Structural characterization and electrical properties of NiNb2−xTaxO6 (0≤x≤2) and some Ti-substituted derivatives

A structural and electrical characterization of the system NiNb2−xTaxO6 (0≤x≤2) is presented. For x≤0.25 materials with the columbite-type structure typical of NiNb2O6 have been obtained whereas for x≥1 tri-rutile-like oxides were obtained. The electrical properties are similar in both cases; they are semiconducting with very low electrical conductivity and very high activation energy, though slight differences were found as a function of Ta content. Improvement of conductivity by reducing the stoichiometric materials could not be achieved due to decomposition. In this connection, partial substitution of Nb or Ta by Ti has been carried out in order to create oxygen vacancies. Tantalum was partially replaced by Ti to a significant extent in the tri-rutile structure inducing a slight increasing of conductivity. However, for the columbite case neither Nb nor Ta could be partially replaced. This behavior is quite different from that reported for other similar columbites such as MnNb2O6−δ, which exhibits high electrical conductivity upon substitution of niobium by titanium.