First-principles calculations on surface hydroxyl impurities in BaF2

OH impurities located near the (111) BaF2 surface have been studied by using density functional theory (DFT) with hybrid exchange potentials, namely DFT-B3PW. Twenty surface OH configurations were studied, and the hydroxyls located on the first surface layer are the energetically most favorable configurations. For the (111) BaF2 surface atomic layers, the surface hydroxyls lead to a remarkable XY-translation and a dilating effect in the Z-direction, overcoming the surface shrinking effect in the perfect slab. Bond population analysis shows that the surface effect strengthens the covalency of surface OH impurities. The studies on band structures and density of states (DOS) of the surface OH -impurity systems demonstrate that there are two defect levels induced by OH impurities. The O p(x) and p(y) orbitals form two superposed occupied O bands, located above the valence bands (VB), and the H s orbitals do the major contribution to an empty H band, located below the conduction bands (CB). Because of the surface effect, the O bands move downward, toward the VB with respect to these bands in the bulk case, and this leads to narrowing of the VB -> O gap and widening of the O -> H gap which corresponds to the first optical absorption. (C) 2011 Elsevier B. V. All rights reserved.