Lattice thermal conductivity of cubic GeTe with vacancy defects

Germanium telluride (GeTe) and its variants are promising compounds as high figure of merit thermoelectric materials due to their low lattice thermal conductivity. The strong anharmonicity and the intrinsic Ge vacancies are shown to be the origin of the low thermal conductivity. While the anharmonic effect on the lattice thermal conductivity has been systematically studied using the perturbation theory, the vacancy disorder has been treated perturbatively mostly as an extreme case of phonon-isotope scattering. This simplification ignores realistic features such as the nonbonding character and the detailed local environments near the vacancies. In this study, we calculate the lattice thermal conductivity of the cubic GeTe by considering the anharmonicity and the vacancy disorder on the same footing using the machine-learning potential molecular dynamics. We obtain the spectral function via the nonperturbative approaches, the velocity autocorrelation function, and the phonon unfolding scheme to investigate the effect of the vacancies on the lattice thermal conductivity. We find that the reduction in the lattice thermal conductivity by the vacancies originates from the momentum-dependent scattering of the acoustic phonon modes and the momentum-independent scattering of the optical phonon modes as determined by the strength of the anharmonicity.