Anharmonic lattice dynamics and structural phase transition of SnTe monolayer from first principles

In this work we investigate the role of quartic anharmonicity on the lattice- and thermo-dynamic properties of rectangular (γ) and square (β) phases of two-dimensional (2D) SnTe monolayer (ML) by using self-consistent phonon (SCP) theory, based on the first-principles calculations. For both phases, as compared to the usual harmonic approximation (HA), the renormalized phonon frequency at the optical modes (4-10) is found to be increased upon the inclusion of quartic anharmonicity via the SCP method, where the effects of cubic anharmonicity are neglected. At the experimentally observed transition temperature (T_c= 270 K), the difference in the vibrational free-energy between the square and rectangular phases of SnTe ML, calculated by using the anharmonic SCP correction is found to be much closer to the structural energy gain as compared to that obtained by using only the quasi-harmonic contribution. This validates the significance of SCP approach over the HA to explain the lattice dynamics properties and predict theT_cfor SnTe ML and similar 2D compounds. The calculated lattice thermal conductivity of square SnTe ML (e.g. 10.67 W m^-1K^-1at 300 K) is higher than that of the rectangular SnTe ML (e.g. 6.72 W m^-1K^-1at 300 K) due to the relatively higher corresponding thermodynamic parameters: specific heat capacity, group velocity, and phonon lifetime obtained for the square SnTe ML. Particularly, the low energy phonon modes are found to transport most of the heat in the system and, hence, played major role to the total lattice thermal conductivity.