Disorder effect and thermoelectric properties of Bi1−xCaxCu1−ySeO with Cu vacancy

BiCuSeO exhibits remarkable low thermal conductivity among oxides. The lone pair of the Bi ³⁺ and the point defects brought about by elemental doping are thought to be the origin of the low thermal conductivity. In a recent study, however, the Cu atoms in BiCuSeO play a more significant role in reducing the lattice thermal conductivity than the above two factors. In this work, two series of polycrystalline Bi _1−xCa _xCu _0.975SeO (x = 0.025, 0.050, 0.075, 0.100, 0.125, 0.150) and Bi _0.95Ca _0.05Cu _1−ySeO ( y = 0.015, 0.025, 0.035) are synthesized by solid-state reaction. All the materials are of single phase. Point defects introduced by partial elemental substitution and atom vacancy are expected to reduce the lattice thermal conductivity, however, which is not followed by the prediction for Bi _1−xCa _xCu _0.975SeO with 0.050 ≤ x ≤ 0.125 and Bi _0.95Ca _0.05Cu _1−ySeO with y = 0.015, 0.025, and 0.035 in the temperature range where the resistivity decreases while the thermopower increases with temperature. This is in sharp contrast with those BiCuSeO with partial elemental substitution or Cu deficiency exhibiting either both the electrical resistivity and thermopower increase or decrease with increasing temperature. Our results support that the Cu atoms in BiCuSeO play a more significant role in reducing the lattice thermal conductivity than the lone pair of the Bi ³⁺ and the point defects brought about by elemental doping.