First-principles calculations of surface H centers in BaF2

H center, a hole trapped at an interstitial anion site, placed on the (111) surface of Barium fluoride BaF2 has been studied by using density functional theory (DFT) with hybrid exchange potentials, namely, DFT-B3PW. Two different configurations of surface H center are investigated carefully. Both surface H-center systems have strong relaxations because of the surface effect. In the configuration that the interstitial fluorine atom is within the surface, named case 1 in this paper, the unpaired electron is almost equally distributed onto the two atoms of the H center, which is quite different from the bulk H-center case. The other configuration with one of the F atoms of the H center located above the BaF2 (111) surface (case 0) has a more polarized charge distribution as compared to that obtained in the bulk case and case 1. The calculation on total energies of different surface H-center configurations implies that H centers have a trend to locate near the surface. The creation of a surface H center in BaF2 results in a new beta-hole band located at the Gamma point 3.99 and 2.70 eV, for the cases 0 and 1, respectively, above the top of valence bands. According to our calculations on density of states, the constituents of the defect bands are cleared and the beta-hole band is primarily composed of p(z) orbitals localized on the H center.