Correlation between the Parameters of the Dispersion Relations of Refractive Indices and Coordination Numbers of Cations
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CAL PROPERTIES
Correlation between the Parameters of the Dispersion Relations of Refractive Indices and Coordination Numbers of Cations for the Langasite-Family Crystals N. I. Sorokin* Shubnikov Institute of Crystallography, Federal Scientific Research Centre “Crystallography and Photonics,” Russian Academy of Sciences, Moscow, 119333 Russia *e-mail: [email protected] Received July 6, 2020; revised July 6, 2020; accepted July 8, 2020
Abstract—Correlation between the parameters of the dispersion relations of refractive indices and coordination numbers of cations in the structure is found for langasite-family La3Ga5SiO14 optical crystals (Ca3Ga2Ge4O14 structure type, sp. gr. P321). Keywords: langasite, oxides, single crystals, refractive index, coordination number DOI: 10.1134/S1063783420110359
1. INTRODUCTION Complex langasite-family oxides (La3Ga5SiO14) belong to the Ca3Ga2Ge4O14 structure type (sp. gr. P321). Their chemical formula can be written as A3BC3D2O14, where A, B, C, and D cations are located at the sites of four types (regular systems of points of the sp. gr. P321). The ability of the langasite structure to wide isomorphous substitutions of cations makes the chemical composition more complicated and provides many new crystals. Langasite-family crystals are multifunctional materials, which exhibit piezoelectric, optical, laser, luminescence, acoustic, and magnetic properties. They are intensely studied in rapidly developing fields of piezoelectronics, acoustoelectronics, photonics, and physics of multiferroics (see references in [1]). The studies devoted to the formation of new langasite crystals and analysis of their atomic structures and physical properties are under way. The optical properties of A3BC3D2O14 crystals are characterized by the absence of electronic and vibrational transitions in the spectral range of 0.2–8 μm (transparency window). Fundamental characteristics of optical crystals, which underlie structural refractometry, are refractive index n and its dispersion relation n(λ) [2]. Refractive indices n(λ) must be known for calculation of the operating characteristics of devices based on optical materials and application of refractometric data in structural chemistry. One of the problems of structural refractometry is to reveal correlations between the optical (refractive index) and crystallochemical (chemical bonding,
atomic coordination, and structure type) characteristics of ionic crystals [2, 3]. Although more than two hundred compounds and solid solutions belong to the langasite family, the experimental data on refractive indices are known for less than ten of its representatives [4–6]. In order to measure the refractive indices of langasites, they must be grown in the form of bulk single crystals of high optical quality. The growth of these crystals is a complex technological problem, which requires great resource expenditures. In addition to the experimentally measured refractive indices, these parameters for fifteen new crystals were calculated theoretically in [1] using molecula
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