Investigation of thermoelectric materials: Substitution effect of Bi on the Ag_1-xPb₁₈MTe₂₀ (M = Sb, Bi) (x = 0, 0.14, 0.3)

We performed comparative investigations of the Ag_1-xPb ₁₈MTe₂₀ (M = Bi, Sb) (x = 0, 0.14, 0.3) system to better understand the roles of Sb and Bi on the thermoelectric properties. In both systems, the electrical conductivity nearly keeps the same values, while the Seebeck coefficient decreases dramatically in going from Sb to Bi. Compared to the lattice thermal conductivity of PbTe, that of AgPh₁₈BiTe ₂₀ is substantially reduced. The lattice thermal conductivity of the Bi analog, however, is higher than that of AgPb₁₈SbTe₂₀ and this is attributed largely to the decrease in the degree of mass fluctuation between the nanostructures and the matrix (for the Bi analog). As a result the dimensionless figure of merit ZT of Ag_1-xPb₁₈MTe ₂₀ (M = Bi) is found to be smaller than that of Ag _1-xPb₁₈MTe₂₀ (M = Sb).