Ab initio calculations of hydroxyl impurities in CaF ₂

OH ^- in CaF ₂ crystal and the (111) surface have been studied by using density functional theory (DFT) with hybrid exchange potentials, namely, DFT-B3PW. Three bulk and 20 surface OH ^- configurations were investigated, and we found that Configs OH ₍₁₁₁₎ for the bulk case and HO11(\) and HOfull(\) for the surface case are the energetically most favorable configurations. For the (111) CaF 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 there is a considerable covalency between the oxygen and hydrogen atoms, and the surface effect strengthens the covalency of surface OH ^- impurities. The studies on band structures and density of states of the surface OH ^--impurity systems demonstrate that there are two defect levels induced by OH ^- impurities. The O p orbitals form two superposed occupied O bands, located above the valence bands (VBs), and the H s orbitals do the major contribution to an empty H band, located below the conduction bands. Because of the surface effect, the O bands move downward, toward the VBs with respect to the relevant 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. Additionally, the study on the formation of OH ^- impurities shows that isolated hydroxyls are favorite to substitute fluorine ions and adsorb on the surface energetically in CaF ₂. On the other hand, the formation of OH ^- impurities may also be due to the aggregation of separated oxygen and hydrogen impurities in CaF ₂. The formation of OH ^- impurities could be avoided in CaF ₂ crystal if we can control the concentration of the oxygen atoms near the surface, because oxygen does the primary contribution to the formation of OH ^- impurities in CaF ₂.