Raman scattering study of cation-deficient Ba-n(MoNb)(n-delta)O3n-x and related perovskite-like oxides

The Raman scattering spectra of Ba3MoNbO8.5 and Ba7Nb4MoO20, two cation-def icient perovskite-like oxides, are presented for the first time and analyze d in an attempt to confirm the distribution of octahedra and tetrahedra pro posed in recent structural investigations. The Raman spectra are discussed in relation to spectra of compounds of the hexagonal perovskite-like oxides with the general formula A(n)B(n-delta)O(3n-x) (delta greater than or equa l to 1, x greater than or equal to 0). While the spectra of all compounds a re found to be dominated by strong tetrahedral bands near 300 and 800 cm(-1 ), modes of octahedral symmetry are evident between 400 and 700 cm(-1) in t hose compounds which contain octahedral [BO6] groups according to structura l studies. The Raman spectrum of Ba3Nb2O8 contains modes in the region attr ibuted to vibrations of octahedral symmetry although it is stoichiometrical ly identical to the palmierite structure which has only tetrahedrally coord inated B-sites. This observation is in disagreement with structural work pr eviously reported in the literature but provides strong corroborating evide nce for a new study which indicates a variant stacking arrangement of hexag onal and cubic close-packed layers giving rise to an equal number of octahe drally and tetrahedrally coordinated B-sites in this compound.