https://hdl.handle.net/11256/47 2026-05-01T13:13:50Z https://hdl.handle.net/11256/997 Ws2-WTe2 Burton, Benjamin 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)WTe_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. 2016-01-01T00:00:00Z https://hdl.handle.net/11256/996 MoS2-MoTe2 Singh, Arunima; Burton, Benjamin A first principles phase diagram calculation, that included van der Waals interactions, was performed for the 3D bulk system (1−𝑋)·𝑀𝑜𝑆2−(𝑋)·𝑀𝑜𝑇𝑒2. Surprisingly, the predicted phase diagram has at least two ordered phases, at 𝑋≈0.46, even though all calculated formation energies are positive; in a ground-state analysis that examined all configurations with 16 or fewer anion sites. The lower-temperature I-phase is predicted to transform to a higher-temperature 𝐼′-phase at 𝑇≈500K, and 𝐼′ disorders at 𝑇≈730K. Both these transitions are predicted to be first-order, and there are broad two-phase fields on both sides of the ordered regions. Both the I- and 𝐼′-phases are predicted to be incommensurate, i.e., aperiodic: I-phase in three dimensions; and 𝐼′-phase in two dimensions.