Optical properties of brannerite-type vanadium oxides, MV2O6 (M = Ca, Mg, Mn, Co, Ni, Cu, or Zn)

Abstract

The optical properties of brannerite-type oxides MV2O6 (M ​= ​Ca, Mg, Mn, Co, Ni, Cu, or Zn) and solid solutions synthesized by conventional high temperature solid state reaction method are compared in the present study. The structural phase transformation from pseudo-brannerite to brannerite was found at x ​≥ ​0.6 in the solid solution Ca1-xMnxV2O6 (x ​= ​0 - 1). In the new solid solution of Zn1-xCoxV2O6 (x ​= ​0 - 1), the phase transformation from brannerite to NiV2O6-type occurred at x ​≥ ​0.8. A systematic decrease in the optical band gap with increasing Mn content in Ca1-xMnxV2O6 resulted in the change in the colors of the samples. In Zn1-xCoxV2O6, the prominent d-d transitions of Co2+ ion in addition to the band gap narrowing altered the colors. The variation in the colors of brannerite-type oxides can be understood based on the changes in the optical band gap, ligand-to-metal (O2--V5+) and/or metal-to-metal (M2+-V5+) charge transfer transitions, and d-d transitions.