Multicell model of La 3 Ga 5 GeO 14 crystal: A new approach to the description of the short-range order of atoms
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Multicell Model of La3Ga5GeO14 Crystal: A New Approach to the Description of the Short-Range Order of Atoms A. P. Dudka Shubnikov Institute of Crystallography, Crystallography and Photonics Federal Scientific Research Center, Russian Academy of Sciences, Moscow, 119333 Russia e-mail: [email protected] Received August 11, 2016
Abstract—The multicell model alternative to the model of mixed atomic sites used now is proposed for a single crystal of La3Ga5GeO14 belonging to the langasite family. The multicell consists of four unit cells. In three identical cells of the structure, atoms adapt to the Ge atom occupying one of the two 2d positions on the threefold symmetry axis. In the fourth cell, atoms surround the Ge atom located at the 1a position. The multicell model allows one to study the short-range order of atoms by the methods of classical structure analysis based on Bragg scattering. Four high-resolution data sets measured at 295 and 111.5 K are used in the study. The results are obtained with high relative precision (space group P321, Z = 1; at 295 K a = 8.2020(6) Å and c = 5.1065(6) Å, R/wR = 0.81/0.73% for 3829 unique ref lections; at 111.5 K a = 8.1939(1) Å and c = 5.1022(4) Å, R/wR = 0.85/0.76% for 3880 reflections). DOI: 10.1134/S106377451703004X
INTRODUCTION Nowadays crystals of the langasite family are the most promising piezoelectric materials for the design of transducers of physical quantities and devices of acoustoelectronics of new generation; they can replace with advantage conventionally used piezoelectric materials, namely, piezoceramics and α-quartz [1]. The discovery of magnetic ordering in iron-containing langasites in 2010 [2] made these compounds promising multiferroics and initiated a new wave of publications devoted to crystals of this family. The Ca3Ga2Ge4O14 structural type (CGG, space group Р321, Z = 1) was discovered in [3]; the family takes its name from the langasite crystal La3Ga5SiO14 (LGS) [4]. Isomorphous substitution Si → Ge results in the La3Ga5GeO14 crystal (LGG) studied in this work. In distinction to crystals of LGS or La3Ta0.5Ga5.5O14 and La3Nb0.5Ga5.5O14 [5], whose structure and properties have been studied in hundreds of works mentioned in review [6], publications devoted to LGG are few in number. The structure of LGG was determined in [7] (relative precision R ~ 4% for 1693 reflections); the growth and laser properties of these crystals were also described there; optical and spectroscopic properties were measured in [8–10]; and optical characteristics were summarized in [11, 12]. The recent results on luminescence in LGG : Cr3+, Yb3+, Er3+ offer promise for application of the crystals in medicine [13]. Four types of polyhedra are distinguished in the structure of crystals belonging to the langasite family:
the La atom (site 3e) is located at the center of a distorted Thomson cube, a Ga atom (site 1a) is located in an octahedron, another Ga atom (site 3f) occupies a large tetrahedron, and the Ge (in LGG) or Si (in LGS) atom shares the 2d s
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