High thermoelectric performance in pseudo quaternary compounds of (PbTe)_0.95−x(PbSe)_x(PbS)_0.05 by simultaneous band convergence and nano precipitation

Lead chalcogenides have long been studied as promising thermoelectric materials, operating at the mid-temperature range of 500–950 K. Here, we studied thermoelectric properties of pseudo-quaternary compounds of (PbTe)_0.95−x(PbSe)_x(PbS)_0.05 (x = 0.0, 0.05, 0.10, 0.15, 0.20, 0.35, and 0.95) with 1% Na-doping, synthesized by melting and rapid quenching of pristine compounds of PbTe, PbSe, and PbS and followed by hot press sintering. The lattice parameters and transmission electron microscopy confirmed that the PbSe makes solid solution with PbTe leading to PbTe_1-xSe_x matrix while the PbS precipitates in the matrix. In terms of two valence bands model, the energy band gap between conduction and valence L-band was decreased and the energy difference between L- and Σ-bands was increased with increasing Se concentration. The band convergence at high temperature may be associated with the enhancement of power factor. The PbS nano-scale precipitation in the matrix attributed to the decrease of lattice thermal conductivity. From the Matthiessen's rule, the lattice thermal conductivity was described by the nano precipitation as well as alloy scattering of phonons. The simultaneous emergence of band convergence and nano-precipitation in the quaternary compounds of (PbTe)_0.95−x(PbSe)_x(PbS)_0.05 gives rise to exceptionally high zT value of 2.3 at 800 K for x = 0.20. The high zT value also showed enhancement of practical thermoelectric performances such as engineering zT_eng, device efficiency η, output power density P_d, and device zT_d. In addition, high zT compounds have good compatibility with the n-type I-doped PbTe compound, which can be applied to practical waste heat power generation.