Structural properties and Raman spectra of columbite-type NiNb2−xVxO6 synthesized under high pressure

The complete set of structural parameters of the new series of compounds NiNb2−xVxO6 (0 ​≤ ​x ​≤ ​2) with the unusual columbite-type structure is presented here. In the samples containing vanadium, this crystalline structure was stabilized by synthesis in conditions of high pressure and high temperature. Here we report the first Raman spectrum for the NiV2O6-Pbcn polymorph and extend the list of the previously observed modes for the NiNb2O6. The evolution of the vibrational Raman spectrum produced when the vanadium is substituted for niobium along the series is also presented and discussed. This evolution is interpreted by taking into account the changes in the local structural environment of the niobium/vanadium atoms and its influence over the nickel-oxygen bonds around them. The presence of vanadium atoms favors an increase of the symmetry in the arrangement of oxygen atoms around the nickel-ones; in counterpart, the vanadium is in an octahedral environment which is more distorted than that of the niobium. Because of these apparently subtle differences, the homogeneous distribution of vanadium in the solid solution NiNb2−xVxO6 is not possible.