Properties of F+, F and F- electron centers and adsorptivity of atomic H on LiF and NaH isoelectronic crystals:: an ab initio study

An attempt has been made to examine the bulk and surface properties of exciton bands near F+, F and F- centers (alpha, beta and gamma bands), diffusion of electron centers (F+, F and F-) and adsorptivity of atomic H over the defect free and defect containing surfaces of LiF and NaH isoelectronic crystals using an ab initio embedded cluster model at the second order Moller-Plesset perturbation level. The LiF and NaH clusters were embedded in simulated Coulomb fields that closely approximate the Madelung potentials of the host crystals. The isoelectronic LiF and NaH clusters in crystals were found to exhibit distinct differences in the title properties. The defect free and F+ band gaps and exciton bands of LiF were significantly greater than those of NaH. The LiF crystal was more sensitive to the relaxation effects than NaH. The activation energy barriers to the electron center diffusion hops in LiF were always greater than those in NaH. The H atom adsorbs more strongly on the defect free. F+ and F- surfaces of NaH relative to LiF. The reported changes in band structure due to surface imperfection explain the dramatic increase of atomic H adsorption over F and F- surfaces of LiF and NaH as well as the preferred stability of atomic H over the defect free and F+ defect containing surfaces of NaH in the course of adsorbate substrate interactions. The reported differences in properties are possibly attributed to the differences in the lattice interionic interactions and the extended charge distribution of the hydride anion. (C) 2001 Elsevier Science B.V. All rights reserved.