0000000000017508
AUTHOR
Patrick W. M. Jacobs
A First-Principles Study of the Ag/a-Al2O3(0001) Interface
Ab initio simulations of the Ag/a-Al2O3(0001) interface have been performed for periodic slab models. We have considered Al- and O-terminated corundum surfaces, low and high substrate coverages by silver, as well as the two preferred Ag adsorption sites. The two different terminations give rise to qualitatively different results: silver physisorption on the Al-terminated substrate and chemisorption on O-terminated one. The latter could be treated as a possible model for the defective Al-terminated substrate, where the outermost aluminium ions are removed (completely or partly). This makes O-terminated surface highly reactive towards a deposited metal, in order to restore initial corundum st…
On the mechanism of the interaction between oxygen and close-packed single-crystal aluminum surfaces
Abstract Using periodic first principles simulations we investigate the interaction of oxygen molecules with both regular Al(111) and Al(001) surfaces as well as a stepped Al(111) substrate. The limitation of this approach is the use of thin metallic slabs with a limited range for their coverage by adsorbed oxygen. The advantage is the detailed modeling that is possible at an atomic level. On the regular Al(111) surface, we have been able to follow the oxidation process from the approach of O 2 molecules to the surface, through the chemisorption and absorption of O atoms, up to the formation of first Al 2 O 3 formula units. An energetically feasible mechanism for the formation of these Al 2…
Theoretical Simulations of Surface Relaxation for Perovskite Titanates
The (100) and (110) surface relaxations are calculated for SrTiO3 and BaTiO3 perovskite thin films Using a semiempirical shell model, the positions of atoms in 16 near-surface layers placed atop a slab of rigid ions are calculated. Surface rumpling and surface-induced dipole moments perpendicular to the surface are calculated for different surface terminations. Surface relaxation is found much larger for the (110) surface. Our results for the (100) surfaces are compared with ab initio calculations and LEED experiments.
<title>Large-scale computer simulations of metal/oxide interfaces with defects</title>
Ab initio slab simulations have been performed for silver adhesion to the perfect and defective MgO(001) surfaces. For 1/4 Ag monolayer (ML) coverage of perfect substrate, we observe small silver adhesion energies over both O2- and Mg2+ ions on a regular MgO(001) substrate (0.23 and 0.22 eV per Ag atom, respectively), with negligible interfacial charge transfer towards metal atoms. For larger Ag coverages (beginning with 1/2 ML), silver adsorption over regular O2- ions is much more favorable. We demonstrate that point surface defects on a magnesia surface increase markedly the metal adhesion energy and cause a redistribution of the electron density across the interface. The results for elec…
The adhesion properties of the Ag/α-Al2O3() interface: an ab initio study
Ab initio computer simulations of the atomic and electronic structure of the Ag/a-Al2O3(0 0 0 1) (corundum) interface have been performed for a periodic two-dimensional slab model using the Hartree–Fock method and a posteriori electron correlation corrections.We have considered both Al- and O-terminated corundum substrate surfaces.The dependence of the adhesion energy on the interfacial distance has been analyzed for the two most favorable Ag adsorption positions over corundum and for two different metal coverages (a 1/3 monolayer (ML) of the Ag(1 1 1) crystallographic plane and a full Ag(1 1 1) monolayer).The two different terminations (Al- and O-) give rise to qualitatively different resu…
Theoretical simulations of the radiation-induced defect processes in insulating materials
Abstract The results of two basic kinds of computer simulations of radiation-induced processes in insulating materials, one based on quantum-mechanical and pair-potential (atomistic) approaches, and the other a phenomenological theory of diffusion-controlled reactions, are presented. It is shown that, by combining different techniques (atom-atom potentials and semi-empirical quantum chemical methods) the optimized geometry and the electronic structure of a family of hole centres in crystalline corundum (α-Al2O3) could be found. Their energetics are analyzed; V2−, V−V−Mg hole centres all have a common basic element, namely the diatomic molecule O23−, which is responsible for their similar ab…
Modelling of silver adhesion on MgO(100) surface with defects
We show how surface defects (especially Fs 0 and Vs 0 centres) can play a major role in the adhesion of Ag (at 1:4 and 1:1 coverages) on the MgO(100) surface. Our calculations use a periodic (slab) model and an ab initio Hartree-Fock approach with a posteriori electron correlation corrections. We are able to analyse the interatomic bond populations, effective charges and multipole moments of ions, in combination with the interface binding energy and the equilibrium distances. Both surface defects cause strong redistributions of the electron density which increase the binding energy of metal atoms by more than an order of magnitude. This implies radiation-induced strengthening of metal adhes…
Semi-empirical indo and shell-model calculations for perovskites
Abstract Structural, phonon and some elastic and dielectric properties have been calculated for various paraelectric or ferroelectric phases of the perovskites KNbO3 and SrTiO3, using either the semi-empirical INDO (Intermediate Neglect of Differential Overlap) method or a temperature-dependent shell model. The INDO method was used to calculate the energy changes resulting from [100], [110] or [111] displacements of Nb atoms in the cubic perovskite cell of KNbO3, at 0K. The conventional shell model gives a good account of the elastic, dielectric and phonon properties of the cubic phase of strontium titanate at room temperature, but difficulties remain in modelling the permittivity and elast…
Ab InitioModeling of Metal Adhesion on Oxide Surfaces with Defects
Our ab initio studies show that surface defects cause redistribution of the electron density which can increase substantially the binding energy of metal atoms to oxide surfaces. The results for electron $({F}_{s}^{0})$ and hole $({V}_{s}^{0})$ centers in the adhesion of Ag atoms (at 1:4 and 1:1 coverages) to a MgO(100) surface, combined with previous studies for charged defects, support earlier ideas of the mechanism of radiation-enhanced adhesion of nonreactive metals on oxide substrates. The results suggest that some optical control of adhesion energies is possible through charge transfer.
Quantum chemical simulations of hole self-trapping in semi-ionic crystals
A novel formalism is presented for reliable calculations of the energetics of hole self-trapping in semi-ionic solids with mixed valence bands. Unlike previous model-Hamiltonian-type approaches, it is based on self-consistent quantum chemical INDO simulations of the atomistic and electronic structure of a self-trapped hole, making no a priori assumptions about a particular form of its localization (if any). This formalism is applied to the problem of hole self-trapping in corundum crystals (a -A1203). The hole self-trapping is found to be energetically favorable in the form of a diatomic 02 molecule with strong covalent bonding quite similar to the self-trapped hole (VK-center) in alkali ha…
Calculations of the atomic structure of the KNbO3 (110) surface
Abstract The O-terminated KNbO 3 (110) surface is modeled using a semi-empirical shell model and two different short-range interatomic potentials. We find this surface to be unstable with respect to a strong reconstruction and K-termination. This conclusion is confirmed by preliminary calculations using the ab initio linear combination of atomic orbitals (LCAO) formalism.
Comparative theoretical study of the Ag–MgO (100) and (110) interfaces
We have calculated the atomic and electronic structures of Ag–MgO(100) and (110) interfaces using a periodic (slab) model and an ab initio Hartree–Fock approach with a posteriori electron correlation corrections. The electronic structure information includes interatomic bond populations, effective charges, and multipole moments of ions. This information is analyzed in conjunction with the interface binding energy and the equilibrium distances for both interfaces for various coverages. There are significant differences between partly covered surfaces and surfaces with several layers of metal, and these can be understood in terms of electrostatics and the electron density changes. For complet…
Modeling of Point Defects in Corundum Crystals
Several different approaches including Hartree-Fock ab initio cluster calculations, semiempirical INDO calculations, and atom-atom potentials were used for modeling of the spatial and electronic structure as well as migration mechanisms of both intrinsic defects (self-trapped and defect-trapped holes, O and Al vacancies) and impurities (transition-metal ions like Co, Fe, Mg, Mn, Ti). The atomic structure of all hole centers is found to be similar to V[sub K] centers in alkali halides (two-site model); their formation is energetically favorable. The energy required for 60[degree] hole reorientations inside the basic oxygen triangles is found to be similar to both the energy for hops between …
Semi-empirical defect calculations for the perovskite KNbO3
A new parametrization of the classical shell model for the cubic phase of the perovskite KNbO3 has been derived and used to calculate the structural, elastic and dielectric properties of this material. Using this parametrization, the defect formation and migration energies, as well as atomic displacements, have been calculated. In parallel, the quantum mechanical method of the intermediate neglect of the differential overlap (INDO) has been applied to the same problem. The migration energies for the O vacancy obtained by these quite different methods are reasonably close (0.68 eV and 0.79 eV, respectively) and also agree with the only experimental estimate available of approximately 1 eV. A…
Formation, diffusion, and aggregation of radiation-induced defects in MgO and α-Al 2 O 3
MgO and M2O3 (corundum) are important ceramic materials with several technological applications including theirpossible use in fusion reactors' . Weare therefore studying the properties ofpoint defects (vacancies, interstitials, andtheir aggregates) and electronic defects (trapped holes and electrons) in these materials. Pair-potential (PP) calculationswere carried out using the Mott-Littleton (ML) two-region strategy, as implemented in the computer code CASCADEdue to Leslie. Details ofthe method, including the construction ofPPs, are in the literature3. For semi-empirical INDO(intermediate neglect ofdifferential overlap) calculations computer codes developed at the University of Riga were …
Modelling of point defects in α-AL2O3
Abstract The semiempirical method of the Intermediate Neglect of Differential Overlap (INDO), in the program SYMSYM, has been used for calculations on perfect and imperfect corundum crystals. For the perfect crystal the periodical Large Unit Cell (LUC) model was used while the Molecular Cluster (MC) model was used in defect calculations. By means of the MC model, we have investigated the optical properties of electronic centers (F+, F, F−, FMg, F− Mg) in corundum. Calculated optical properties of these defects are compared with experimental values and new bands are predicted to exist in the absorption spectrum of Mg-doped corundum. The energy levels of F-type and Mg-impurity related centers…