Raman spectra of vanadates [formula omitted] ([formula omitted] Mn, Co, Ni, Zn) crystallized in the non-usual columbite-type structure

Abstract

We report the first Raman spectra for the vanadates with chemical formula MV2O6 (M=Mn, Co, Ni, Zn) crystallized in a columbite-type structure (Pbcn space group). The columbite-type structure, which is quite unusual for the compounds MV2O6, was obtained by synthesis at high temperature and pressure from precursor powder samples in either P1‾ or C2/m structures. The cell parameters, interatomic distances, and distortion indexes of the octahedra in the Pbcn structure, were obtained from the Rietveld refinement of powder X-ray diffraction patterns measured at room temperature for each sample. The Raman modes were obtained from high quality spectra recorded in the range of wavenumbers between 50 and 1170 cm-1; these were assigned by comparison with the known spectra of the isostructural niobates MNb2O6. Common sequences of peaks can be identified in the Raman spectra of all the studied samples, but the position of those peaks in wavenumber ω‾ does not follow the behavior that is expected from looking only at the interatomic distances between the atoms involved in the vibration. The influence of the electronegativity of the atoms M in the force constants of the V–O bonds is proven in the evolution of the Ag Raman mode observed at the highest wavenumber.