Anharmonic lattice dynamics of SnS across phase transition: A study using high-dimensional neural network potential

SnS that exhibits strong lattice anharmonicity and a structural phase transition between the Pnma and Cmcm phases has emerged as a high-performance thermoelectric material. Herein, the lattice dynamics of SnS have been investigated by molecular dynamics to reveal the soft mode mechanisms across the phase transition. We construct a first-principles-based machine-learning potential, which is capable of reproducing the dynamical nature of the structural phase transition of SnS. We reproduce an explicit softening of the zone-center phonon mode and unveil a similar behavior at the zone boundary U = (0.5, 0.0, 0.5) of SnS, which are attributed to the large anharmonicity induced by the phase transition. Our results reveal a non-monotonic temperature dependence of the linewidth of the soft modes beyond the perturbative regime. The zone-center optical soft mode collapses across the structural phase transition, corresponding to a second-order nature. These results pave the way for further phonon engineering of SnS and the related IV–VI thermoelectrics.