First-principles calculations on surface hydroxyl impurities in BaF ₂

OH ^- impurities located near the (1 1 1) BaF ₂ 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 (1 1 1) BaF ₂ 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.