Temperature-renormalized phonon and electron transport in thermoelectric Mg3Sb2: Dominant role of anharmonic phonon modes

Layered thermoelectric ${\mathrm{Mg}}_{3}{\mathrm{Sb}}_{2}$ has inspired increasing interest due to its inherently peculiar phonon and electron properties. Here, we propose the crucial role of temperature-induced renormalization of phonon and electron transports in the thermal, electronic, and thermoelectric performance by considering the peculiar temperature-dependent anharmonic phonon vibrational modes. After the phonon renormalization is included, the lattice thermal conductivity has a very weak temperature dependence of $\ensuremath{\sim}{T}^{\ensuremath{-}0.62}$ that agrees better with the experimental results than other theoretical pictures, in sharp contrast to the traditional harmonic ${T}^{\ensuremath{-}1}$ trend. This is because the strong quartic anharmonicity induces significant hardening of low-lying acoustic phonon modes at the Brillouin zone boundary with temperature and finally suppresses the scattering rate by reducing the phonon scattering phase space. The fundamental band gap anomalously increases with temperature rather than decreasing as in most semiconductors because the greatly strengthened electron-acoustic-phonon coupling by locally asymmetric atomic vibrations drops the valence band maximum significantly. Furthermore, the temperature dependence of thermopower is effectively improved by the temperature-renormalized electronic structures, and then, combined with the phonon renormalization, excellent thermoelectric performance in good agreement with the experimental data is described. Our work establishes the relationship of temperature-renormalized phonon and electron transports versus intrinsic anharmonic acoustic phonon modes, which is helpful for describing the related physical properties more precisely at elevated temperatures.