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

Hollandite oxides, A(x)Bi(y)V(8)O(16) (A = Na, K, Sr, Pa, Tl, Pb) with varying x, y ratios have been synthesized and studied. The feasibility of obtaining such materials under our normal solid stale 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 A(n+) co-cations. Crystal structures refined from single crystals of nominal compositions A(approximate to 0.5)Bi(approximate to 1.1)V(8)O(16) evidenced a segregation between A(n+) and Bi3+ ions. They respectively occupy the center of a square prism (K, Sr, Pa, 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 Angstrom. The preparation of polycrystalline ABIV(8)O(16) samples, in which the tunnels are filled, indicates a disordered distribution of A- and Pi-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.