Structure and electrical conductivity of multicomponent metal oxides having scheelite structure

Abstract

The investigated bismuth vanadate, BiVO4, bismuth vanadomolybdate, Bi0.85V0.55Mo0.45O4, and pure bismuth molybdate, Bi2Mo3O12, oxides prepared by novel spray drying technique having the scheelite structure were found to be monoclinic in the case of BiVO4 and Bi2Mo3O12, whereas Bi0.85V0.55Mo0.45O4 is body-centered tetragonal. The distribution of intergranular porosity across polycrystalline samples is inhomogeneous, being larger parallel to the flat compression of the powder samples. The high polarizability of Bi3+ cations with their lone-pair electrons influences stability of the disordered oxygen sublattice. All as-prepared systems undergo a slight structural change of the oxygen arrangement during the sample heating in the temperature region of 340–390°C, probably due to an order ⇆ disorder transition, resulting in a decrease of lattice oxygen ion mobility and decrease of electrical conductivity. The total bulk electrical conductivity of both vanadate systems is essentially ionic at low temperatures and high oxygen partial pressures. Diffusion of the lattice oxygen ions in the Bi1−x/3V1−x Mo x O4 scheelite structure is indirectly related to the introduction of cation vacancies. The highest conductivity was observed in the multicomponent oxide Bi0.85V0.55Mo0.45O4, both as-prepared and thermally treated. Above 350°C the conductivity of BiVO4 and partially also Bi0.85V0.55Mo0.45O4 oxides is affected by the n-type electron contribution.