First principles studies of the self trapped hole and the fluorine adsorption on the SrF₂(1 1 1) surface

By using density functional theory (DFT) with hybrid exchange potentials, namely DFT-B3PW, the ground states of self trapped hole and adsorbed fluorine atom on the strontium fluoride (1 1 1) surface are investigated. The self trapped hole at an interstitial anion site is denoted by H-center. In both the H-center and fluorine adsorption cases, the strong relaxations due to the surface effects are observed. In the H-center case, the unpaired electron distributes almost equally over two H-center atoms. This equivalent distribution of the unpaired electron is totally different from that of the bulk H-center [J. Phys. Chem. A 114 (2010) 8444]. The other case with an adsorbed fluorine atom lying outside the slab has a more polarized charge distribution with respect to the H-center case. The surface effects and the polarizations of H centers can be well explained with the calculated electric fields on the surfaces. A new β-hole band located 2.80 eV above the top of valence band (VB) is observed in the case of fluorine adsorption, and a new β-hole band located 4.26 eV above the VB is also observed in the H-center case. Specifically, the β-hole bands are primarily composed of p _z-orbitals, which are localized on the defect points.