Effects of uniaxial and hydrostatic pressure on the valence band maximum in Sb2Te3:: An electronic structure study

Sb2Te3, in the form of alloys with Bi2Te3 and Bi2Se3, forms the basis of room temperature thermoelectric materials. While experimental pressure studies have investigated changes to thermoelectric properties, few theoretical investigations have examined how the band structure changes as a function of pressure. Electronic structure calculations have been performed on Sb2Te3 in order to understand the nature of the valence band maximum. We used lattice constants and atomic positions which have previously been relaxed under pressure in a work by another set of authors. Using these values we find significant changes to the valence band maximum as a function of pressure which the previous authors did not investigate. The valence band maximum lies off of the high symmetry lines which are usually plotted. Hydrostatic pressure shows very little change in the position of the valence band maximum up to 4 GPa, but a profound shift occurs when pressure is applied uniaxially. The valence band maximum shifts from one off-axis position to another by 2 GPa, so that around 1.5 GPa multiple valence band maxima occur which may enhance the thermoelectric properties. The effective masses of this new peak are larger, consistent with the existence of a light-hole upper valence band (UVB) and a heavy-hole lower valence band (LVB). This electronic topological transition may explain the significant increase seen in the thermoelectric properties of Sb1.5Bi0.5Te3 as a function of uniaxial pressure around 1-2 GPa by increasing both the band degeneracy and effective masses.