Compositional modification of Sr2TiCoO6 double perovskites by Mo and La for high temperature thermoelectric applications

Double perovskites (A2B/B//O6) are an interesting family of materials, which are amenable to compositional modification. Recently double perovskites have shown promising thermoelectric properties especially at high temperature. In this report, we investigated the combination of two different cubic double perovskites with similar lattice constant but one (Sr2TiCoO6) showing good Seebeck coefficient and the other (Sr2TiMoO6) with high electrical conductivity. Dense ceramic samples of Sr2TiCo0.5Mo0.5O6 and La0.2Sr1.8TiCo0.5Mo0.5O6 were synthesized by solid state reaction method. Rietveld refinement of XRD data confirmed the cubic structure with Pm3¯m space group in these ceramics. Doping of Mo in Sr2TiCoO6 helped in increasing its conductivity; however Seebeck coefficient showed the n-type behavior unlike the p-type conductivity observed in Sr2TiCoO6 ceramics. Aliovalent doping of La in Sr2TiCo0.5Mo0.5O6 further increased its conductivity and Seebeck coefficient demonstrated temperature driven p-type to n-type conduction switching behavior. Conductivity mechanism of these ceramics was found to be governed by small polaron hopping model. Temperature driven p-n switching observed in the thermopower (S) measurement of La0.2Sr1.8TiCo0.5Mo0.5O6 was further explained by an analytical model.