First-principles analysis of the local atomic environment in calcium lanthanum sulfide ceramics

Abstract

Density functional theory is utilized to investigate the energetics and mechanisms of sulfur loss across different stoichiometries of La2S3:CaS ceramics. These atomistic results reveal that the local atomic environment around each sulfur atom, consisting of six cation sites, can be used to predict the vacancy formation energy of the atom. Three nominal compositions are considered, 50:50, 65:35, and 90:10 La2S3:CaS, for two different atomic configurations, La2S3-like and La3S4-like. Relations were developed utilizing multivariable linear models. It is shown that the number of cation vacancies and lanthanum atoms in the local environment are among the most important factors in determining the energetics of a sulfur vacancy. Bader charge analysis reveals this is related to the degree of localization of charge change around the vacancy site and how close to the preferred oxidation states the lanthanum is. Results also suggest that while defect-free La2S3-like structures are energetically favorable, once a critical vacancy content is reached, the La3S4-like structure becomes the lower energy structure, which agrees qualitatively with recent experimental observations. This has important implications on how defect content and stoichiometry in this system are related. These findings also provide useful guidance on how these materials might best be processed.