A.M. El-Mahdy

An ab initio approach to bulk and surface properties of many-body energies and adsorptivity in MgO crystal

Abstract A finite MgO crystal, in which Coulomb potential in the bulk region closely approximates the Madelung potential in the host crystal, is constructed. A method was then suggested for generating the crystal surface and matrix Hartree–Fock calculations were carried out for isolated clusters and for clusters embedded in the bulk and the surface of the crystal. Overlap effects of nearest neighbor ions are tested. The convergence properties of many-body energies were examined as well as the effective pair potentials for some symmetrical distortions in the central region. The adsorptivity of Mg 2+ and O 2− ions as well as the nucleation mechanism of (Mg 2− O 2− ) 2 tetramer on crystal surface are discussed in relation to crystal growth and screw dislocation.

Excitons, electron center diffusion and adsorptivity of atomic H on LiH (0 0 1) surface: Ab initio study

Abstract An attempt has been made to examine the bulk and surface properties of exciton bands near F+, F and F− centers (α, β and γ bands), diffusion of electron centers (F+, F and F−) and adsorptivity of atomic H over the undefected and defected (F+, F and F−) surfaces of LiH using an ab initio embedded cluster method at the Hartree–Fock approximation and Moller–Plesset second-order perturbation correction. The results confirm the exclusive dependence of the exciton bands on the type of the electron center. The activation energy for bulk diffusion increases monotonically in the series F+→F→F−. Bulk and surface relaxation effects are more important for F+ than for F and F− centers. The introduction of F or F− center changes the nature of adsorption from physisorption to chemisorption. The introduction of F− center changes the nature of LiH surface from an insulating surface to a semiconducting surface. As F and F− centers are introduced, the HOMO and LUMO levels of the substrate shift to higher energies and the band gaps become narrower. These changes in the electronic structure make charge transfer between adsorbate and substrate energy levels and spin pairing with F center more facile in the course of adsorbate–substrate interactions.

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

Abstract An attempt has been made to examine the bulk and surface properties of exciton bands near F + , F and F − centers (α, β and γ 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.

Many body expansion and ion diffusion in LiH crystal

A finite LiH lattice whose Madelung potential in the central region closely approximates the Madelung potential in the host crystal is constructed. Hartree-Fock calculations were then carried out on lithium hydride clusters both within the crystal and as isolated species. The many body expansion terms and the probability of ionic motions which result in diffusion are examined and relaxation around diffused ions is taken into account. Calculations confirm that the many body expansion terms in the crystal environment are both convergent and smaller than for the isolated clusters. The two-ion rotation mechanism is the most probable. The barrier height for cation diffusion is less than for anion diffusion suggesting easier transfer of cations rather than anions within the LiH crystal. The results are correlated with those reported previously on LiF crystal.

An ab initio approach to many-body energies and Be2+V- dipoles in LiH crystal and clusters

A finite LiH crystal whose Coulomb potential in the central region closely approximates the Madelung potential in the unit cell of the host crystal is constructed. The beryllium ion is then introduced to initiate the Be2+V- dipole and to examine the perfect and defect properties of LiH clusters both within the crystal and as isolated species. These include the convergence properties of many-body energies, the defect formation mechanism and hydride ion migration. Crystal field and overlap effects are examined. Lattice relaxation around defect sites is allowed and the optimal relaxation mode is assigned. Dipole aggregates, the cluster-lattice interaction, defect formation energies, the free rotation of the Be2+V- dipole in two perpendicular planes, the energy of rotation of the cation vacancy around Be2+ and the tendency of Be2+ to associate with the cation vacancy are examined and explained in relation to their scientific and technological importance.

Bulk dislocation-U defect interaction, surface excitons and adsorptivity of atomic H on dislocated surfaces of LiH crystal: ab initio calculations

An ab initio embedded cluster method was used to examine the bulk dislocation-U defect interaction, surface excitons and the adsorptivity of atomic H on dislocated surfaces of LiH using the Hartree-Fock approximation and the second-order Moller-Plesset perturbation correction. In the LiH crystal bulk, the results confirm: (1) U1 and U2 centres make dislocations more facile, (2) dislocation processes do not reduce the ionic conductivity of highly populated edge centred hydride interstitials and (3) the dislocation-U defect interaction increases monotonically in the series face→volume→edge centred interstitial structures. On LiH crystal surfaces the results confirm: (1) the exclusive dependence of band gaps and exciton bands on dislocation, (2) the strongest adsorption of atomic H on a surface is associated with X-dislocations, (3) dislocations are unable to change the nature of physical adsorption to chemical adsorption and (4) the mobility of atomic H over the Z-dislocated surface is more facile than that over the X-dislocated surface. As X-surface dislocation proceeds, the HOMO and LUMO levels of the substrate shift to higher energies and the band gap becomes narrower. This change in the electronic structure suggests that charge transfer from the X-dislocated surface is more facile in the course of adsorbate-substrate interaction.