Investigating the anharmonic lattice dynamics of tin sulfide using DFT and molecular dynamics simulations

Monolayer SnS exhibits two phases with similar crystal structures and low thermal conductivities, making it a promising candidate for phase change memory and thermoelectric applications. The high temperature square phase (single layer of the bulk Cmcm crystal) can be described as a thermal average of two degenerate configurations of the low temperature rectangular phase (single layer of the bulk Pnma crystal). In this work, we investigate the lattice dynamics of monolayer SnS to probe the thermally induced transition between phases. Using a frozen phonon approach with density functional theory (DFT), we confirm the existence of a displacive phase transition associated with strong anharmonicities of the square phase. To further understand the phase transition dynamics, we conducted DFT-based molecular dynamics (MD) simulations at finite temperature. Using the MD results, we estimate the free energy and thermal conductivity of the phases via the Temperature Dependent Effective Potential (TDEP) method. We find a strong dependence of the zero stress lattice parameters on the temperature, highlighting the importance of finite temperature effects in the study of the anharmonic phase transition dynamics of monolayer SnS.