Computational investigations of thermoelectric properties of lead telluride, magnesium silicide, and magnesium stannide under high pressure and anisotropic stress

Abstract

I present a comprehensive picture of the electrical transport properties for PbTe, Mg2Si, and Mg2Sn under high pressure and stress.

The electronic band structure and thermoelectric properties of p- and n-doped lead telluride, PbTe, were calculated within the framework of density functional theory, as implemented in Wien2k, and Boltzmann transport theory, as implemented in the BoltzTraP code. Thermoelectric properties were calculated as a function of hydrostatic pressure, as well as uniaxial stress along several crystal directions. Signicant enhancements of the thermoelectric power factor can be obtained by using anisotropic stress, and the underlying changes in the electronic band structure are analysed. We identify two key eects in PbTe: redistribution of carriers between dierent carrier pockets, each of which has anisotropic transport properties; and reshaping of the carrier pockets themselves.

Individual carrier pockets of PbTe have anisotropic transport properties. Stress along [111] is shown to redistribute dopant charges between the carrier pockets. Due to this eect, compressive [111] stress improves the thermoelectric power factor along [111] of p-PbTe, and tensile [111] stress improves the power factor along [111] of n-PbTe. Compressive stress along [001], which causes an intervalleytransfer-eect, is shown to reshape the valence band of PbTe, improving the p-type power factor along [001].

Along with specic results for PbTe, general rules are presented for identifying other materials which could have their thermoelectric power factor increased by stress along specic directions. Guided by these rules, the thermoelectric properties of magnesium silicide and magnesium stannide under strain are presented. The same key eects, intervalley-transfer and valley-reshaping, are observed. The thermoelectric power factors of Mg2Si, and Mg2Sn are shown to be greatly improved by anisotropic strain.