Raman Scattering Study of Cation-Deficient Ban(MoNb)n-δO3n-x and Related Perovskite-like Oxides

The Raman scattering spectra of Ba 3 MoNbO 8.5 and Ba 7 Nb 4 MoO 20 , two cation-deficient perovskite-like oxides, are presented for the first time and analyzed in an attempt to confirm the distribution of octahedra and tetrahedra proposed 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-δ O 3n-x (δ ≥ 1, x ≥ 0). While the spectra of all compounds are 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 those compounds which contain octahedral [BO 6 ] groups according to structural studies. The Raman spectrum of Ba 3 Nb 2 O 8 contains modes in the region attributed to vibrations of octahedral symmetry although it is stoichiometrically identical to the palmierite structure which has only tetrahedrally coordinated B-sites. This observation is in disagreement with structural work previously reported in the literature but provides strong corroborating evidence for a new study which indicates a variant stacking arrangement of hexagonal and cubic close-packed layers giving rise to an equal number of octahedrally and tetrahedrally coordinated B-sites in this compound.