Electrical conduction behavior of La and Mn substituted strontium titanate

Abstract

Attempts were made to synthesize various compositions in the system Sr1−xLaxTi1−xMnxO3 (x⩽0.50) by solid state ceramic route. A few compositions were also synthesized by citrate nitrate route. Solid solution forms in all the compositions investigated. Crystal structure remains cubic up to x=0.10 similar to SrTiO3, while for compositions with x⩾0.20, it is orthorhombic similar to LaMnO3. Average grain size is small in all the compositions. Average grain size decreases with increasing x. Seebeck coefficient (α) and dc resistivity were measured as a function of temperature. For compositions with x⩽0.30, Seebeck coefficient is initially positive. It becomes negative at higher temperature. For x=0.50, it is negative over the entire temperature range. Based on Seebeck coefficient and activation energy of conduction in different temperature ranges, it is proposed that for x=0.01, at higher temperatures conduction occurs due to migration of doubly ionized oxygen vacancies. In all the compositions conduction occurs either by hopping of holes among Mn4+ and Mn3+ or electrons among Mn2+ and Mn3+ sites. For compositions with x⩽0.30, at low temperature, conduction occurs due to hopping of holes, it changes to conduction due to hopping of electrons at higher temperature. For composition with x=0.50 conduction occurs due to hopping of electrons.