Relative stabilities of layered perovskite and pyrochlore structures in transition metal oxides containing trivalent bismuth

Abstract

In order to investigate the factors determining the relative stabilities of layered perovskite and pyrochlore structures of transition metal oxides containing trivalent bismuth, several ternary and quaternary oxides have been investigated. While d 0 cations stabilize the layered perovskite structure, cations containing partially-filled d orbitals (which suppress ferroelectric distortion of MO 6 octahedra) seem to favor pyrochlore-related structures. Thus, the vanadium analogue of the layered perovskite Bi 4Ti 3O 12 cannot be prepared; instead the composition consists of a mixture of pyrochlore-type Bi 1.33V 2O 6, Bi 2O 3, and Bi metal. The distortion of Bi 1.33V 2O 6 to orthorhombic symmetry is probably due to an ordering of anion vacancies in the pyrochlore structure. None of the other pyrochlores investigated, Bi 2NbCrO 7, Bi 2NbFeO 7, TlBiM 2O 7 ( M = Nb, Ta), shows evidence for cation ordering in the X-Ray diffraction patterns, as indeed established by structure refinement of TlBiNb 2O 7.