Crystal chemistry of mixed Bi³⁺-Aⁿ⁺ (Aⁿ⁺=Na⁺, K⁺, Sr²⁺, Ba²⁺, Tl⁺, Pb²⁺) vanadium hollandite materials

Abstract

Hollandite oxides, A_xBi_yV₈O₁₆ (A=Na, K, Sr, Ba, Tl, Pb) with varying x, y ratios have been synthesized and studied. The feasibility of obtaining such materials under our normal solid state conditions is to be highlighted considering the fact that previous work on potassium-only compounds required high pressure and high temperature. It is therefore noteworthy that introduction of Bi³⁺ in the framework is easier than for Aⁿ⁺ co-cations. Crystal structures refined from single crystals of nominal compositions A_≈0.5Bi_≈1.1V₈O₁₆ evidenced a segregation between Aⁿ⁺ and Bi³⁺ ions. They respectively occupy the center of a square prism (K, Sr, Ba, Pb cases) and the center of a quasi-square plane (Bi³⁺) while Na⁺ and Tl⁺ behave differently. This involves, since the tunnels are almost fully occupied, the impossibility of cohabitation of the same tunnels by the two antogonist species which would lead to an unrealistic A-Bi distance of 1.5 Å. The preparation of polycrystalline ABiV₈O₁₆ samples, in which the tunnels are filled, indicates a disordered distribution of A- and Bi-only tunnels since no supercell diffraction lines were observed. Finally electric conductivity measurements on mixed K-Bi materials are in agreement with a hopping semiconductivity due to the V³⁺-V⁴⁺ mixed valence of the transition metal.

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