Structure-property relationship for complex-substituted molybdates and tungstates of the scheelite family
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E OF INORGANIC COMPOUNDS
Structure–Property Relationship for ComplexSubstituted Molybdates and Tungstates of the Scheelite Family G. M. Kuz’michevaa, E. A. Zagorul’koa, N. B. Bolotinab, V. B. Rybakovc, E. V. Zharikovd, e, D. A. Lise, and K. A. Subbotine a
Lomonosov Moscow State University of Fine Chemical Technologies, pr. Vernadskogo 86, Moscow, 119571 Russia email: [email protected] b Shubnikov Institute of Crystallography, Russian Academy of Sciences, Leninskii pr. 59, Moscow, 119333 Russia c Moscow State University, Moscow, 119992 Russia d Mendeleyev University of Chemical Technology of Russia, Miusskaya pl. 9, Moscow, 125047 Russia e Prokhorov General Physics Institute, Russian Academy of Sciences, ul. Vavilova 38, Moscow, 117942 Russia Received February 13, 2013
Abstract—A highprecision Xray diffraction study of crystals with the nominal compositions (Na0.5Y0.495Er0.005)WO4 and (Na0.5Gd0.25La0.25)MoO4 and the subsequent refinement and analysis of their compositions showed that an increased number of vacancies at the tungsten sites contributes to the cracking of crystals. It was also found that an associate—a color center (a vacancy at the molybdenum site and an elec tron localized near the vacancy)—gives the crystals a yellow color. The formation of regularly oriented isos tructural domains with coherent boundaries as a bulk defect was observed for the first time for crystals of the scheelite family. DOI: 10.1134/S1063774513050052
INTRODUCTION
one hand, and the type and concentration of the point defects in these structures, on the other hand, is almost unknown. In some studies the additional optical absorption was attributed to different factors. Thus, the black color of tungstates and molybdates was explained by oxygen vacancies and electrons compensating these vacancies (VOn• + e ') [8]; the lilac color of molybdates was explained by electrons on Mo6+ ions (Mo 'Mo ) [8]; the green color of tungstates was explained by elec trons on W6+ ions ( W W' ) [5]; and the yellow color of molybdates was explained by associates of a vacancy at n' the molybdenum site and a hole located nearby (VMo , • х nh ) serving as color centers [8]. In the present work, we studied crystal with the fol lowing nominal compositions: (Na0.5Y0.495Er0.005)WO4 (I); a sample characterized by an abnormally low crack resistance (I.1) and a sam ple whose crack resistance is typical of this class of compounds (I.2) (Table 1); (Na0.5Gd0.25La0.25)MoO4 (II); a nonannealed pale gray sample (II.1) and a yellowish sample annealed in air (II.2) (Table 2). The aim of the present study is to perform a high precision Xray diffraction study of these crystals of the scheelite family under the same experimental con ditions and determine the details of their composition and structure.
Both nominally pure and rareearthdoped crystals of double molybdates and tungstates with the general + 3+ formula (Na 10.5 R0.5)T O 4 (R = La, Gd, Y, Bi; T = Mo, W) belong to the scheelite family (sp. gr. I41/a, Z = 4; Na and R atoms occupy dodecahedral sites wit
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