ELECTRICAL-CONDUCTION BEHAVIOR IN K2NIF4-TYPE CA2MNO3.98 BELOW ROOM-TEMPERATURE

Electrical transport in polycrystalline ceramics of K2NiF4-type Ca2MnO 3.98 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 susceptibilit ies. The bulk conductivities were obtained using the complex-plane imp edance analyses. The magnetic properties indicate that antiferromagnet ically ordered magnetic moments are canted and there is a weak ferroma gnetic contribution below the Neel temperature of 113 K, The thermoele ctric power changes from positive Values to negative ones at around 20 0 K with increasing temperature. Above about 200 K, the dielectric rel axation peak due to a hopping process involving small polarons manifes ts itself. The temperature dependencies of the dielectric: relaxation process and the bulk conductivities suggest that the charge carriers r esponsible for the conduction are strongly localized. This is directly supported by the fact that the activation energy for the bulk conduct ion is nearly equal to that for the dielectric relaxation, The experim ental results above about 200 K have been interpreted in terms of a ho pping process involving small polarons. At lower temperatures, either multiphonon jumping processes of small polarons are frozen out or char ge transport between impurity states takes place.