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

Abstract

Hollandite oxides, A _x Bi _y V ₈ O ₁₆ (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 Bi ³⁺ in the framework is easier than for A ⁿ⁺ co-cations. Crystal structures refined from single crystals of nominal compositions A _{≈ 0.5} Bi _{≈ 1.1} V ₈ O ₁₆ evidenced a segregation between A ⁿ⁺ and Bi ³⁺ ions. They respectively occupy the center of a square prism (K, Sr, Ba, Pb cases) and the center of a quasi-square plane (Bi ³⁺ ) 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 ABiV ₈ O ₁₆ 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 V ³⁺ -V ⁴⁺ mixed valence of the transition metal.