First Principles Phase Diagram Calculation for the 2D TMD system $WS_{2}-WTe_{2}$

TL;DR

This study uses first principles calculations with van der Waals interactions to map the phase diagram of the WS2-WTe2 system, revealing a pronounced asymmetry in solubility driven by metastable configurations.

Contribution

It provides a detailed phase diagram of the WS2-WTe2 system using extensive cluster expansion calculations, highlighting the importance of metastable states in phase stability.

Findings

All formation energies are positive.

A miscibility gap with asymmetrical solubility is predicted.

Metastable configurations influence phase behavior.

Abstract

First principles phase diagram calculations, that included van der Waals interactions, were performed for the bulk transition metal dichalcogenide system $(1-X) \cdot WS_{2} - (X) \cdot WTe_{2}$. To obtain a converged phase diagram, a series of cluster expansion calculations were performed with increasing numbers of structure-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 $\approx 150 N_{Str} \leq 376$, false ordered ground-states are predicted. With $N_{Str} \geq 399$, 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.