Abstract
An embedded cluster method was used to examine the relative stabilities of H center orientations in the bulk of LiH as well as the effects of the H center on the adsorptivity of atomic H using the second order Moller-Plesset perturbation level of ab initio theory. The clusters were embedded in a simulated Coulomb field that closely approximates the Madelung potential of the host. The < 111 > orientation was found to be more stable than the < 110 > orientation by ca. 0.871 eV. An appreciable spin density redistribution on the H-2(-) molecular ion of the < 111 > and < 110 > orientations is observed. The H center improves the adsorptivity of atomic H on some sites by ca, 15.5 eV but disproves the adsorptivity on others by ca. 5.76 eV and enhances the electrical properties of the crystal. The diffusivity of atomic H over the surface is severely restricted by H center imperfection and the electrostatic curves explain the exclusive effects of H center on the adsorptivity and mobility of atomic H. As the H center is introduced, the top of the valence band and the bottom of the conduction band shift to lower energies and the valence-conduction band gap is reduced. This change in the electronic structure makes spin pairing between the singly occupied atomic orbital of H and the singly occupied molecular orbital of substrate more facile in the course of adsorbate-substrate interactions. (C) 2001 Elsevier Science B.V. All rights reserved.