An XAS study of the local structure in Czochralski-grown optical scheelite-type sodium-gadolinium molybdates
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ORIGINAL RESEARCH
An XAS study of the local structure in Czochralski-grown optical scheelite-type sodium-gadolinium molybdates Galina M. Kuz’micheva 1 & Evgeny V. Khramov 2 & Irina A. Kaurova 1 Received: 21 August 2020 / Accepted: 14 September 2020 # Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract The local environment of Mo6+ ions in the structures of scheelite-type crystals (space group I41/a) with the initial compositions (Na1/2Gd1/2)MoO4 (NGM 1:1) and (Na2/7Gd4/7□1/7)MoO4 (NGM 1:2; □ - vacancies) was determined by X-ray absorption spectroscopy (XAS) for the first time. Different coordination environments of Na1+ and Gd3+ ions caused primarily by the difference in their formal charges have been established based on the comparison of Mo–Gd and Mo–Na interatomic distances. According to resent results and structural data known for the (Na1-хREx)MO4 crystals (RE3+ = rare-earth cations), the character of distribution of different Na1+ and RE3+ polyhedra over NGM crystal structures is influenced by the sizes and electronegativities of Na1+, RE3+, and М6+ (M6+ = Mo, W) ions, the Na1+:RE3+ ratio, and vacancies in the M site. Disordered distribution of Na1+ and RE3+ polyhedra in NGM occurs without changing the symmetry of crystals (space group I41/a) and ordered one is accompanied by reduction of crystal symmetry. Keywords Crystals . Scheelite family . X-ray absorption spectroscopy . Isomorphism . Statistical structure . Local structure
Introduction Powellite CaMoO4 and phases with the initial composition (Na0.5RE0.5)MO4 (RE3+ is rare-earth metal ion; M6+ is W or Mo) crystallize in the scheelite structure (space group I41/a, Z = 4) [1] or its derivatives [2, 3]. In the scheelite structure, the Ca cations and (Na,RE) «cations» occupy the Wyckoff site 4a (symmetry S4) with coordinates 0 0 0 forming the dodecahedra with refracted upper and lower faces with two different Ca–O or (Na,RE)–O interatomic distances with CN Ca or (Na,RE) = 4 + 4 (CN is a coordination number); i.e., oxygen ions are located on two spheres with different cation-anion distances. The M6+ cations occupy the Wyckoff site 4b Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11224-020-01641-6) contains supplementary material, which is available to authorized users. * Irina A. Kaurova [email protected] 1
MIREA - Russian Technological University, 78 Vernadsky Ave., Moscow, Russia 119454
2
National Research Center «Kurchatov Institute», 1 Akademika Kurchatova pl., Moscow, Russia 123182
(symmetry S4) with coordinates 0 0 0.5 and form elongated tetrahedra with four equal M–O distances (CN M = 4); i.e., oxygen ions are on the same sphere. Oxygen ions are coordinated, in turn, by two Ca or (Na,RE) atoms and one M6+ atom and occupy general site 16c (symmetry С1) with coordinates x y z. Interest in these objects is determined, on the one hand, by their high scientific importance for solving such fundamental problems as the establishment of correlations between composition, structure, a
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