Substitution of Bi for Sb and its role in the thermoelectric properties and nanostructuring in Ag_1-xPb₁₈MTe₂₀ (M = Bi, Sb) (x = 0, 0.14, 0.3)

We have performed a comparative investigation of the Ag _1-xPb₁₈MTe₂₀ (M = Bi, Sb) (x = 0, 0.14, 0.3) system to assess the roles of Sb and Bi on the thermoelectric properties. Detailed charge transport data including electrical conductivity, the Seebeck coefficient, the Hall coefficient, and thermal conductivity are presented. Optical reflectivity data support the conclusions of the transport studies. For comparable nominal compositions, the carrier concentrations are lower in the Sb analogs and the mobilities are higher. The Seebeck coefficient decreases dramatically in going from Sb to Bi. High resolution transmission electron microscopy (TEM) images of both samples reveal that all systems contain compositional fluctuations at the nanoscopic level and are nanostructured. Compared to PbTe, the lattice thermal conductivity of AgPb₁₈BiTe ₂₀ is substantially reduced. The lattice thermal conductivity of the Bi analog is, however, higher than the Sb analog, and this correlates with 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-xPb₁₈MTe₂₀ is found to be substantially smaller than that of Ag_1-xPb₁₈MTe₂₀. Electronic structure calculations performed within the density functional theory and generalized gradient approximation show marked differences in the band structure near the Fermi level between the two analogs providing useful insights on the carrier transport in these systems.