First principles phase diagram calculation for the 2D TMD system WS2−WTe2

First principles phase diagram calculations, that included van der Waals interactions, were performed for the bulk transition metal dichalcogenide system (1 -X) WS 2 -(X) WT e 2 . To obtain a converged phase diagram, a series of cluster expansion calculations were performed with increasing numbers of structural energies, (N str ) up to N str = 435, used to fit the cluster expansion Hamiltonian. All calculated formation energies are positive and all ground-state analyses predict that formation energies for supercells with 16 or fewer anion sites are positive; but when 150 N str 376, false ordered ground-states are predicted. With N str 399, only a miscibility gap is predicted, but one with dramatic asymmetry opposite to what one expects from size-effect considerations; i.e. the calculations predict more solubility on the small-ion S-rich side of the diagram and less on the large-ion Te-rich side. This occurs because S-rich low-energy metastable ordered configurations have lower energies than their Te-rich counterparts which suggests that elastic relaxation effects are not dominant for the shape of the miscibility gap.