Thermoelectric transport properties of PbTe under pressure

In this work, we present a comprehensive picture of structural, dynamical, electronic, and transport properties of PbTe at ambient and high pressures. The first-principles linear-response calculations show that there exists an anharmonic instability of the optical branch phonon at the Brillouin-zone (BZ) center and soft phonons at the BZ boundary X point. The k-dependent soft modes may lead to substantial changes in the thermal conductivity when the pressure is applied. The electronic band structure of both B1 and Pnma phases are investigated by full potential method with various exchange-correlation functionals. Under pressure there is a band-gap closure as well as reopening within B1 structure whereas for Pnma phase only the gap closure is observed. Their thermoelectric transport properties are studied by exploring their energy bands based on Boltzmann transport theory. We found that n-doped Pnma phase at 6.7 GPa has better thermoelectric performance than B1 phase at ambient condition, while for the p-doped case, B1 phase has much better thermoelectric properties. Energy band gap does play an important role in thermoelectric performance. At 300 K, modifications of thermoelectric properties caused by band-gap variation can be observed only at a low doping level, at 600 K the influence can be detected in mid-to-high doping levels. The detailed analysis of thermoelectric properties as respect to temperatures and carrier concentrations reveal that in the low-doping case the optimal performance occurs in 300-450 K temperature range but for mid-to-high doping cases the optimal working temperature increase to higher range. With the pressure applied, the thermoelectric response shows many interesting features. The thermoelectric figure of merit (ZT) for B1 phase achieves its maximum at middoping region with similar to 8 GPa for p doping and above 18 GPa for n doping. In the Pnma case, ZT values are more sensitive to doping than to pressure, and there is small difference between the 300 and 600 K results. These findings are expected to be useful in searching an optimal combination of doping level, working temperature, and pressure in order to achieve higher ZT in PbTe-based materials.