Crystal chemistry of mixed Bi3+-A(n+) (A(n+) = 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 vary ing x, y ratios have been synthesized and studied. The feasibility of obtai ning such materials under our normal solid stale conditions is to be highli ghted considering the fact that previous work on potassium-only compounds r equired 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 squar e 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 tunne ls by the two antogonist species which would lead to an unrealistic A-Bi di stance of 1.5 Angstrom. The preparation of polycrystalline ABIV(8)O(16) sam ples, in which the tunnels are filled, indicates a disordered distribution of A- and Pi-only tunnels since no supercell diffraction lines were observe d. Finally electric conductivity measurements on mixed K-Bi materials are i n agreement with a hopping semiconductivity due to the V3+-V4+ mixed valenc e of the transition metal.