Strong anharmonicity with large mode-Grüneisen parameter in Na-doped PbTe

Understanding the lattice dynamics in $p$-type PbTe-based materials is vital to their phonon engineering and to further lowering thermal conductivity in this prominent thermoelectric family. Here we report a systematic study of the high-pressure behavior of ${\mathrm{Pb}}_{0.99}{\mathrm{Na}}_{0.01}\mathrm{Te}$ using Raman scattering and x-ray diffraction measurements together with first-principles calculations. Our calculations reveal the important contributions of the optical modes to the heat transfer. The evolution of both the phonon vibrational and structural properties with pressure is established within the initial phase, which has a cubic NaCl structure. The experimentally determined average Gr"uneisen parameter of about 4 in this phase is larger than those of most other high-performance thermoelectric materials, demonstrating the giant anharmonicity in such a rock-salt lattice. We show that the primary contributor to this strong anharmonicity originates from the transverse optical mode, which possesses the highest mode-Gr"uneisen parameter due to its sensitive dependence on the lattice change. Our findings suggest that managing the highly volume-dependent phonons in thermoelectrics is the key to further tailoring their heat conduction.