating time, temperature at which the material is heated to the melting stage, and cooling rate all have a significant impact on the growth results of alkaline earth metal borate birefringent crystals.

[0099] Therefore, in actual production, these parameters need to be optimized and adjusted according to specific needs and conditions.

[0100] To obtain high-quality crystal products.

[0101] Crystal performance testing

[0102] The crystal prepared in Example 2 was subjected to crystal performance testing. The crystal belongs to the trigonal crystal system, space group P63CM, and its cell parameters are as follows: Figure 1 Image of Ba2Mg2Tb2(BO3)4O crystal.

[0103] Transmission spectroscopy was performed on a 1.5 mm thick Ba₂Mg₂Tb₂(BO₃)₄O crystal sheet, and the results are presented below. Figure 2 As can be clearly observed from the figure, the transmittance of Ba₂Mg₂Tb₂(BO₃)₄O crystal remains consistently above 80% in the wavelength range of 250 to 2000 nm. Even at wavelengths approaching 200 nm, its transmittance still reaches 40%. Notably, the ultraviolet absorption edge of Ba₂Mg₂Tb₂(BO₃)₄O crystal is located below 200 nm, a value similar to 189 nm for α-BBO. Therefore, Ba₂Mg₂Tb₂(BO₃)₄O crystal demonstrates great potential as a material for manufacturing deep ultraviolet optical devices.

[0104] Deliquescence test

[0105] To investigate the deliquescent properties of Ba₂Mg₂Tb₂(BO₃)₄O crystals, a 5g sample of the crystals, which had reached constant weight, was exposed to air for approximately one week. Observation revealed no moisture on the sample surface. A remeasurement showed the sample remained at 5 grams, with a mass change of less than 0.001 grams. This test result indicates that Ba₂Mg₂Tb₂(BO₃)₄O crystals possess good stability and are not prone to deliquescence.

[0106] Application Examples

[0107] The Ba2Mg2Tb2(BO3)4O crystal prepared in Example 2 was processed into a triangular prism shape, and its refractive index was measured at 12 different wavelengths within the wavelength range of 404.7 nm to 1068 nm. The relevant data are summarized in Table 1. The accuracy of these refractive index measurements is estimated to be ±2 × 10⁻⁴. Subsequently, using the least squares method and combining the obtained ne and no values, we derived the Selmeyer dispersion equation, the specific expression of which is:

[0108]

[0109] In the Selmeyer dispersion equation above, λ represents wavelength, and the unit is... To visually represent n. e (Extra-optical refractive index) and n o The variation of (ordinary optical refractive index) with wavelength, such as Figure 3 As shown in the table, at a laser wavelength of 589 nm, the no value of this Ba2Mg2Tb2(BO3)4O crystal is 1.9059, the ne value is 2.0160, and its birefringence is calculated to be -0.1101. This indicates that the crystal possesses good optical properties. Table 1 shows the refractive index at the selected wavelength.

[0110] Table 1 Refractive index at selected wavelengths

[0111] 1.068 1.8789 1.9795 -0.1006 0.9592 1.8821 1.9839 -0.1018 0.8617 1.8851 1.9878 -0.1027 0.8072 1.8874 1.9910 -0.1036 0.6943 1.8958 2.0022 -0.1064 0.6563 1.9000 2.0080 -0.1080 0.5893 1.9059 2.0160 -0.1101 0.578 1.9098 2.0212 -0.1114 0.5461 1.9153 2.0288 -0.1135 0.4962 1.9279 2.0460 -0.1181 0.4861 1.9314 2.0508 -0.1194 0.4047 1.9681 2.1019 -0.1338

[0112] Similarly, the crystals prepared in Examples 1 and 3 were subjected to crystal performance testing and application, and the characteristics they exhibited were similar to those of the crystals prepared in Example 2.

[0113] As illustrated by the above embodiments, the birefringent crystal Ba2Mg2Tb2(BO3)4O prepared by this invention possesses several significant advantages: low-cost growth, stable physical and mechanical properties, high transmittance in the ultraviolet and deep ultraviolet regions, excellent birefringence, and low deliquescence. Furthermore, the water-insoluble nature of Ba2Mg2Tb2(BO3)4O crystals facilitates processing and cutting, further enhancing its practical value. These characteristics make Ba2Mg2Tb2(BO3)4O-based birefringent crystals promising for industrial application in optical communication devices, meeting the demands of ultraviolet and deep ultraviolet birefringence technologies and driving the development of related industries.

Claims

1. A birefringent crystal of an alkaline earth metal borate, characterized in that, Its chemical formula is Ba2Mg2Tb2(BO3)4O, it belongs to the trigonal crystal system, space group P63CM, and its unit cell parameters are a = 9.4231 Å and c = 18.8681 Å.

2. A method for growing alkaline earth metal borate birefringent crystals, characterized in that, Includes the following steps: (1) Weigh BaCO3, Mg2(OH)2CO3, Tb(NO3)3·6H2O and H3BO3 according to the molar ratio 2:1.02-1.07:2:4.2-4.5, put the weighed raw materials into the grinding equipment, and grind them thoroughly until a uniform powder mixture is formed; (2) Place the powder mixture in a 100mL platinum crucible, put it in a programmable temperature controlled furnace, and preheat it at 500℃ for more than 24 hours until the components are completely decomposed; (3) Place the platinum crucible in a programmable temperature controlled furnace and heat it to 940-960℃ and keep it for 6 hours to promote further reaction of the raw materials and melt them to obtain molten liquid; (4) Then the molten liquid is cooled to 700℃ at a rate of 1.5-1.6℃ / h to carry out preliminary crystal nucleation and growth. After the crystal grows to the predetermined size, it is lifted out of the melt and finally cooled to room temperature at a rate of 15-20℃ / h to obtain Ba2Mg2Tb2(BO3)4O birefringent crystal. During steps (2), (3) and (4), oxygen is introduced into the temperature-controlled furnace at a flow rate of 0.6 to 0.7 mL / min.

3. The method for growing alkaline earth metal borate birefringent crystals according to claim 2, characterized in that, In step (1), the purity of BaCO3, Mg2(OH)2CO3, Tb(NO3)3·6H2O and H3BO3 is not less than 99%.

4. The application of the alkaline earth metal borate birefringent crystal according to claim 1, characterized in that, The crystal Ba2Mg2Tb2(BO3)4O is used to fabricate ultraviolet / deep ultraviolet optical devices.

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