STH centers in LiF and NaH crystals: ab initio calculations

An attempt has been made to compare between the energetic properties of the self trapped hole STH center in LiF and NaH isoelectronic crystals, using an embedded cluster model and second order Moller-Plesset perturbation correction of ab initio theory. LiF and NaH clusters were embedded in simulated Coulomb fields that closely approximate the Madelung potentials of the host crystal and STH reorientation, band structure and adsorptivity of atomic H were examined. LiF and NaH clusters in crystals were found to exhibit distinct differences in the energetic properties. While the off-center configuration in the bulk of LiF was more stable than the on-center configuration, the on-center configuration in the bulk of NaH was more stable. The activation energy barriers were calculated to be 0.07 and 0.30 eV for bulk and surface diffusion of STH in LiF, and 0.66 and 0.60 eV for bulk and surface diffusion of STH in NaH. On basis of X-2(-) bond elongation, LiF surface crystalline potentials were found to be less important than those of NaH. The order of stability of atomic H over LiF and NaH surfaces was totally reversed under the effect of STH imperfection. The STH enhances the adsorptivity of atomic H by ca. 4.49 and 3.85 eV on LiF and NaH surfaces respectively, and changes the nature of adsorption from physical adsorption to chemical adsorption. The dramatic increase in adsorption energies was found to be attributed to spin pairing rather than the differences in energy gaps between adsorbate and substrate electrons in the course of adsorbate substrate interactions. (C) 2001 Elsevier Science Ltd. All rights reserved.