Theory of minerals at extreme conditions: Predictability of structures and properties
Basic theory behind first-principles simulations has been reviewed by many authors, e.g. Jung & Oganov (2005b) in this volume, or by Oganov et al. (2002) and Cohen (1999); the main focus of this chapter is on applications of such simulation techniques, illustrating the power of modern simulation approaches. There are two main problems thatsimulations have to be able to solve in order to be useful:
prediction of crystal structure topology, i.e. of the structure type of the stable and possible metastable phases for a given chemical composition;
once structural topology is known, optimisation of the structure for given P–T conditions and calculation of physical properties.
While the second problem is practically solved for most properties, the first problem still poses great challengesand has no general practical solution. In principle, one should explore the entire energy surface and locate all local minima and the global minimum. However, the dimensionality of this surface is so overwhelmingly high that it is difficult to explore it efficiently. Nevertheless, there have been some recent successes in this direction and it seems that soon this problem may become tractable.
The method of Martonak et al. (2003) seems promising and using it Oganov et al. (inprep.) have been able to predict, at a fully ab initio level, several highpressure forms of MgSiO3. Since the problem of structure type prediction is still far from its solution, here we review what can be done once the structure type is known – e.g., how accurate the theoretically optimised structures are, how accurate the predicted simulations helped to resolve several important problems in mineral sciences.
Figures & Tables
Mineral behaviour at extreme conditions
Extreme conditions and their effects on matter and materials are currently fashionable topics in modern science. Perhaps the fascination derives from the unimaginable dimensions that grab our attention and push the boundaries of our imagination. Imagine the pressures in extremely dense neutron stars where electrons and protons are fused together and atoms collapse to the density of an atomic nucleus; imagine temperatures of thousands of degrees Kelvin at the solar surface, or multimegabar and terapascal pressures deep within the interior of our planets. But even a simple droplet of water represents an extreme environment when it comes into contact with an otherwise stable crystal of rock salt, causing the crystal to dissolve as external conditions are drastically changed. We have an inherent desire to understand these diverse kinds of phenomena in nature, the mechanisms of the material changes involved, as well as the extreme conditions which are becoming increasingly demanded to achieve the extraordinary performance of new engineering materials. This rapidly evolving area of science is necessarily interdisciplinary, as it combines fundamental physics, chemistry and biology with geoplanetary and materials science, in addition to increasingly becoming one of the keys to engineering and technology aimed at process optimisation. Current experimental methods permit materials to be studied at pressures of several megabars, temperatures of tens of thousands of degrees Kelvin, and to achieve magnetic fields of several thousand teslas. Moreover, the rapid surge in computer technology has, in turn, permitted the solution of many previously intractable problems, and now even allows the behaviour of matter to be predicted far beyond the range of conditions currently accessible to experimentation. Previously unknown phenomena such as the formation of new phases, new forms of electronic and magnetic order, melting, atomic and electronic excitation, ionisation or the formation of a plasma state might result from exposing matter to extreme conditions well beyond those which were characteristic of the equilibria at the time of formation. With this volume of EMU Notes in Mineralogy we have endeavoured to provide up-to-date reviews of our understanding of the behaviour of minerals and geomaterials at exterior conditions that are sufficiently extreme to induce changes. In total 18 chapters reflect the diversity of this theme, but also demonstrate how strongly interdisciplinary this domain of modern mineralogy has become, bringing together physicists, chemists and geologists as well as experimentalists and computer scientists. The present volume contains the contributions of the lectures presented at the 7th EMU School, held at the University of Heidelberg from June 19 to June 25, 2005.