Key phonon modes to determine the phase transition of two dimensional Janus transition metal dichalcogenides: a DFT and tight-binding study

Phase stability and the phase transition of Janus transition metal chalcogenides (TMDs) have become interesting issues that have not been fully resolved since their successful synthesis. By fitting the results from first principles calculations, a tight-binding dynamics matrix of the 1T′ phase is constructed and the eigenvectors are also obtained. We propose a method to project the atomic motion causing the phase transition from 2H to 1T′ onto these eigenvectors, and identify four key phonon modes which are the major factors to trigger phase transition. Temperature excitation is used to excite the key modes and the free energy criterion is used to determine the phase stability. The relatively large enthalpy difference between the 2H and 1T′ phases favours the 2H one as the stable phase at low temperature. While the 1T′ phase has a quick increase in vibrational free energy with rising temperature, especially for 1T′ Janus TMDs which have a quicker increase in the total free energy than that of 1T′ non-Janus TMDs, making them show a lower phase transition temperature. Our work will deepen our understanding of the phase transition behavior of 2D Janus TMDs, and the tight-binding dynamics matrix and the method to obtain the key modes will be a useful tool for further study of the phase transitions of 2D Janus TMDs and other related materials.