Identification of the soft mode in Na doped PbTe

As a leading thermoelectric material, PbTe is widely used in energy conversion applications largely due to its intrinsic low bulk thermal conductivity. However, the current phonon scattering mechanisms responsible for its thermal transport properties in the whole temperature region are not sufficient. Here, from Raman scattering, specific heat, thermoelectric transport, as well as Hall effect measurements, a systematic experimental work on p-type Na-PbTe is presented. A transverse optical (TO) mode softening with increasing temperature is observed by Raman scattering below 120 K, which can account for the anomalous behaviour of electrical resistivity, Seebeck coefficient and magnetoresistance. Combining with the constant gap detected by the jump of heat capacity near 120 K, we identify a soft TO mode from Landau theory. The soft TO mode induced an anomalous phonon scattering process in the high temperature zone shows great phonon anharmonicity beyond the conventional three-phonon and four-phonon processes. Our results reveal that the soft TO mode is vital to further suppress lattice thermal conductivity in PbTe to improve energy conversion efficiency.