Enhanced thermoelectric performance of PbTe within the orthorhombic Pnma phase

The thermoelectric properties of PbTe within the NaCl (B1) and the orthorhombic Pnma phases are extensively studied by ab initio calculations using the full-potential linearized augmented plane-wave method and the semiclassical Boltzmann theory. The calculations of n- and p-type Seebeck coefficients for the B1 structure suggested that the energy band gap plays an important role in determining the thermoelectric properties, but only at lower carrier concentrations. We found that the n-doped Pnma phase at 6.5 GPa has a larger Seebeck coefficient than that of the n-doped B1 phase at zero pressure, but has a comparable electric conductivity. This fact could be well understood by the large density of states in the conduction band and the large anisotropy in the band structure and constant energy surface. Our calculations also predicted that the largest n-type ZT values at 300 and 600 K of the Pnma phase at 6.5 GPa can reach up to 0.9 and 1.59, respectively, which are two times larger than those in B1 phase at zero pressure. The current theory strongly suggests that the Pnma structure of PbTe is an excellent thermoelectric material. It is desirable to synthesize the Pnma phase of PbTe at ambient pressure by making use of its high performance.