Small polaron hopping in spinel manganates

Abstract

The temperature dependence of small polaron hopping conduction in ceramic spinel NiMn2O4+ thermistor material has been investigated. We used a theoretical framework based on a random resistor network model to describe small polaron nearest-neighbor hopping NNH and variable range hopping VRH, following the principles of Shklovskii and Efros. We find that in printed thick films and in pressed pellets resistivity is described best by a VRH model of the form T2p expT0 /Tp, whereas in thin NiMn2O4+ films resistivity is better described by NNH, T expT0 /T. Steady state dc resistance vs temperature measurements for NiMn2O4+ thick films, thin films, and pellets have been carried out and the parameters p and T0 determined. For thick films p was found to be 0.5, indicating VRH with an approximately parabolic distribution of the density of states DOS around Fermi level. For pellets p was 0.65, and in thin films 1 indicating NNH. The increase of p was interpreted as an increase of disorder in the system, leading to strong electron localization effects and narrowing Mn3+/Mn4+ bandwidth. In thick films and pellets the DOS was determined by a parametrization related to the p value, giving 1020–1021 eV−1 cm−3. The characteristic temperature T0 was in the range of 2105 K in thick films, 3104 K in pellets, and 5103 K in thin films.