Metal halide nonlinear optical crystal and preparation method thereof
The synthesis of Rb2CdCl2I2 crystals via solution evaporation spontaneous crystallization solves the problem of synthesizing mixed anionic metal halides and realizes a nonlinear optical crystal with high laser damage threshold and wide transmission range, suitable for laser frequency conversion and infrared communication.
Patent Information
- Application Number
- CN202511690284.6
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-11-18
- Publication Date
- 2026-02-10
AI Technical Summary
In the existing technology, the synthesis of mixed anion metal halide nonlinear optical crystals is difficult, single crystal growth is difficult, and existing materials have low laser-induced damage thresholds or two-photon absorption problems, making it impossible to cover atmospheric transparent windows.
Metal halide nonlinear optical crystal Rb₂CdCl₂I₂ was synthesized by solution evaporation spontaneous crystallization. By controlling the temperature and solution evaporation conditions, a crystal with excellent optical properties that is easy to synthesize and store was grown. The chemical formula is Rb₂CdCl₂I₂, which belongs to the orthorhombic crystal system and has a space group of I4mm.
The prepared Rb2CdCl2I2 crystal has a wide bandgap (3.70 eV), a high laser damage threshold (19×AgGaS2), and an ultrawide transmittance range (0.35 ~103.2 μm), making it suitable for devices such as laser frequency converters and infrared communication.
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Figure CN121496574A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of nonlinear optical crystals. More specifically, it relates to an Rb₂CdCl₂I₂ nonlinear optical crystal and its synthesis and preparation. Background Technology
[0002] Nonlinear optical materials have shown great application potential in modern laser science and technology, serving as key components for frequency conversion. Based on their transmission band and application range, nonlinear optical crystal materials can be categorized into ultraviolet, visible, and infrared nonlinear optical materials. Among these, mid- and far-infrared (MFIR) nonlinear optical crystals have become indispensable in military and civilian fields such as laser communication, remote sensing, industrial manufacturing, environmental monitoring, and medical surgery. Currently, the commercially available MFIR material AgGaS2 (E g = 2.6 eV), AgGaSe2 (E g =1.8 eV) and ZnGeP2 (E g The inherent limitations of infrared nonlinear optical materials (with a band gap of 1.7 eV) exist: AgGaS2 and AgGaSe2 exhibit low laser-induced damage thresholds (LIDTs), while ZnGeP2 exhibits strong two-photon absorption and cannot cover the atmospheric transparency window of 8–14 μm. Therefore, developing novel infrared nonlinear optical materials with large band gaps and wide transmission ranges remains one of the urgent scientific frontiers.
[0003] Metal halides are promising candidates due to their excellent band gaps (high LIDTs) and wide infrared transmission range, and the "mixed anion" strategy has proven effective in breaking structural symmetries. Although the significant electronegativity differences of mixed periodic anions present greater synthetic challenges, they exhibit clear advantages over anions of the same period: increased band gap width, expanded transmission range, and enhanced polyhedral distortion in nonlinear response. However, due to the high synthetic difficulty, research on mixed anion metal halides is relatively limited in current technologies, and single-crystal growth is also challenging. Summary of the Invention
[0004] This invention aims to address the technical problem of the difficulty in synthesizing mixed anionic metal halides using existing methods, and provides a metal halide nonlinear optical crystal and its preparation method. The metal halide nonlinear optical crystal of this invention is a single crystal, which is easy to synthesize and grow, easy to grind, easy to store, stable in air, and not easily deliquescent. Simultaneously, it exhibits excellent optical properties, possessing a wide bandgap (3.70 eV), a high laser damage threshold (19 × AgGaS2), an ultrawide transmission range (0.35 ~ 103.2 μm), moderate birefringence, and ease of fabrication.
[0005] The metal halide nonlinear optical crystal of the present invention has the chemical formula Rb2CdCl2I2. The structure of the nonlinear optical crystal does not have a center of symmetry, belongs to the orthorhombic crystal system, has a space group of I4mm, and has cell parameters of a = 5.31210(10), b = 5.31210(10), c = 17.7062(10), and Z = 2.
[0006] The above-mentioned method for preparing metal halide nonlinear optical crystals is a solution evaporation spontaneous crystallization method, specifically carried out according to the following steps:
[0007] 1. Weigh RbCl and CdI2 according to the stoichiometric ratio of the metal halide Rb2CdCl2I2; dissolve the weighed RbCl and CdI2 in deionized water to obtain a mixed solution;
[0008] 2. Transfer the mixed solution into a polytetrafluoroethylene liner, then place the liner into a stainless steel reactor and tighten it. Place it in an oven and heat it to 200-230 ℃ at a rate of 25-30 ℃ / h and keep it at that temperature for 24-48 h. Then cool it down to room temperature at a rate of 5-10 ℃ / h.
[0009] 3. The product in the polytetrafluoroethylene liner was filtered, and the filtrate was placed in a non-sealed container and allowed to stand and evaporate at a temperature of 33-37 °C. Crystal particles grew at the bottom of the container until the crystal size showed no significant change, at which point the growth ended. The resulting colorless blocky crystals were washed with deionized water to obtain a metal halide nonlinear optical crystal. The chemical formula of this crystal is Rb₂CdCl₂I₂.
[0010] Furthermore, the purity of RbCl and CdI2 mentioned in step one reaches analytical grade (AR) or higher.
[0011] Furthermore, the non-sealed container mentioned in step three is formed by placing the filtrate inside the container, sealing it with a membrane, and punching small holes in the membrane to create a non-sealed state.
[0012] Furthermore, the temperature described in step three is 35°C. At this temperature, the crystal growth rate is suitable, other impurities and byproducts can be avoided, and the synthesized product has high purity.
[0013] Furthermore, the statement in step three that the crystal size does not change significantly means that the increase in crystal size is less than 5% within 2 days.
[0014] The chemical reaction formula for preparing the metal halide nonlinear optical crystal Rb₂CdCl₂I₂ according to the present invention is: 2RbCl₂ + CdI₂ = Rb₂CdCl₂I₂
[0015] The nonlinear optical crystal of this invention is a metal halide, Rb₂CdCl₂I₂, synthesized using a solution evaporation method. During solution evaporation, the temperature is maintained at 33–37 °C, lower than that of the melt method. This results in crystals with high purity and good crystallinity. The crystals grow easily and are transparent without inclusions, making them easy to process. This nonlinear optical crystal, Rb₂CdCl₂I₂, has a wide bandgap of 3.70 eV and an ultra-wide transmittance range of 0.35–103.2 μm, covering the two key mid-infrared atmospheric windows of 3–5 μm and 8–14 μm, without strong absorption caused by chemical bond vibrations. Meanwhile, this infrared nonlinear optical crystal Rb₂CdCl₂I₂ also exhibits a high laser damage threshold. The laser damage thresholds for six different particle sizes (38–48 µm, 48–75 µm, 75–108 µm, 108–150 µm, 150–212 µm, and 212–270 µm) are 55.9, 56.2, 56.1, 56.2, 56.4, and 56.6 MW·cm⁻¹, respectively. -2 It is approximately 19 times larger than AgGaS2 of the same particle size range, and therefore can be applied to various devices such as laser frequency converters and infrared communication infrared laser devices. Attached Figure Description
[0016] Figure 1 This is a single crystal photograph of Rb2CdCl2I2 prepared in Example 1.
[0017] Figure 2 This is a structural diagram of the Rb2CdCl2I2 crystal prepared in Example 1.
[0018] Figure 3 The X-ray diffraction pattern of the Rb2CdCl2I2 crystal powder prepared in Example 1 is compared with the theoretical value.
[0019] Figure 4 Thermogravimetric curve of Rb2CdCl2I2 crystal powder prepared in Example 1.
[0020] Figure 5 The optical band gap diagram is shown for the Rb2CdCl2I2 crystal prepared in Example 1.
[0021] Figure 6 This is the infrared spectrum of the Rb2CdCl2I2 crystal prepared in Example 1.
[0022] Figure 7 The relationship between the particle size of the Rb2CdCl2I2 crystal prepared in Example 1 and the laser damage threshold is shown.
[0023] Figure 8 This is a comparison diagram of the X-ray diffraction pattern of the Rb2CdCl2I2 crystal powder prepared in Example 2 and the theoretical value. Detailed Implementation
[0024] The beneficial effects of the present invention will be verified using the following examples.
[0025] Example 1: The preparation method of the metal halide nonlinear optical crystal in this example is carried out according to the following steps:
[0026] 1. Weigh 0.2184 g of analytical grade RbCl and 0.3662 g of analytical grade CdI2 and add them to 30 mL of deionized water. Stir and dissolve evenly to obtain a mixed solution.
[0027] 2. Transfer the mixed solution to a 50 mL polytetrafluoroethylene liner, then place the liner into a stainless steel reactor and tighten it. Place the reactor in an oven and heat it to 230 °C at a rate of 30 °C / h and hold it for 24 h. Then cool it down to room temperature at a rate of 10 °C / h.
[0028] 3. The product within the polytetrafluoroethylene liner was filtered, and the filtrate was placed in a beaker, sealed with a membrane, and several small holes were punched in the membrane to create a loose seal. The beaker was then placed in an oven at 35 ℃ to allow evaporation. Crystal particles grew at the bottom of the beaker. After 10 days of growth, the crystal size showed no significant change, and the growth was considered complete. The resulting colorless, blocky crystals were washed with deionized water to obtain a metal halide nonlinear optical crystal. The chemical formula of this crystal is Rb₂CdCl₂I₂.
[0029] The single-crystal photograph of Rb₂CdCl₂I₂ prepared in this embodiment is shown below. Figure 1 As shown, from Figure 1 It can be seen that the maximum size of a single crystal can reach 2×2×1 mm. 3 . Figure 2 This is a crystal structure diagram of Rb₂CdCl₂I₂ prepared in Example 1. In the asymmetric unit of Rb₂CdCl₂I₂, there are two crystallographically independent Rb atoms, one Cd atom, one Cl atom, and two I atoms. Rb₁ and Rb₂ atoms are coordinated with I and Cl atoms, respectively, forming two [RbCl₄I₅] structural units. The [Rb₁Cl₄I₅] and [Rb₂Cl₄I₅] groups are interconnected through four shared Cl atom faces, constructing a three-dimensional framework structure. Cd atoms are connected to four Cl atoms and two I atoms, forming asymmetric [CdCl₄I₂] octahedral units. Each [CdCl₄I₂] octahedron connects to the other three octahedrons through shared Cl atoms, forming a two-dimensional planar chain structure. These chains are arranged in the aob plane to form a layered structure. Rb atoms are located in the voids surrounded by four octahedrons, connecting the layers along the c-axis through Rb-I bonds, thus forming a three-dimensional framework structure.
[0030] The metal halide nonlinear optical crystal Rb₂CdCl₂I₂ prepared in Example 1 was ground into powder, and the X-ray diffraction pattern of the Rb₂CdCl₂I₂ powder was compared with the theoretical value. Figure 3 It can be seen that the XRD pattern of the metal halide Rb₂CdCl₂I₂ product prepared in Example 1 is basically consistent with the theoretically calculated pattern, with no diffraction peaks indicating impurities. This shows that the Rb₂CdCl₂I₂ prepared in Example 1 has high purity. The Rb₂CdCl₂I₂ crystal belongs to the orthorhombic crystal system, space group I4mm, with cell parameters a = 5.31210(10), b = 5.31210(10), c = 17.7062(10), Z = 2, and unit cell volume V = 499.64 Å. 3 .
[0031] The metal halide nonlinear optical crystal Rb₂CdCl₂I₂ prepared in Example 1 was ground into powder, and the thermogravimetric curve of the Rb₂CdCl₂I₂ powder was tested as shown in the figure. Figure 4 As shown, from Figure 4 It can be seen that the thermal stability of Rb2CdCl2I2 is about 500℃, and it continues to lose weight until it reaches a plateau at about 810℃.
[0032] The optical bandgap diagram of the Rb₂CdCl₂I₂ crystal prepared in Example 1 was determined using ultraviolet-visible diffuse reflectance spectroscopy, as shown below. Figure 5 As shown, from Figure 5 It can be seen that the crystal Rb2CdCl2I2 has a band gap of 3.70 eV.
[0033] Figure 6 This is the infrared spectrum of the Rb₂CdCl₂I₂ crystal prepared in Example 1. Figure 6 It can be seen that the Rb2CdCl2I2 crystal does not exhibit strong absorption caused by chemical bond vibrations in the entire 2.5 μm~25 μm transmission range, and the transmission region covers the two key mid-infrared atmospheric windows of 3~5 μm and 8~14 μm.
[0034] Figure 7 This study investigated the relationship between the particle size of Rb₂CdCl₂I₂ crystals prepared in Example 1 and the laser damage threshold of commercial AgGaS₂ crystals. The laser damage thresholds of Rb₂CdCl₂I₂ and AgGaS₂ crystals were measured in six different particle size distribution ranges: 38–48 µm, 48–75 µm, 75–108 µm, 108–150 µm, 150–212 µm, and 212–270 µm. Figure 7 As shown, the LIDT values of Rb₂CdCl₂I₂ are 55.9, 56.2, 56.1, 56.2, 56.4, and 56.6 MW·cm⁻¹, respectively. -2The LIDT values for the corresponding particle size groups of AGS are 2.8, 2.8, 2.9, 3.1, 3.0, and 3.2 MW·cm⁻¹. -2 Within the same particle size range, the LIDT of Rb2CdCl2I2 is approximately 19 times that of AGS, demonstrating excellent resistance to laser damage.
[0035] Example 2: The preparation method of the metal halide nonlinear optical crystal in this example is carried out according to the following steps:
[0036] 1. Weigh 0.2184 g of analytical grade RbCl and 0.3662 g of analytical grade CdI2 and add them to 30 mL of deionized water. Stir and dissolve evenly to obtain a mixed solution.
[0037] 2. Transfer the mixed solution to a 50 mL polytetrafluoroethylene liner, then place the liner into a stainless steel reactor and tighten it. Place the reactor in an oven and heat it to 230 °C at a rate of 30 °C / h and hold it for 24 h. Then cool it down to room temperature at a rate of 10 °C / h.
[0038] 3. Filter the product in the polytetrafluoroethylene liner, divide the filtrate into two equal parts, place them in two beakers respectively, seal them with a membrane, and make several small holes in the membrane to form a non-sealed state; then place one beaker in an oven at 30 ℃ to stand and evaporate, crystal particles grow at the bottom of the beaker. After 10 days of growth, the crystal size does not change significantly, and the growth ends; place the other beaker in an oven at 40 ℃ to stand and evaporate, crystal particles grow at the bottom of the beaker. After 10 days of growth, the crystal size does not change significantly, and the growth ends; wash the obtained colorless block crystals with deionized water to obtain two metal halide nonlinear optical crystals.
[0039] The two metal halide nonlinear optical crystals Rb₂CdCl₂I₂ prepared in Example 2 were ground into powders, and their X-ray diffraction patterns were measured and compared with theoretical values. Figure 8 As shown, from Figure 8 It can be seen that the XRD patterns of the target products at volatilization temperatures of 30 ℃ and 40 ℃ show diffraction peaks of impurity CdI2 and other byproducts, indicating low purity. This also shows that excessively high or low volatilization crystallization temperatures reduce crystal quality.
Claims
1. A metal halide nonlinear optical crystal, characterized in that, The chemical formula of the nonlinear optical crystal is Rb2CdCl2I2. The structure of the nonlinear optical crystal does not have a center of symmetry, belongs to the orthorhombic crystal system, space group I4mm, and the cell parameters are a = 5.31210(10), b = 5.31210(10), c = 17.7062(10), Z = 2.
2. The method for preparing a metal halide nonlinear optical crystal according to claim 1, characterized in that, The method is performed according to the following steps:
1. Weigh RbCl and CdI2 according to the stoichiometric ratio of the metal halide Rb2CdCl2I2; dissolve the weighed RbCl and CdI2 in deionized water to obtain a mixed solution; 2. Transfer the mixed solution into a polytetrafluoroethylene liner, then place the liner into a stainless steel reactor and tighten it. Place it in an oven and heat it to 200-230 ℃ at a rate of 25-30 ℃ / h and keep it at that temperature for 24-48 h. Then cool it down to room temperature at a rate of 5-10 ℃ / h.
3. Filter the product from the polytetrafluoroethylene liner. Place the filtrate in a non-sealed container and allow it to stand and evaporate at 33-37°C. Crystal particles will grow at the bottom of the container until the crystal size shows no significant change, at which point the growth ends. Wash the resulting colorless blocky crystals with deionized water to obtain a metal halide nonlinear optical crystal. The chemical formula of this crystal is Rb₂CdCl₂I₂.
3. The method for preparing a metal halide nonlinear optical crystal according to claim 2, characterized in that, The purity of RbCl and CdI2 mentioned in step one reaches analytical grade or above.
4. A method for preparing a metal halide nonlinear optical crystal according to claim 2 or 3, characterized in that, The non-sealed container mentioned in step three is formed by placing the filtrate inside the container, sealing it with a membrane, and punching small holes in the membrane to create a non-sealed state.
5. A method for preparing a metal halide nonlinear optical crystal according to claim 2 or 3, characterized in that, The temperature mentioned in step three is 35°C.
6. A method for preparing a metal halide nonlinear optical crystal according to claim 2 or 3, characterized in that, The statement in step three that the crystal size did not change significantly means that the increase in crystal size was less than 5% within 2 days.