High thermoelectric performance in XAgSe2 (X = Sc, Y) from strong quartic anharmonicity and multi-valley band structure

Abstract

The thermal transport and thermoelectric characteristics of the chalcogenide XAgSe₂ (X = Sc, Y) are comprehensively investigated through first-principles calculations. To treat the quartic anharmonicity completely, which encompasses four-phonon scattering and temperature-driven phonon frequency shift, the Boltzmann transport equation is solved on top of self-consistent phonon theory. Our investigation reveals that the soft X–Ag bond and the resonant bond in the pseudocage composed of X and Se atoms lead to ultralow lattice thermal conductivity. Moreover, the multi-valley electronic band structure displays good dispersion and high degeneracy, resulting in high power factors. Remarkably, p-type YAgSe₂ captures zT values of 3.70 at 900 K, breaking the long-standing record for zT < 3. The results show that these materials have great potential for thermoelectric applications.