First Principles Simulations Testhttps://hdl.handle.net/11256/472026-04-11T11:13:06Z2026-04-11T11:13:06ZWs2-WTe2Burton, Benjaminhttps://hdl.handle.net/11256/9972021-10-29T16:01:59Z2016-01-01T00:00:00ZWs2-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.
2016-01-01T00:00:00ZMoS2-MoTe2Singh, ArunimaBurton, Benjaminhttps://hdl.handle.net/11256/9962021-10-29T15:48:40ZMoS2-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.