Defect structure and charge transfer in undoped and doped lanthanum cobaltites

The results of electronic conductivity and Seebeck coefficient are presented for perovskite-type undoped cobaltite LaCoO3–d, doped with strontium La0.7Sr0.3CoO3–d, and doped with copper LaCo0.7 Cu0.3O3–d. The electronic conductivity and the Seebeck coefficient were studied as a function of pO2 in the range 10–6 £ pO2/atm £ 1 at temperatures between 1073 and 1323 K. The charge transfer mechanism in the oxides studied is revealed within the framework of localized charge carriers approach on the basis of the defect structure modeling. LaCoO3–d and La0.7Sr0.3 CoO3–d are shown to be the typical small polaron hopping conductors with p-type small polarons at Co sites, Co Co, as major carriers and n-type small polarons, CoCo / , as minor carriers in the temperature range investigated. The charge transfer in copperdoped cobaltite LaCo0.7Cu0.3O3–d seems to be carried out by means both hopping of n-type small polarons, CuCo / and CoCo / , at copper and cobalt sites, respectively, and p-type small polarons at Co sites. The mobilities and nonconfigurational entropies of charge transfer are determined for all types of charge carriers.