High thermoelectric properties of p-type BiCuSeO co−doped with Ca²⁺ and Ba²⁺

In this work, we prepare Ca²⁺ and Ba²⁺ co−doped BiCuSeO using a solid−state reaction, followed by spark plasma sintering. Ca²⁺/Ba²⁺ co−doping significantly increases the electrical conductivity through the increased hole concentration. In addition to the increased power factor, Ca²⁺/Ba²⁺ co−doping reduces the phonon thermal conductivity through grain refinement, increased porosity, and mass and strain field fluctuations. The combination of the largest power factor and the low phonon thermal conductivity for Bi_0.80Ca_0.10Ba_0.10CuSeO yields the largest dimensionless figure−of−merit (ZT). The ZT of Bi_0.80Ca_0.10Ba_0.10CuSeO (0.60) at 600 °C is two times larger than that of the undoped BiCuSeO (0.30) at 600 °C. We systematically discuss the effect of the Ca²⁺/Ba²⁺ co−doping on the thermoelectric properties of BiCuSeO oxyselenide.