Electronic transport properties of PbTe and AgPbmSbTe2+m systems

Transport calculations using the Boltzmann equation within energy-dependent relaxation time approximations were performed for PbTe and AgPbmSbTe2+m (LAST-m) systems. We have used both the nonparabolic Kane model for the energy dispersion and the energy dispersion given by ab initio electronic structure calculations. For PbTe we find that the temperature dependence of the density of states effective mass m(d) is very important in order to have good agreement with experiment for electrical conductivity sigma and thermopower S. Transport calculations in n-type PbTe using the energy dispersion given by the ab initio electronic structure results in overestimation of sigma and underestimation of S because the temperature dependence of m(d) cannot be incorporated in the calculation of the chemical potential. Transport calculations in n-type LAST-m systems using the nonparabolic Kane model for the energy dispersion show a small enhancement of the power factor (sigma S-2) in 0-500 K temperature range relative to PbTe. The observed large ZT values of the LAST-12 and LAST-18 systems are a combination of a small enhancement of the power factor and a strong reduction in the thermal conductivity due to the formation of Ag-Sb microstructures.