Effect of Ti substitution on the electric properties of MnNb₂O_6−δ

Ti-substituted MnNb2O6 samples have been prepared to enhance conductivity via introduction of oxygen vacancies or hole formation. The single phase MnNb2-xTixO6-delta columbite existence range is narrow (0 < x < 0.2). The effect of the substitution on electrical properties has been evaluated and compared to both nominal MnNb2O6 as prepared in air and reduced MnNb2O6-delta (delta similar to 0.02). Compared to the former, the effect of the dopant on both the sintering process and the electrical conductivity is beneficial. A homogeneous brick layer microstructure is obtained for a low extent of niobium substitution and short time of sintering. Conductivity of the Ti-substituted material is several orders of magnitude higher than that of the unsubstituted. On the other hand reduced MnNb2O6-delta (delta similar to 0.02) exhibits a higher conductivity at high temperatures. Ti-substituted MnNb2O6 behaves as a hole/electron electronic conductor, yielding quite similar conductivities in air and 5% H-2 at 900 degrees C but with differing dominant charge carriers. A small contribution to conductivity independent of the p(O2) suggests a possible ionic conductivity of approximately 6.0 x 10(-5) ohm(-1) cm(-1). Magnetic properties are also affected by the substitution. Although the antiferromagnetic transition at low temperature due to Mn2+ ions still persists, a decrease in Neel temperature is observed as titanium content increases.