Using periodic density functional theory, the structure and migration energies were

calculated for fluorine vacancies and interstitials in the bulk and at the stoichiometric bulk-truncated surface of these alkali-earth fluorides. A study was

then made of the adsorption of water and hydrogen, in both molecular and

dissociated form, at the ideal surface, and at neutral and charged vacancies in the

surface and sub-surface layers. The results demonstrated that molecular adsorption

was strongly favoured in nearly all cases. The migration paths and barriers were

also studied for the most probable configurations on the surfaces, and it was found

that water was highly mobile on the surface; even when adsorbed at defects. In

general, CaF2 and SrF2 exhibited a similar behaviour with respect to water, while

the adsorption energies and migration barriers for BaF2 were smaller.

Structure and Diffusion of Intrinsic Defects, Adsorbed Hydrogen, and Water

Molecules at the Surface of Alkali-Earth Fluorides Calculated Using Density

Functional Theory. A.S.Foster, T.Trevethan, A.L.Shluger: Physical Review B,

2009, 80[11], 115421