First-principles study of the polar O-terminated ZnO surface in thermodynamic equilibrium with oxygen and hydrogen

Using density-functional theory in combination with a thermodynamic formalism we calculate the relative stability of various structural models of the polar O-terminated (000-1)-O surface of ZnO. Model surfaces with different concentrations of oxygen vacancies and hydrogen adatoms are considered. Assuming that the surfaces are in thermodynamic equilibrium with an O2 and H2 gas phase we determine a phase diagram of the lowest-energy surface structures. For a wide range of temperatures and pressures we find that hydrogen will be adsorbed at the surface, preferentially with a coverage of 1/2 monolayer. At high temperatures and low pressures the hydrogen can be removed and a structure with 1/4 of the surface oxygen atoms missing becomes the most stable one. The clean, defect-free surface can only exist in an oxygen-rich environment with a very low hydrogen partial pressure. However, since we find that the dissociative adsorption of molecular hydrogen and water (if also the Zn-terminated surface is present) is energetically very preferable, it is very unlikely that a clean, defect-free (000-1)-O surface can be observed in experiment.Comment: 10 pages, 4 postscript figures. Uses REVTEX and epsf macro

THE CALCULATED PROPERTIES OF GRAIN BOUNDARIES IN NICKEL OXIDE

Les calculs des propriétés des joints de coincidence en NiO faits par une simulation atomistique dans un réseau statique sont discutés. Les calculs considèrent les interfaces de torsion et de flexion, les interactions avec les défauts et les impuretés, les régions de charge spatiale et diffusion le long du joint.Atomistic static simulations of the properties of coincidence grain boundaries in NiO are discussed. The calculations include the structure of pure tilt and twist interfaces, the interactions with defects and impurities, space-charge regions and diffusion along the boundary

The stabilization of oxide and oxide-metal interfaces by defects and impurities

Lattice defects and impurities can play a dominant role in determining the stability and structure of interfaces involving oxide materials. A particularly important example is the oxide-metal interface whose critical feature is a large mis-match in the dielectric constant between the two materials. The resulting image interactions derived from charges in the oxide provide the principle binding between the materials. We show that the systematics of wetting by metals and other features of the interface stability can be interpreted in terms of a simple model based on these image interactions. Atomistic lattice simulations of other oxide interfaces and surfaces confirm these general features and emphasise the role of both defects and impurities. Examples discussed include the structures of hetero-interfaces, grain boundaries, point defects and impurities near oxide surfaces. The last example shows that impurity segregation can stabilise a surface such that the surface energy is negative

UNDERSTANDING OXIDE-METAL INTERFACES

One of the most important features of the interface between an oxide and a metal is the sharp change in dielectric constant between the two materials. This suggests that the stabilisation of charges (lattice ions and defect centres) in the non-metal by the polarisation of the metal may contribute strongly to the interfacial adhesion. This effect can be treated in classical electrostatics by the method of images. We present the application of this theory to the adhesion of bulk materials and to the properties of thin films and coatings. The predictions of the continuum theory are compared with atomistic calculations of interfaces. Finally, we consider how, in light of this theory, how interfacial adhesion can be influenced and controlled

An appraisal of the molecular model for the vk centre

AbstractThe Vk centre in halide crystals is often described in terms of an X2− molecule ion. Recent molecular calculations for the halogen molecule negative ions have enabled a detailed comparison to be made between the theoretical anion and the results from optical and spin resonance studies on the crystals. The optical absorption line widths are naturally dependant on the host lattice, but the excitation energies and spin resonance constants may be readily interpreted in terms of the molecule. Certain anomalies in the spin resonance data for the heteronuclear defects, in particular ClI− and BrI−, show that the simple wavefunction constructed from “s” and “p” atomic orbitals does not adequately describe the polarisation of the large halogen atoms, and a more flexible wavefunction will be needed to calculate the spin resonance constants for these ions

Key Issues Influencing the Performance of Near-Field Barriers

ABSTRACTIn developing a national strategy for the disposal of radioactive wastes, each country will consider isolation options that are appropriate to the types of waste that are produced. The options are developed in response to specific national regulatory requirements, and thus will focus on different aspects of performance. However, there are a number of technical concerns that are common to all programmes. The major issues concerning the behaviour of the chemical and physical barriers in the near field of a radioactive waste repository are discussed in this paper.The description of key issues has been divided into the following categories: barrier design, barrier evolution, scientific understanding through modelling, and validation of performance. The near-field barriers are selected and designed to provide appropriate radionuclide containment and control. Factors affecting the evolution of these barriers, such as container degradation and gas generation, determine the subsequent release of radionuclides to the human environment. Modelling repository evolution is therefore an integral feature of performance assessments, and issues such as the treatment of inhomogeneities and non-equilibrium chemistry may need to be addressed. However, the use of mathematical and computer models implies a requirement for validation. The use of demonstration experiments and natural analogues builds confidence in the predictions of repository performance models, and provides a degree of validation for otherwise inaccessible timescales.</jats:p