Diffraction techniques: Shedding light on structural changes at extreme conditions
Ronald Miletich, Clivia Hejny, Günter Krauss, Ullrich Angela, 2005. "Diffraction techniques: Shedding light on structural changes at extreme conditions", Mineral behaviour at extreme conditions, Ronald Miletich
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Following the discovery of X-ray scattering on crystalline materials, crystallography and the experimental application of diffraction techniques have resulted in our current understanding of the atomic arrangements and bonding in condensed phases.
Soon after Max von Laue’s famous experiments in 1912 (Fig. 1), for which he received the Nobel Prize in Physics in 1914, a period of pioneering work began. Starting with father and son Bragg in the early twenties, the “century of crystal-structure determination” brought insight to the atomic view of solid matter, which only had been a matter of speculation before the discovery of scattering by crystals. Scattering techniques were de-veloped over the subsequent years, not only using X-ray radiation, but also involving electron and neutron scattering phenomena. Nowadays, with the evolution of powerful radiation sources, such as neutron spallation sources or synchrotron radiation facilities, the nature of atomic structure can be visualised even for the most complex macromolec-ular systems which include thousands of atoms.
The pioneering work of structure solution was carried out at ambient conditions, starting with basic structures such as of sodium chloride, zincblende or diamond. Structure solution was not straightforward from the beginning. The so-called “phase problem”, i.e. the fact that the amount of phase shift on wave interference could not be determined experimentally from diffraction data, kept crystallographers busy for decades. It resulted in fundamental approaches, such as the Fourier summation of a set of squared (but not phased) amplitudes as introduced by A.L. Patterson (1934). H. Hauptman and J. Karle (1950) employed statistics and probability distributions as applied in the “direct methods” to overcome the phase problem.
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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.