Development of methods of X-ray diffraction analysis for determining the composition and structure of sillenite-family c
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E OF INORGANIC COMPOUNDS
Development of Methods of XRay Diffraction Analysis for Determining the Composition and Structure of SilleniteFamily Crystals T. I. Mel’nikovaa, G. M. Kuz’michevaa, N. B. Bolotinab, and N. V. Sadovskayac a
State University of Fine Chemical Technology, Moscow, Russia email: [email protected] b Shubnikov Institute of Crystallography, Russian Academy of Sciences, Leninskii pr. 59, Moscow, 119333 Russia c Karpov Institute of Physical Chemistry, Moscow, Russia Received December 3, 2012
Abstract—A methodology for refining the crystal structure of sillenites of nominal composition Bi24M2O40 based on the choice of the correct initial model and thermal atomic parameters is reported. The validity of the approach proposed is demonstrated by examples of crystals with M = Si, Fe, or V, for which the real com position is found with allowance for the composition of each structural site. Individual structural details are confirmed by IR and Raman spectroscopy data. DOI: 10.1134/S1063774514020151
INTRODUCTION Bi24M2O40 compounds and Bi24(M ',M' ')2O40 solid solutions (M, M ', and M' ' are elements of II–VIIIB groups) are crystallized into a sillenite structure (γ Bi2O3 with z = 13 or Bi24Bi2O40 – δ with z = 1, sp. gr. I23). Bi atoms in the Bi24M2O40 structure (24f site, symmetry 1; x ∼ 0.8, y ∼ 0.9, and z ∼ 0.7) are located in a distorted halfoctahedron ([BiO5E] polyhedron, where E denotes an unshared electron pair); М (M ', M '') atoms (2а site, symmetry 23; coordinates (0, 0, 0)) are positioned in a tetrahedron; and oxygen atoms form three different regular systems of points: O1 (site 24f, symmetry 1; x ∼ 0.1, y ∼ 0.2, and z ∼ 0.5), O2, and O3 (site 8c, symmetry 3) with the coordinates (x, x, x), where х ∼ 0.2 and 0.9, respectively [1]. An analysis of the Xray and neutron diffraction data on sillenite polycrystals and single crystals revealed some contradictions in the study of their structure and composition and in the methodical fea tures of structure refinement, in particular, (i) different compositions of phases obtained from charges with the same nominal composition (Bi24V2O4): Bi24(BiV)O40 [2], Bi24[Bi0.06V1.78䊐0.16]O40O(i)0.54 [3], + 5+ and (Bi23.4(1)䊐0.6)[Bi30.04(2) V1.96 ]O40 [4]; (ii) differences in their crystal structures: splitting of the O3 crystallographic site with formation of alter nating “small” and “large” [V5+O4]– and [Bi3+O4]– tetrahedra in Bi24[(BiO4)(VO4)]O32 and Bi24(BiV)O40 [2] structures, vacancies in the tetrahedral site jointly with incorporated О4 atom in the Bi24[Bi0.06V1.78䊐0.16]O40O(i)0.54 structure [3], and
vacancies in the halfoctahedral Bi1 site in the + 5+ (Bi23.4(1)䊐0.6)[Bi30.04(2) V1.96 ]O40 structure [4]; (iii) the absence of reliable data on the coordina tion of Bi ions; (iv) the occurrence of an additional peak with coordinates ∼(0.02, 0.02, 0.02) (8с site) in the residual electron density maps at large values of the “atom” thermal parameter in the tetrahedral site in the Bi324+ (Fe13+䊐1)(Bi13+)i(O39䊐0.1) structure, where a ternary “umbrellalike” trigon
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