rus
Issue #2/2022
V. A. Komornikov, I. S. Timakov, A. A. Kulishov
Rb2CuCl4 · 2H2O Crystal for Optical Applications
Rb2CuCl4 · 2H2O Crystal for Optical Applications
DOI: 10.22184/1993-7296.FRos.2022.16.2.126.134
The phase equilibria in the RbCl – CuCl2 · 2H2O – H2O system were examined in the temperature range of 25–50°C. The concentration limits of crystallization, the temperature dependence, and the congruent nature of solubility of the Rb2CuCl4 · 2H2O compound were determined. The Rb2CuCl4 · 2H2O crystal was obtained by the temperature reduction method and its optical transmittance spectrum was studied for the first time.
The phase equilibria in the RbCl – CuCl2 · 2H2O – H2O system were examined in the temperature range of 25–50°C. The concentration limits of crystallization, the temperature dependence, and the congruent nature of solubility of the Rb2CuCl4 · 2H2O compound were determined. The Rb2CuCl4 · 2H2O crystal was obtained by the temperature reduction method and its optical transmittance spectrum was studied for the first time.
Теги: band filter crystal growth optical spectrum water-soluble crystal водорастворимый кристалл зонный фильтр оптический спектр рост кристаллов
Rb2CuCl4 · 2H2O Crystal for Optical Applications
Komornikov V. A., Timakov I. S., Kulishov A. A.
Federal Research Center “Crystallography and Photonics” of the Russian Academy of Sciences, Moscow, Russia
The phase equilibria in the RbCl – CuCl2 · 2H2O – H2O system were examined in the temperature range of 25–50°C. The concentration limits of crystallization, the temperature dependence, and the congruent nature of solubility of the Rb2CuCl4 · 2H2O compound were determined. The Rb2CuCl4 · 2H2O crystal was obtained by the temperature reduction method and its optical transmittance spectrum was studied for the first time.
Keywords: Optical spectrum, crystal growth, water-soluble crystal, band filter
Received on: 04.03.2022
Accepted on: 18.03.2022
Introduction
The crystals of transition element salts can be used in photonics as the band filters in various portions of the spectrum [1]. A good example of this phenomena is the crystals of copper sulfate pentahydrate (CuSO4 · 5H2O) [2]. The optical transmittance spectrum of these crystals makes it possible to selectively emphasize the spectral region of 280–570 nm. Such properties of the copper sulfate crystal are due to the inner coordination environment of copper atoms in the crystal structure. The copper atoms are located in an octahedral environment of four oxygen atoms of water molecules and two oxygen atoms of sulfate groups [3].
In order to expand the field of application of optical crystals in photonics, it is necessary to study the possibilities of controlled shifting of the crystal’s bandpasses to one or another portion of the spectrum. The most efficient method to control the spectral specifications is to change the inner coordination environment of the transition metal atom in the crystal, while maintaining the octahedral symmetry of this environment.
An applicable ligand suitable for this purpose is the chloride ion (Cl–). This ligand, similar to water and sulfate anion, belongs to the group of soft ligands, i. e. it does not change the symmetry of the coordination environment of copper atoms in the crystal structure. Thus, during the formation of complex octahedra in the crystal structures of copper chloride, chlorine enters into a competitive interaction with water to generate octahedra with the mixed ligand composition.
Due to the given features of the inner coordination environment in the copper chloride crystals, the crystal of rubidium-copper dichloride dihydrate Rb2CuCl4 · 2H2O provokes some interest. This crystal belongs to the tetragonal crystal system, space group of symmetry: P42 / mnm, with the following parameters: a = 7.596(2), c = 8.027(3) Å, V = 463.1(2) Å3, Z = 2, Dx = 2.957 g / cm3 [4]. Moreover, if the structural crystal data are presented in the literature, then the reliable data relating to the transmission spectra in the visible and UV regions for this crystal are not provided.
Having considered the chemical nature of the crystal, the most efficient way for its generation for studying the transmission spectrum is the controlled growth using a low-temperature aqueous solution. Thus, the purpose of this paper is to determine the main parameters of Rb2CuCl4 · 2H2O crystallization process and to examine its optical spectrum in the visible and UV regions.
Materials and methods
In order to study the phase equilibria, in this paper we used rubidium chloride (RbCl, ultra high purity) and copper (II) chloride dihydrate (CuCl2 · 2H2O, pure). To obtain the Rb2CuCl4 · 2H2O crystal, the same reagents were used for the polycrystalline compound synthesis, followed by the subsequent recrystallization.
The phase equilibria studies were performed by the simultaneous parallel microcrystallizations at the temperatures of 25.0, 30.0, 40.0, 50.0 °C. This examination was carried out in a special laboratory shaking thermostat with a movable vessel ejection rack (for mixing in the temperature-controlled vessels) and a programmable temperature PID controller. A series of mother liquors were prepared in the idential sealed vessels (crystallizers) with a variable ratio of initial dry components and a minimum amount of distilled water (5 ml). The solubility of weighed components was then determined by repeated addition of water in small portions (1–5 ml / day) until the saturated solutions were obtained with a minimum sediment content at the crystallizer bottom. After determining the solubility, the mother liquors were additionally kept for two days at the specified temperature. Such exposure was necessary to strike the dynamic interphase equilibrium between the saturated solution and sediment in the crystallizer. After such exposure, temperature of the crystallizer tanks was reduced according to the same procedure by 5°C from the initial value, at which the solubility was determined, for several days. The crystals generated by this approach had the dimensions of 3–7 mm and were easily decanted from the mother liquor.
The crystals thus obtained were used to determine the phase composition of the solid phases being in equilibrium with the mother liquor by the X-ray powder diffraction method.
The X-ray diffraction analysis (XRD) of the pulverized single-crystalline samples was performed at a room temperature using a Rigaku Miniflex 600 X-ray diffractometer (Japan) (radiation of Cu with no filter, continuous shooting mode – 1 degree / minute, step value – 0.02° in the angle range of 2θ 5–65°, without sample rotation and in the ambient atmosphere).
The single-crystal growth of Rb2CuCl4 · 2H2O was performed on the basis of an aqueous solution by a scheduled temperature reduction of the saturated solution in accordance with the established temperature dependence of the solubility. The polished crystal obtained at the stage of phase equilibria examination in the RbCl – CuCl2 · 2H2O – H2O system with the dimensions of 2 × 3 × 5 mm was used as a seed crystal. The process was carried out in a glass crystallizer on an open immovable platform with reverse stirring; the temperature was monitored using a programmable PID controller.
The optical transmission spectra were studied using a Cary 300 spectrophotometer in the wavelength range of 200–800 nm.
Experiment performance
The phase equilibria study in the RbCl – CuCl2 · 2H2O – H2O system was performed in the entire range of ratios (RbCl) : (CuCl2 · 2H2O) with a step of 5 mole percent at the temperatures of 25, 30, 40 and 50°C. Three crystallization branches were found in the system at all temperatures: the crystallization branches of the initial components CuCl2 · 2H2O and RbCl, and the crystallization branch of rubidium-copper dichloride (Rb2CuCl4 · 2H2O). The relevant X-ray diagrams are shown in Fig. 1.
The solubility was increased with the rise of temperature for all compounds of the studied system. The solubility nature of double chloride in Rb2CuCl4 · 2H2O was congruent. The study results of phase equilibria at all temperatures are given in Table. Fig. 2 demonstrates the equilibrium diagram of the system at the temperatures of 25 and 50°C.
The crystallization region of double chloride in Rb2CuCl4 · 2H2O was quite wide in terms of the ratio RbCl : CuCl2 · 2H2O: from 25 : 75 to 80 : 20 in molar fractions without regard to the solvent. The solubility nature was congruent.
Based on the study results of phase equilibria in the RbCl – CuCl2 · 2H2O – H2O system at various temperatures, the Rb2CuCl4 · 2H2O solubility was also determined for a solution with a stoichiometric ratio of the initial components. This temperature dependence is shown in Fig. 3 and is described by the linear equation CRb2CuCl4 · 2H2O = = 21,42 + 0,1461t.
Using the obtained temperature dependence of the solubility, a single crystal of Rb2CuCl4 · 2H2O was grown by the controlled temperature reduction method. The growth was obtained in a temperature range of 38–25°C using a 250 ml solution for 21 days. As a result of growth under the indicated conditions, a crystal was obtained with the dimensions of 25 × 25 × 17 mm and a weight of 15 grams (see Fig. 4).
The plates with the same thickness (~3 mm) were cut out from the resulting crystal in accordance with the diagram given in Fig. 4. The abovementioned plates were used to study the optical transmission spectra and evaluate the anisotropy of optical properties on a qualitative basis (see Fig. 5).
It has been determined that in the wavelength range of 200÷800 nm the main bandwidth is located in a rather wide range of values (375÷625 nm). In this case, the transmittance factor is dependent on the crystallographic index of the facet. The sample that has shown the maximum transmission value apparently belongs to the facet with index (001), since this facet is perpendicular to the main crystallographic axis of the crystal that coincides with the optical axis. Accordingly, a sample with the lower transmission value belongs to the {101} facet group.
Discussion and conclusions
The study of phase formation in the RbCl – CuCl2 · 2H2O – H2O system has revealed a wide concentration range of Rb2CuCl4 · 2H2O crystallization and the congruent nature of its solubility. The established temperature dependence of solubility is quite significant: 0.22% weight / degree that can be equivalent to the crystal growth by 0.35 g using a 100 ml solution with a temperature decrease by 1 °C.
The congruent nature of solubility means that Rb2CuCl4 · 2H2O can be recrystallized to dry state without phase composition losses that is very convenient when purifying the reagent by fractional crystallization. The given features make it possible to obtain the Rb2CuCl4 · 2H2O crystals using a relatively simple method of a saturated solution temperature decrease.
The Rb2CuCl4 · 2H2O crystal is tetragonal and optically uniaxial that makes it relatively easy to orient it to obtain the band filters with a bandwidth of 375–625 nm without any birefringent effect.
In comparison with the studied CuSO4∙5H2O crystal, the transmission band for Rb2CuCl4 · 2H2O is shifted to the long wavelength region (280–570 nm for sulfate and 375÷625 nm for chloride). This is primarily due to the difference in the inner coordination environment of copper atoms in the structures of such crystals. As it has been already mentioned, the octahedral environment of copper in copper sulfate is formed by six oxygens from two sulfate anions and four water molecules. In the Rb2CuCl4 · 2H2O crystal, the coordination environment of copper atoms is generated by four chlorine atoms and two oxygens of water molecules. Having considered that the sulfate anion and chlorine anion are weaker than water in terms of the fission strength in the octahedral field, the overall decrease in the water amount in the copper coordination environment leads to the transmission band shifting to the long wavelength region.
With due regard to the structural features of Rb2CuCl4 · 2H2O, its growth (relative ease of preparation), and optical properties, this crystal can be promising in terms of its practical application as an optical band filter. This crystal can be useful as a model site in studying the impact of isomorphous and isovalent replacement with other transition metals on the actual crystal structure and optical properties.
ACKNOWLEDGMENT
The paper was supported by the Ministry of Science and Higher Education as a part of works under the state terms of reference of the Federal Research Center “Crystallography and Photonics” of the Russian Academy of Sciences. The equipment of the Common Use Center of the Federal Research Center “Crystallography and Photonics” of the Russian Academy of Sciences was applied in the work.
AUTHORS
Komornikov Vladimir Andreevich, Cand.of Science(Chemistry);
e-mail: v.a.kom@mail.ru, Senior Researcher, Laboratory of Crystallization Processes, Institute of Crystallography, Federal Research Center «Crystallography and Photonics» RAS, https://crys.ras.ru, Moscow, Russia.
ORCID: 0000-0001-8965-7604
Timakov Ivan Sergeevich, research engineer, Laboratory of Crystallization Processes, Institute of Crystallography, Federal Research Center «Crystallography and Photonics» RAS, https://crys.ras.ru, Moscow, Russia.
ORCID: 0000-0002-3728-7004
Kulishov Artyom Andreevich, junior researcher, Laboratory of Crystallization Processes, Institute of Crystallography, Federal Research Center «Crystallography and Photonics» RAS, https://crys.ras.ru, Moscow, Russia.
ORCID: 0000-0001-5092-5026
Komornikov V. A., Timakov I. S., Kulishov A. A.
Federal Research Center “Crystallography and Photonics” of the Russian Academy of Sciences, Moscow, Russia
The phase equilibria in the RbCl – CuCl2 · 2H2O – H2O system were examined in the temperature range of 25–50°C. The concentration limits of crystallization, the temperature dependence, and the congruent nature of solubility of the Rb2CuCl4 · 2H2O compound were determined. The Rb2CuCl4 · 2H2O crystal was obtained by the temperature reduction method and its optical transmittance spectrum was studied for the first time.
Keywords: Optical spectrum, crystal growth, water-soluble crystal, band filter
Received on: 04.03.2022
Accepted on: 18.03.2022
Introduction
The crystals of transition element salts can be used in photonics as the band filters in various portions of the spectrum [1]. A good example of this phenomena is the crystals of copper sulfate pentahydrate (CuSO4 · 5H2O) [2]. The optical transmittance spectrum of these crystals makes it possible to selectively emphasize the spectral region of 280–570 nm. Such properties of the copper sulfate crystal are due to the inner coordination environment of copper atoms in the crystal structure. The copper atoms are located in an octahedral environment of four oxygen atoms of water molecules and two oxygen atoms of sulfate groups [3].
In order to expand the field of application of optical crystals in photonics, it is necessary to study the possibilities of controlled shifting of the crystal’s bandpasses to one or another portion of the spectrum. The most efficient method to control the spectral specifications is to change the inner coordination environment of the transition metal atom in the crystal, while maintaining the octahedral symmetry of this environment.
An applicable ligand suitable for this purpose is the chloride ion (Cl–). This ligand, similar to water and sulfate anion, belongs to the group of soft ligands, i. e. it does not change the symmetry of the coordination environment of copper atoms in the crystal structure. Thus, during the formation of complex octahedra in the crystal structures of copper chloride, chlorine enters into a competitive interaction with water to generate octahedra with the mixed ligand composition.
Due to the given features of the inner coordination environment in the copper chloride crystals, the crystal of rubidium-copper dichloride dihydrate Rb2CuCl4 · 2H2O provokes some interest. This crystal belongs to the tetragonal crystal system, space group of symmetry: P42 / mnm, with the following parameters: a = 7.596(2), c = 8.027(3) Å, V = 463.1(2) Å3, Z = 2, Dx = 2.957 g / cm3 [4]. Moreover, if the structural crystal data are presented in the literature, then the reliable data relating to the transmission spectra in the visible and UV regions for this crystal are not provided.
Having considered the chemical nature of the crystal, the most efficient way for its generation for studying the transmission spectrum is the controlled growth using a low-temperature aqueous solution. Thus, the purpose of this paper is to determine the main parameters of Rb2CuCl4 · 2H2O crystallization process and to examine its optical spectrum in the visible and UV regions.
Materials and methods
In order to study the phase equilibria, in this paper we used rubidium chloride (RbCl, ultra high purity) and copper (II) chloride dihydrate (CuCl2 · 2H2O, pure). To obtain the Rb2CuCl4 · 2H2O crystal, the same reagents were used for the polycrystalline compound synthesis, followed by the subsequent recrystallization.
The phase equilibria studies were performed by the simultaneous parallel microcrystallizations at the temperatures of 25.0, 30.0, 40.0, 50.0 °C. This examination was carried out in a special laboratory shaking thermostat with a movable vessel ejection rack (for mixing in the temperature-controlled vessels) and a programmable temperature PID controller. A series of mother liquors were prepared in the idential sealed vessels (crystallizers) with a variable ratio of initial dry components and a minimum amount of distilled water (5 ml). The solubility of weighed components was then determined by repeated addition of water in small portions (1–5 ml / day) until the saturated solutions were obtained with a minimum sediment content at the crystallizer bottom. After determining the solubility, the mother liquors were additionally kept for two days at the specified temperature. Such exposure was necessary to strike the dynamic interphase equilibrium between the saturated solution and sediment in the crystallizer. After such exposure, temperature of the crystallizer tanks was reduced according to the same procedure by 5°C from the initial value, at which the solubility was determined, for several days. The crystals generated by this approach had the dimensions of 3–7 mm and were easily decanted from the mother liquor.
The crystals thus obtained were used to determine the phase composition of the solid phases being in equilibrium with the mother liquor by the X-ray powder diffraction method.
The X-ray diffraction analysis (XRD) of the pulverized single-crystalline samples was performed at a room temperature using a Rigaku Miniflex 600 X-ray diffractometer (Japan) (radiation of Cu with no filter, continuous shooting mode – 1 degree / minute, step value – 0.02° in the angle range of 2θ 5–65°, without sample rotation and in the ambient atmosphere).
The single-crystal growth of Rb2CuCl4 · 2H2O was performed on the basis of an aqueous solution by a scheduled temperature reduction of the saturated solution in accordance with the established temperature dependence of the solubility. The polished crystal obtained at the stage of phase equilibria examination in the RbCl – CuCl2 · 2H2O – H2O system with the dimensions of 2 × 3 × 5 mm was used as a seed crystal. The process was carried out in a glass crystallizer on an open immovable platform with reverse stirring; the temperature was monitored using a programmable PID controller.
The optical transmission spectra were studied using a Cary 300 spectrophotometer in the wavelength range of 200–800 nm.
Experiment performance
The phase equilibria study in the RbCl – CuCl2 · 2H2O – H2O system was performed in the entire range of ratios (RbCl) : (CuCl2 · 2H2O) with a step of 5 mole percent at the temperatures of 25, 30, 40 and 50°C. Three crystallization branches were found in the system at all temperatures: the crystallization branches of the initial components CuCl2 · 2H2O and RbCl, and the crystallization branch of rubidium-copper dichloride (Rb2CuCl4 · 2H2O). The relevant X-ray diagrams are shown in Fig. 1.
The solubility was increased with the rise of temperature for all compounds of the studied system. The solubility nature of double chloride in Rb2CuCl4 · 2H2O was congruent. The study results of phase equilibria at all temperatures are given in Table. Fig. 2 demonstrates the equilibrium diagram of the system at the temperatures of 25 and 50°C.
The crystallization region of double chloride in Rb2CuCl4 · 2H2O was quite wide in terms of the ratio RbCl : CuCl2 · 2H2O: from 25 : 75 to 80 : 20 in molar fractions without regard to the solvent. The solubility nature was congruent.
Based on the study results of phase equilibria in the RbCl – CuCl2 · 2H2O – H2O system at various temperatures, the Rb2CuCl4 · 2H2O solubility was also determined for a solution with a stoichiometric ratio of the initial components. This temperature dependence is shown in Fig. 3 and is described by the linear equation CRb2CuCl4 · 2H2O = = 21,42 + 0,1461t.
Using the obtained temperature dependence of the solubility, a single crystal of Rb2CuCl4 · 2H2O was grown by the controlled temperature reduction method. The growth was obtained in a temperature range of 38–25°C using a 250 ml solution for 21 days. As a result of growth under the indicated conditions, a crystal was obtained with the dimensions of 25 × 25 × 17 mm and a weight of 15 grams (see Fig. 4).
The plates with the same thickness (~3 mm) were cut out from the resulting crystal in accordance with the diagram given in Fig. 4. The abovementioned plates were used to study the optical transmission spectra and evaluate the anisotropy of optical properties on a qualitative basis (see Fig. 5).
It has been determined that in the wavelength range of 200÷800 nm the main bandwidth is located in a rather wide range of values (375÷625 nm). In this case, the transmittance factor is dependent on the crystallographic index of the facet. The sample that has shown the maximum transmission value apparently belongs to the facet with index (001), since this facet is perpendicular to the main crystallographic axis of the crystal that coincides with the optical axis. Accordingly, a sample with the lower transmission value belongs to the {101} facet group.
Discussion and conclusions
The study of phase formation in the RbCl – CuCl2 · 2H2O – H2O system has revealed a wide concentration range of Rb2CuCl4 · 2H2O crystallization and the congruent nature of its solubility. The established temperature dependence of solubility is quite significant: 0.22% weight / degree that can be equivalent to the crystal growth by 0.35 g using a 100 ml solution with a temperature decrease by 1 °C.
The congruent nature of solubility means that Rb2CuCl4 · 2H2O can be recrystallized to dry state without phase composition losses that is very convenient when purifying the reagent by fractional crystallization. The given features make it possible to obtain the Rb2CuCl4 · 2H2O crystals using a relatively simple method of a saturated solution temperature decrease.
The Rb2CuCl4 · 2H2O crystal is tetragonal and optically uniaxial that makes it relatively easy to orient it to obtain the band filters with a bandwidth of 375–625 nm without any birefringent effect.
In comparison with the studied CuSO4∙5H2O crystal, the transmission band for Rb2CuCl4 · 2H2O is shifted to the long wavelength region (280–570 nm for sulfate and 375÷625 nm for chloride). This is primarily due to the difference in the inner coordination environment of copper atoms in the structures of such crystals. As it has been already mentioned, the octahedral environment of copper in copper sulfate is formed by six oxygens from two sulfate anions and four water molecules. In the Rb2CuCl4 · 2H2O crystal, the coordination environment of copper atoms is generated by four chlorine atoms and two oxygens of water molecules. Having considered that the sulfate anion and chlorine anion are weaker than water in terms of the fission strength in the octahedral field, the overall decrease in the water amount in the copper coordination environment leads to the transmission band shifting to the long wavelength region.
With due regard to the structural features of Rb2CuCl4 · 2H2O, its growth (relative ease of preparation), and optical properties, this crystal can be promising in terms of its practical application as an optical band filter. This crystal can be useful as a model site in studying the impact of isomorphous and isovalent replacement with other transition metals on the actual crystal structure and optical properties.
ACKNOWLEDGMENT
The paper was supported by the Ministry of Science and Higher Education as a part of works under the state terms of reference of the Federal Research Center “Crystallography and Photonics” of the Russian Academy of Sciences. The equipment of the Common Use Center of the Federal Research Center “Crystallography and Photonics” of the Russian Academy of Sciences was applied in the work.
AUTHORS
Komornikov Vladimir Andreevich, Cand.of Science(Chemistry);
e-mail: v.a.kom@mail.ru, Senior Researcher, Laboratory of Crystallization Processes, Institute of Crystallography, Federal Research Center «Crystallography and Photonics» RAS, https://crys.ras.ru, Moscow, Russia.
ORCID: 0000-0001-8965-7604
Timakov Ivan Sergeevich, research engineer, Laboratory of Crystallization Processes, Institute of Crystallography, Federal Research Center «Crystallography and Photonics» RAS, https://crys.ras.ru, Moscow, Russia.
ORCID: 0000-0002-3728-7004
Kulishov Artyom Andreevich, junior researcher, Laboratory of Crystallization Processes, Institute of Crystallography, Federal Research Center «Crystallography and Photonics» RAS, https://crys.ras.ru, Moscow, Russia.
ORCID: 0000-0001-5092-5026
Readers feedback
