Ab Initio Calculations of Hydroxyl Impurities in CaF2

作者：Shi, H.^*; Chang, L.; Jia, R.; Eglitis, R. I.

来源：Journal of Physical Chemistry C, 2012, 116(10): 6392-6400.

DOI：10.1021/jp211075g

• ELECTRONIC-STRUCTURE
• OPTICAL-PROPERTIES
• CALCIUM-FLUORIDE
• HARTREE-FOCK
• F-CENTERS
• SURFACE
• ENERGY
• EXCHANGE
• ACCURATE
• CRYSTALS

OH- in CaF2 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) (f)or the bulk case and HO11(\) and HOfull(\) for the surface case are the energetically most favorable configurations. For the (111) CaF2 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 CaF2. On the other hand, the formation of OH- impurities may also be due to the aggregation of separated oxygen and hydrogen impurities in CaF2. The formation of OH- impurities could be avoided in CaF2 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 CaF2.

• 出版日期2012-3-15
• 单位北京理工大学