On the electrical properties of synthetic manganocolumbite MnNb₂O_6-δ

A manganocolumbite with nominal composition MnNb2O6 has been prepared under air in order to characterize its electrical behavior. Although powder X-ray diffraction indicates the presence of a single phase, electrical and microscopic characterization reveals two different bulk responses. The origin of these two bulk elements seems to relate to the presence of two compositions with slight excess of either manganese or niobium. These are stabilized instead of MnNb2O6, indicating the instability of the nominal composition. However, these variations of composition are not large enough to be detected by our structural characterization methods, although the effect is easily detected by impedance spectroscopy and from particle shape in scanning electron microscopy (SEM). Under reducing conditions, monophasic samples with Nb/Mn ratio close to 2 are obtained. From magnetic measurements the main magnetic contribution is determined to be from Mn2+ in all cases. The oxygen deficiency of reduced MnNb2O6-delta was only just determinable by thermogravimetry, indicating that delta similar to 0.02. On annealing at 1150 degrees C in 5% H-2, a decrease in unit cell volume of 0.22%, an increase in electrical conductivity of 4 orders of magnitude, and the appearance of a Pauli contribution also indicate a small degree of Nb reduction.