Introduction by G. Wortmann, Universität Paderborn (Germany) and M. Krisch, ESRF

The investigation of materials under extreme conditions such as high pressure and high temperature constitutes an important field in modern condensed matter research, covering a large variety of disciplines including solid-state physics, materials design, and earth and planetary science. The high brilliance of undulator-based X-ray sources, together with important progress in focusing optics, makes it possible to obtain spot sizes in the few micrometre range, and thus to perform experiments routinely at multi-megabar (1 Mbar = 100 GPa) pressures. Coupling pressure with temperature and/or magnetic fields further extends our knowledge about the phase diagram as well as the electronic and magnetic properties of materials.

High-pressure instrumentation comprises, besides the widely utilised diamond anvil cell (DAC), large volume cells such as, the Paris-Edinburgh cell, where temperatures up to 2300 K at 10 GPa can be reached. This temperature range can now be further extended by combining DAC experiments with laser heating thus reaching the conditions of Earth and planetary cores. On the other end of the temperature scale, specially designed cryomagnets, compatible with DACs, give access to magnetic studies in the few Kelvin regime.

X-ray diffraction (XRD) is historically the first and still the most wide-spread synchrotron radiation based high-pressure technique, providing invaluable insight into structural properties and phase stability of solids. The outstanding characteristics of third generation synchrotron X-ray sources have helped to push the frontiers of XRD further, and also extended studies under extreme conditions to other synchrotron techniques. Amongst these are inelastic X-ray scattering (IXS) and nuclear resonant scattering (NRS). For example, IXS and nuclear inelastic scattering (NIS), both with meV resolution, are now applied well above 1 Mbar to study lattice dynamics and sound velocities in samples of actual geophysical interest, such as the high-pressure phase of iron (-Fe) and iron bearing compounds [1-4]. Magnetic and electronic properties under extreme conditions can be studied by nuclear forward scattering (NFS), thus complementing other synchrotron radiation based spectroscopies such as X-ray magnetic dichroism, magnetic scattering and absorption techniques. X-ray emission spectroscopy may probe local magnetic spin moments, as exemplified in a high-pressure study of the high-spin/low-spin transition in a Fe mineral of the Earth´s lower mantle, presented in the following article.

As in the case of -Fe, it is often the complementarity and interplay of different synchrotron radiation techniques that helps to advance the understanding and to identify future challenges. This is demonstrated in the following selected highlights. SmS is a compound well-known for its first-order valence transition. A NFS study detected for the first time magnetic order in SmS at pressures above 2 GPa, where Sm is trivalent. An IXS study of the same system [5] revealed a soft mode in the LA[111] phonon branch in the same pressure range, speculatively connected with the possible occurrence of superconductivity. In a similar way, complementary structural and dynamical studies constitute a very powerful ensemble as is nicely exemplified in the studies of caesium in the bcc and fcc crystal structure as well as of the polymorphism in liquid phosphorus.

The selected highlights on studies under extreme conditions give a representative cut through the various techniques utilised in the High Resolution and Resonance Scattering Group, and show the rich diversity of the investigated topics.

[1] R. Lübbers, H.F. Grünsteudel, A.I. Chumakov and G. Wortmann, Science 287, 1250 (2000).
[2] G. Fiquet, J. Badro, F. Guyot, H. Requardt and M. Krisch, Science 291, 468 (2001).
[3] H.-K. Mao et al., Science 292, 914 (2001).
[4] G. Fiquet et al., accepted in Physics of the Earth and Planetary Interiors.
[5] S. Raymond et al., Phys. Rev. B 66, 220301 (2002) and ESRF Highlights 2002, page 55.