Electronic transport properties ofPbTeandAgPbmSbTe2+msystems

Abstract

Transport calculations using the Boltzmann equation within energy-dependent relaxation time approximations were performed for $\mathrm{PbTe}$ and ${\mathrm{AgPb}}_{m}{\mathrm{SbTe}}_{2+m}$ (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 $\mathrm{PbTe}$ 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 $\ensuremath{\sigma}$ and thermopower $S$. Transport calculations in $n$-type $\mathrm{PbTe}$ using the energy dispersion given by the ab initio electronic structure results in overestimation of $\ensuremath{\sigma}$ 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 $(\ensuremath{\sigma}{S}^{2})$ in $0--500\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ temperature range relative to $\mathrm{PbTe}$. 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 $\mathrm{Ag}\text{\ensuremath{-}}\mathrm{Sb}$ microstructures.