Substantially low thermal conductivity and high thermoelectric figure-of-merit in Bi-doped Sr₂CoRuO₆ double perovskites

Transition metal oxide thermoelectric materials are interesting due to their high thermal stability, low toxicity, and low cost. Although several interesting thermoelectric oxides have been reported thus far, but reducing the lattice thermal conductivity while maintaining the good electrical conductivity to get optimum thermoelectric performance is still a challenge. In addition, many oxide systems are not well explored as yet for their potential to convert waste heat into electricity using the Seebeck effect. Herein, we report the structural and thermoelectric properties of the Sr2−xBixCoRuO6 (0 ≤ x ≤ 0.8) double perovskites. Rietveld refinement of X-ray powder diffraction data shows that the unit cell parameters increase with Bi content (x). The electrical resistivity decreases with x due to the much larger and well-connected grains in Bi containing samples. The Seebeck coefficient changes its sign from negative to positive with increasing Bi content indicating hole doping. The thermal conductivity of the samples decreases with increasing x due to the soft chemical bonding and heavy atomic mass of the doped Bi atoms. These changes in transport properties lead to a large power factor of 1.87 × 10–4 W/m·K2 and a thermoelectric figure-of-merit of 0.24 for the x = 0.8 sample at 668 K. We estimated the heat capacities from the temperature independent Dulong-Petit limit for solids for calculation of the thermal conductivities and therefore some uncertainty in the reported zT values is expected.