Molecular dynamics simulations of lanthanum oxide surfaces

Classical molecular dynamics was used to investigate the equilibrium state of the surface region of as-grown La₂O₃. It is currently thought that bulk and epitaxial thin film La₂O₃ surfaces exhibit amorphous structures in the as-prepared state that yield bulk crystal states upon postdeposition annealing. The focus of the study is to determine if the as-prepared surface region of La₂O₃ is purely amorphous as indicated from prior experimental results. Using simulation cells sufficiently large to accommodate the formation of defects, phase segregation, compositional migration, and site defects, our results show that crystalline phases are evident from simulated X-ray diffraction patterns. Although the phase of these crystallites is unresolved, we suggest that combinations of distorted hexagonal, cubic, and nonstoichiometric phases are formed in the as-prepared state prior to annealing. These crystallites likely serve as nucleation site for long-range ordered crystal growth upon annealing.