Large change in thermopower with temperature driven p-n type conduction switching in environment friendly BaxSr2-xTi0.8Fe0.8Nb0.4O6 double perovskites

Oxide based thermoelectric materials have gained some interest in recent times due to their low thermal conductivity, environment friendly nature and high temperature durability. In the present work, the effect of Nb5+ doping on the thermoelectric properties of BaxSr2-xTiFeO6 based double perovskites was investigated. BaxSr2-xTi0.8Fe0.8Nb0.4O6 (BSTFN) double perovskites with stoichiometric compositions of 0.0 <= x <= 0.25 were synthesized by the solid-state reaction method. Rietveld refinement powder XRD data confirmed single-phase solid solutions with cubic crystal symmetry for all the BSTFN compositions. Dense microstructures with fine grain-sizes were observed in SEM studies of the as-synthesized oxide samples. Thermoelectric parameters like the Seebeck coefficient (S), the electrical conductivity (s) and thermal diffusivity of these oxide samples were measured in the temperature range from 300 K to 1223 K. All the BSTFN compositions exhibited temperature driven p-n type conduction switching along with a colossal change in thermopower. The maximum drop in thermopower (DS) of B1146 mV K-1 was observed for BaxSr2-xTi0.8Fe0.8Nb0.4O6 with composition of x = 0.25. From the temperature dependent conductivity studies it is evident that all the compositions underwent intermediate semiconductor to metal-like transition before it showed conversion from p-type to n-type semiconductor behavior in the thermo-power (S) measurement. The conduction mechanisms of these oxides were further explained by the small polaron hopping model.