CONDUCTIVITY AND NEGATIVE-U FOR IONIC GRAIN-BOUNDARIES

The authors show that charge-transfer excitations (like 2M2+ to M++M3+) can be lowered greatly in energy near grain boundaries, where sites are no longer equivalent. In special cases the excitations may be exothermic ('negative-U' behaviour); likely cases include (320) and (122) grain boundaries in FeO. Consequences include effects on conductivity, segregation of impurities with different valence, and on other charge-state-dependent properties

Atomistic modelling of the metal/oxide interface with image interactions

We calculate the interfacial energy and lowest energy relative position for an Ag (001)/MgO (001) interface. The dominant image terms and short-range repulsions are included in full, and the MgO ions are relaxed to equilibrium using the MIDAS code. An essential new feature is the suppression of charge density fluctuations with wave-vectors greater than a (Fermi wavevector) cutoff. Our results show that the powerful methods based on interatomic potentials, widely used for ionic systems, can be extended to metal/ionic interfaces

A calculation of the structure and energy of the Nb/Al2O3 interface

We have modelled the (111)(Nb)/(0001)(s)Nb/Al2O3 interface using an atomistic, static lattice simulation technique. The interaction between the metal and the oxide combines the short range interaction between the metal atoms and the oxide ions, the Coulomb interaction between the oxide ions and the induced image charge of the metal, and the energy required to immerse the ionic cores in the metal jellium. The short range interaction between the Al3+ ion and the Nb atom was found to be repulsive, but the O2-/Nb interaction was found to be attractive at separations greater than 0.23 nm. As a result the lowest energy interface was found to terminate on an oxygen plane of the Al2O3; crystal, with the Nb atoms placed over the vacant sites in the Al lattice. The interfacial energy of this interface was calculated to be -3.6 J/m(2). As in previous work the results agree well with LDF calculations. The calculated structure is also in good agreement with the interpretation of the HREM images of Nb films grown on the (0001) face of Al2O3 using Molecular Beam Epitaxy. Copyright (C) 1996 Acta Metallurgica Inc

Coherent nanoparticles in calcite A toughening strategy known to metallurgists is also used by the brittlestar

Living organisms use a wide range of minerals to perform a variety of functions, including familiar examples such as bones (for support), teeth (for mastication), and shells (for protection), as well as other less common functions, such as optical, magnetic, and gravity sensing. These biominerals are produced with elements that are present in the local environment under ambient conditions. The ability to mimic biological strategies to improve current materials and processing methods is a long-standing goal of material scientists. On page 1294 of this issue, Polishchuk et al. (1) characterized the properties of a biomineral in the skeleton of the brittlestar, Ophiocoma wendtii. An array of microlenses on their skeletons focus light onto an optical receptor, enabling them to detect shadows and hide from predators. Nanoprecipitates in these lenses also toughen the skeleton, an effect that is achieved in engineered metal alloys only through expensive heat treatments

Low temperature ferroelectric behavior in morphotropic Pb (Zr1−xTix)O3

We provide an insight into the switching of near-morphotropic composition of PZT, using molecular dynamics simulations and electrical measurements. The simulations and experiments exhibit qualitatively similar hysteretic behavior of the polarization for different temperatures showing widening of the P-E loops and the decrease in the coercive field toward high temperatures. Remarkably, we have shown that polarization switching at low temperatures occurs via polarization rotation, that is a fundamentally different mechanism from high-temperature switching, which is nucleation driven

Making tracks in metals

Swift heavy ions lose energy primarily by inelastic electronic scattering and, above an energy threshold, electronic losses result in damage to the lattice. Such high energy radiation is beyond the range of validity of traditional cascade simulations, and predictive damage calculations are challenging. We use a novel methodology, which combines molecular dynamics with a consistent treatment of electronic energy transport and redistribution to the lattice, to model how swift heavy ions form damage tracks. We consider a range of material parameters (electron-phonon coupling strength, thermal conductivity and electronic specific heat) and show how these affect the maximum lattice temperature reached and the extent of residual damage. Our analysis also suggests that fission tracks may form in alloys of archaeological interest

Making tracks: electronic excitation roles in forming swift heavy ion tracks

Swift heavy ions cause material modification along their tracks, changes primarily due to their very dense electronic excitation. The available data for threshold stopping powers indicate two main classes of materials. Group I, with threshold stopping powers above about 10 keV nm(-1), includes some metals, crystalline semiconductors and a few insulators. Group II, with lower thresholds, comprises many insulators, amorphous materials and high T-c oxide superconductors. We show that the systematic differences in behaviour result from different coupling of the dense excited electrons, holes and excitons to atomic (ionic) motions, and the consequent lattice relaxation. The coupling strength of excitons and charge carriers with the lattice is crucial. For group II, the mechanism appears to be the self- trapped exciton model of Itoh and Stoneham ( 1998 Nucl. Instrum. Methods Phys. Res. B 146 362): the local structural changes occur roughly when the exciton concentration exceeds the number of lattice sites. In materials of group I, excitons are not self- trapped and structural change requires excitation of a substantial fraction of bonding electrons, which induces spontaneous lattice expansion within a few hundred femtoseconds, as recently observed by laser- induced time- resolved x- ray diffraction of semiconductors. Our analysis addresses a number of experimental results, such as track morphology, the efficiency of track registration and the ratios of the threshold stopping power of various materials

Modelling radiation effects in solids with two-temperature molecular dynamics

The ability to predict the structural modifications of materials resulting from a broad range of irradiation scenarios would have a positive impact on many fields of science and technology. Established techniques for modelling large atomic systems, such as classical molecular dynamics, are limited by the neglect of the electronic degrees of freedom which restricts their application to irradiation events that primarily interact with atomic nuclei. Ab initio methods, on the other hand, include electronic degrees of freedom, but the requisite computational costs restrict their application to relatively small systems. Recent methodological developments aimed at overcoming some of these limitations are based on methods that couple atomistic models to a continuum model for the electronic energy, where energy is exchanged between the nuclei and electrons via electronic stopping and electron-phonon coupling mechanisms. Such two-temperature molecular dynamics models, as they are known, make it practicable to simulate the effects of electronic excitations on systems with millions, or even hundreds of millions, of atoms. They have been used to study laser irradiation of metallic films, swift heavy ion irradiation of metals and semiconductors, and moderately high ion irradiation of metals. In this review we describe the two-temperature molecular dynamics methodology and the various practical considerations required for its implementation. We provide example applications of the model to multiple irradiation scenarios that accommodate electronic excitations. We also describe the challenges of including the effects of the modification of the interatomic interactions, due to the excitation of electrons, in the simulations and how these challenges can be overcome

A molecular dynamics study of diamond exposed to tritium bombardment for fusion applications

Diamond, with its low atomic number and high thermal conductivity, is being assessed as a possible plasma facing material within a fusion reactor. Molecular dynamics simulations using the AIREBO potential were performed simulating the exposure of diamond to a plasma in conditions similar to those of the divertor region of a tokamak. Diamond surfaces at temperatures of 300 and 600 K were bombarded with 15 eV tritium at a high flux (10(29) ions m(-2) s(-1)). A layer-by-layer etching process was observed which, with the lack of any tritium diffusion though the remaining diamond structure, was responsible for limiting damage, and thus tritium retention, to the top 4-5 diamond layers. Analysis of this damaged region also showed a large amount of residual structure suggesting that bombardment below the physical sputtering threshold (similar to 30 eV) may not lead to amorphisation of the surface. (C) 2010 Elsevier B.V. All rights reserved