Electrical conduction behaviour in K2NiF4-type Ca2MnO3.98 below room temperature

Electrical transport in polycrystalline ceramics of K2NiF4-type Ca2MnO3.98 has been investigated as a function of temperature using complex-plane impedance analyses, dielectric properties, DC conductivities by the four-probe method, thermoelectric power and magnetic susceptibilities. The bulk conductivities were obtained using the complex-plane impedance analyses. The magnetic properties indicate that antiferromagnetically ordered magnetic moments are canted and there is a weak ferromagnetic contribution below the Neel temperature of 113 K, The thermoelectric power changes from positive Values to negative ones at around 200 K with increasing temperature. Above about 200 K, the dielectric relaxation peak due to a hopping process involving small polarons manifests itself. The temperature dependencies of the dielectric: relaxation process and the bulk conductivities suggest that the charge carriers responsible for the conduction are strongly localized. This is directly supported by the fact that the activation energy for the bulk conduction is nearly equal to that for the dielectric relaxation, The experimental results above about 200 K have been interpreted in terms of a hopping process involving small polarons. At lower temperatures, either multiphonon jumping processes of small polarons are frozen out or charge transport between impurity states takes place.