Introduction by M. Hanfland, ESRF

Diffraction experiments have become a powerful tool to study the behaviour of matter at high pressures. They permit the investigation of diverse fields, the examples presented here range from the fundamental properties of solids to the chemistry in the interior of the Earth.

The volume of cubic diamond together with the optical phonon frequency has been measured to 140 GPa [1]. Comparisons with theoretical calculations will considerably improve our understanding of the fundamental behaviour of solids. A more practical aspect of the data is a possible revision of the pressure scale used at very high pressures.

The structure of a modulated guest host phase (Sb-II) has been described using a four-dimensional superspacegroup for the first time [2]. The structural investigation is accompanied by an ab initio band structure calculation, which confirms the energetic stability of the modulated phase in the observed pressure range. Modulated phases at high pressure are becoming quite common. Recently new types of modulated structures were discovered for I, and for Te and Se [3].

The equation of state and structural behaviour of synthetic spinel, a model for phases that are stable under Earth mantle conditions, have been studied to 30 GPa [4]. Structural investigations of the iron-silica interaction at extreme pressures and temperatures [5] probe chemical reactions at the core mantle boundary and provide a possible explanation for the anomalously high electrical conductivity of this region.

A major improvement to the ESRF's high-pressure facilities will be the move of the High-pressure Beamline from ID30 to ID27, planned to start in autumn 2004. The move from a high beta section (ID30, large source size, small divergence) to a low beta section (ID27, small source size, large divergence) will allow greater precision because of a significant decrease in the horizontal beam size. Two experimental hutches will provide separate setups for large-volume cell, diamond-anvil cell and laser heating. Two in-vacuum undulators in series will augment the flux by at least one order of magnitude.

[1] F. Occelli et al., Nature Materials 2, 151 (2003).
[2] U. Schwarz et al., Phys. Rev. B 67, 214101 (2003).
[3] K. Takemura et al., Nature 423, 971 (2003); C. Hejny and M.I. McMahon, Phys. Rev. Lett. 91, 215502 (2003).
[4] D. Levy et al., Am. Mineral. 88, 93 (2003).
[5] L. Dubrovinsky et al., Nature 422, 58 (2003).