The sillenites are a crystalline material group with the general formula Bi₂₄M₂O₄₀ [1] (sp.gr. I23, z=1), which demonstrate interesting physical properties depending on their composition. Bi₂₄M⁴⁺₂O₄₀ are named as ideal sillenites (IS). Deviations from the M⁴⁺ formal charge (FC) lead to a defect structure (DS) based on vacancies (Bi₂₄[M²⁺⁽³⁺⁾_2-y[]_y](O_40-d[]_d); []-vacancy) and/or incorporations of the O in the crystal structure (Bi₂₄[M⁵⁺₂](O_40+d). There are solid solutions, which can describe as Bi₂₄(M’,M”₂)O₄₀. They phases may be both IS and DS that depend on the FC of M’ and M’’ ions. The aim of this paper to determine a composition and structural peculiarities of some sillenites using results of X-ray powder diffraction.

All samples of nominal composition Bi₂₄M₂O₄₀ (M⁴⁺=Si, Mn) and Bi₂₄(M’,M”₂)O₄₀ (M'=Si⁴⁺, M” = Mn, V)  were synthesized by hydrothermal method. The samples ground into powder were examined by X-ray diffraction on HZG-4A diffractometer (room temperature, СuKα, graphite monochromator, a stepwise mode with an exposure time of 15 s and a step of 0.02°, 2Θ range 10°-100°). Refinement of crystal structures (the position parameters, the site occupancies and the anisotropic thermal parameters for all atoms) was accomplished using the Rietveld suite of programs DBWS-9411.

As a results of crystallochemical analysis of X-ray data and the valence force method applying the formula [2]: k_i=ν_c/[Σ(1/r_iⁿ)], where n=2.5 (our data for Bi), k_i (table data), and r_i Å are the experimental values of the interatomic Bi-O and M-O distances main conclusions were made:

- An increase of Bi-O average interatomic distances in BiO₅ polyhedra is associated with a reduction of M-O distances in MO₄ tetrahedra and vice versa.

- A linear dependence and negative deviation from a linear dependence of the cell parameters vs. value x for phases of the systems (1-x)Bi₂₄(Bi³⁺_1.4[]_0.6)(O_38.4[]_1.6) – xBi₂₄Mn⁴⁺₂O₄₀  and (1-x)Bi₂₄Si⁴⁺₂O₄₀ – xBi₂₄Mn⁴⁺₂O₄₀ can be caused by the FC Mn⁴⁺ and by the different compression of initial phases, respectively.

- The cell parameters of «Bi₂₄V⁴⁺₂O₄₀» and «Bi₂₄V⁵⁺₂O₄₁» phases were estimated and they were used to find the FC V in phases of systems (1-x)Bi₂₄(Bi³⁺_1.4[]_0.6)(O_38.40[]_1.60) – x«Bi₂₄V⁴⁺⁽⁵⁺⁾₂O₄₀₍₄₁₎» and (1-x)Bi₂₄Si⁴⁺₂O₄₀ – x«Bi₂₄V⁴⁺⁽⁵⁺⁾₂O₄₀₍₄₁₎». Obviously the anomalous behavior of the cell parameter and the M-O distance of phase with the general formula Bi₂₄(Si⁴⁺,V)₂O₄₀ is due to the presence of FC V⁴⁺ that promotes the green colour of this sample.

- The difference between the FC M’ and M” ions in Bi₂₄(M’,M”₂)O₄₀ phases and the non-optimal ionic radii of M (M’ and M”) ions are a possible reasons of DS formation.

1. S.F. Radaev, V.I. Simonov, Kristallografiya. 1992, 37, 914
2. Y.A. Pyatenko, Kristallografiya. 1972, 17, 773.

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