On symmetry of sillenites
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On Symmetry of Sillenites T. I. Mel’nikovaa, G. M. Kuz’michevaa, V. B. Rybakovb, N. B. Bolotinac, and A. B. Dubovskyd a
Lomonosov State Academy of Fine Chemical Technology, Moscow, Russia email: [email protected] b Moscow State University, Moscow, 119992 Russia c Shubnikov Institute of Crystallography, Russian Academy of Sciences, Leninskii pr. 59, Moscow, 119333 Russia d AllRussia Research Institute for Synthesis of Mineral Raw Materials, ul. Institutskaya 1, Aleksandrov, Vladimirskaya oblast, 601600 Russia Received June 22, 2009
Abstract—A precise Xray diffraction study of a sillenite crystal with the initial composition Bi24(SixMn1 – x)2O40 grown by the hydrothermal method showed the presence of diffraction reflections that make it possible to assign this crystal to the sp. gr. P23, in contrast to the Bi24Si2O40 crystal with the sp. gr. I23. The dis tribution of cations over the two positions of the crystal structure of the general composition Bi24(Bi0.130(3)Si0.277(3)Mn1.593(3))O40 with three atoms (Bi, Si, Mn) in one position of the sp. gr. P 23 is estab lished. The disymmetrization of the Bi24(Bi, Si, Mn)2O40 crystal is explained by the kinetic order–disorder phase transition, which is caused by the presence of several atoms with different crystallochemical properties in one crystallographic position of the structure, in contrast to the Bi24Si2O40 crystal, where this phenomenon is absent. DOI: 10.1134/S1063774510020112
INTRODUCTION Sillenites are phases crystallized into the structures of the γBi2O3 type with z = 13 or (Bi1)24(Bi2)2O40 – δ with z = 1 (sp. gr. I23) [1]. In this structure, Bi1 atoms are in the pentacoordinated position 24f (the coordi nation polyhedron (CP) is a distorted halfoctahe dron, (Bi1)O5; it is supplemented to the distorted octahedron by unshared electron pair 6s2), while Bi2 atoms occupy the position 2а (CP is a tetrahedron, (Bi2)O4). O2 atoms (the coordination number is 3 and CP is a solid angle) and O3 atoms (the coordination number is 4 and CP is a trigonal pyramid) are located on the threefold axes (position 8с), while O1 atoms (the coordination number is 3 and CP is a solid angle) are in the position 24f. Due to the specificity of the sillenite structure, cat ions with tetrahedral coordination are located in the Bi2 position; this is caused by either their crystal lochemical nature or the effect of the crystal structure, which assigns a given coordination number with a cer tain formal charge to ion. All this leads to the forma tion of a large number of compositions of the Bi24M2O40 ± δ and Bi24(M'M'')2O40 ± δ types, which have a variety of properties. An occupation of the same position, Bi2, by several cations having different chemical and crystallochemical properties (size, elec tronegativity, and formal charges) should facilitate the manifestation of the kinetic (growth) order–disorder phase transition [2]. Ordering should depend not only
on the specificity of the structure and component properties, but also on the growth method and condi
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