1. Defects in solids
We are interested in three specific areas of defects in solids all of which are of technological significance, topical and are important areas of chemical research and modelling.
- Colour centres The nucleation of defect clusters occur commonly in irradiated solids but there is no practical general theory which allows one to predict the separation between the nucleation and growth stages. In pursuit of transferable ideas in this field we are investigating the effects of radiation on the clustering of F-centres in irradiated alkali halides. Specifically we are considering the dependence of nucleation and the transition between nucleation and growth of both temperature and dose rate.
- Solute defects and their interactions in ferritic alloys The main aim of this work is to develop computer models which can describe the redistribution of solute elements in b.c.c. and ferritic alloys under irradiation. Thus far the work has focussed on the statistical analysis of models and their parameterization for the diffusion of phosphorous and boron impurities in b.c.c. iron by means of static lattice simulations. In order to understand the complexity of interstitial migration it is necessary to extend the study of these processes by molecular dynamics as well as static simulations.
- Heat of transport Atomic movement in solids are biased when the temperature of the system is non-uniform, with the result that net fluxes of the various atomic species arise. In turn these lead to changes in composition (the Ludwig-Soret effect) which is of practical significance in engineering components subject to large temperature gradients (e.g. turbine blades). In view of the general importance of the problem, the broad aim is to develop ways of computing heats of transport from a knowledge of the interatomic forces acting in the material and thereby to obtain a wide understanding of the factors which determine this quantity. Only very partial insights into these factors exist at the present time. We intend to further develop our recent implementation of the isothermal molecular dynamics method on models of insulating materials to metals and ionic conductors. There is a corresponding experimental programme in this area at the University of Hanover with whom we maintain strong links.
2. Computer simulation of interfaces
The modelling of the ion transfer across material interfaces is important in many physical systems. An example of such a system is in a solid-state microbattery. The diffusion of ions across a solid interface in a two-dimensional triangular lattice system has been studied previously by Monte Carlo techniques. We are aiming to develop more realistic models of such systems. In particular the inclusion of coulombic interactions in the two-dimensional model and to explore the ion diffusion by molecular dynamics. We will be particularly interested in the lattice voltage as a function of ion concentration which for certain systems has been determined experimentally. We also intend to address the three-dimensional problem.
The metal/electrolyte interface. Here we are attempting to develop realistic models of both the metal and electrolyte in order to gain an understanding of the physical nature of the interfacial region. Of particular interest is the ion profile close to the metal surface and the temperature dependence of the capacitance which has been reliably determined by experiment.
3. Lattice dynamics of molecular crystals
Although the theory of lattice vibrations is well-established, its application to molecular crystals has been far from successful. Even for simple molecular crystals, such as the hydrogen halides and the halogens, there are no models which satisfactorily explain the experimental data, e.g. Raman and infrared spectra and the phonon dispersion relations. The current effort involves the development of a physically acceptable model of the crystal that includes a description of the molecular polarisability.