Cation distribution in NiMn2O4 spinel probed by high temperature thermopower measurements

We report on measurements of the thermoelectric power of NiMn2O4 spinels over the temperature range from 100 °C up to 860 °C. The thermopower (Seebeck coefficient S) of samples with different thermal history (slow cooled or quenched from different temperatures) was measured upon heating up to the upper temperature of the spinel stability field in air and upon subsequent cooling in air. All samples exhibit a negative Seebeck coefficient at T < 680 °C, and a cross-over to p-type behavior is observed at higher temperature. Thermopower data were evaluated using the modified Heikes equation considering spin and orbital degeneracy, and the concentrations of Mn3+ and Mn4+ on octahedral position (B-site) were calculated. A model of the temperature-dependent cation distribution of NiMn2O4 was derived. The results are discussed in terms of cation redistribution kinetics and electron configurations of Mn ions.