Displacive phase transitions
Published:January 01, 2005
Among the structural phase transitions, displacive phase transitions comprise those that only require small collective displacements of individual atoms. A small displacement of atoms in this context amounts to fractions of the nearest neighbour interatomic distances, i.e. generally at most a few tenths of an ångstrom. Displacive transitions occur spontaneously and reversibly at specific pressure and temperature conditions. Because of this, their direct observation is inextricably linked to the use of in situ methods, usually requiring a non-trivial sample environment, e.g. high-pressure cells, furnaces or cryostats. This definition puts displacive phase transitions in contrast to those structural phase transitions that involve significant diffusion of atoms, e.g. cation ordering transitions or entirely reconstructive phase transitions.
As this introductory text should serve as a guide to the analysis of experimental data, it will be predominantly concerned with the theory of displacive phase transitions and not with the experimental techniques employed to obtain the necessary data. Alarge number of in-depth review articles and textbooks devoted to the subject has already appeared in the recent past. It is therefore not the aim of this text to introduce every imaginable aspect of displacive phase transitions. Many of the details that are necessarily being omitted can be found elsewhere (e.g. Binder, 1987; Salje, 1992a, 1992c, 1993; Dove, 1997; Carpenter et al., 1998a; Carpenter & Salje, 1998). What this text is trying to achieve is to transport a general picture of the theory and its application to experimental data, accompanied by an explanation of technical terms where they might appear.
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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.