X-Ray Natural Circular Dichroism in Langasite Crystal at the Ga and La Edges
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RACTION AND SCATTERING OF IONIZING RADIATIONS First Russian Crystallographic Congress
X-Ray Natural Circular Dichroism in Langasite Crystal at the Ga and La Edges A. P. Oreshkoa,*, B. V. Milla, E. N. Ovchinnikovaa, A. Rogalevb, F. Wilhelmb, and V. E. Dmitrienkoc a Moscow
State University, Moscow, Russia European Synchrotron Radiation Facility, Grenoble, France c Shubnikov Institute of Crystallography, Federal Scientific Research Centre “Crystallography and Photonics,” Russian Academy of Sciences, Moscow, 119333 Russia *e-mail: [email protected] b
Received June 5, 2017
Abstract—X-ray natural circular dichroism (XNCD) and its structural nature have been investigated in a langasite (La3Ga5SiO14) crystal at an incident radiation energy close to the Ga K-absorption edge and the La L2,3-absorption edges. An XNCD signal was observed mainly beyond the absorption edge, which confirmed the existence of delocalized mixed p–d electronic states in Ga and d–f and d–p electronic states in La. Calculations with application of the multiple scattering method have made it possible to separate the contributions from three crystallographically nonequivalent Ga sites to the absorption spectrum and the XNCD signal and explain adequately the largest contribution of the Ga atom occupying the 1a site to the XNCD signal. DOI: 10.1134/S1063774518020189
INTRODUCTION The main tendency in the development of modern materials science is the search for new multifunctional materials with properties that, however extraordinary, are important for practical application. An example of these materials is crystals of langasite (La3Ga5SiO14) family with the general formula A3BC3D2O14, which may contain cations of different elements [1]. To date, the langasite family is known to include about 200 compounds, whose characteristics vary in wide limits. Their piezoelectric, nonlinear optical, luminescent, and acousto-optic properties are of greatest interest [2–7]. In addition, at low temperatures, compounds containing 3d cations possess an exotic magnetic structure [8–11] and are multiferroics (ferroelectromagnets); i.e., they exhibit simultaneous coexistence of magnetic and optical or electric ordering [12, 13]. The crystal structure of most langasites is described by the same symmetry. At the same time, their physical properties depend on the point symmetry of individual atoms and lattice geometry details [14–16]. Thus, an urgent problem is to investigate the electronic properties of each site in the langasite structure. In this study, we jointly used for the first time the X-ray natural circular dichroism (XNCD) method and ab initio quantum-mechanical simulation to solve this problem. This approach was already applied by us
to analyze the nature of the XNCD signal in a CsCuCl3 single crystal [17, 18] and investigate the local electronic structure of copper ions in a CuB2O4 crystal [19]. A specific feature of this study is related to the fact that gallium atoms occupy three nonequivalent sites in langasite, which simultaneously contribute to the
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