Introduction by S. Pascarelli, ESRF

Studies under extreme conditions of temperature and pressure represent one of the main research topics of the X-ray Absorption and Magnetic Scattering Group. Most of the activities in this area concern X-ray absorption fine structure studies (EXAFS and XANES) on beamlines BM29 and ID24 up to temperatures of 2500 K and pressures of 100 GPa. Very recently it also became possible to perform resonant magnetic X-ray scattering (RMXS) on beamline ID20, at pressures up to 3 GPa, variable in situ, and a range of temperatures reaching 1.5 K. The scientific issues addressed by these experiments cover many different fields and it is impossible to cite all of them. It is interesting however to highlight the trends that have emerged in the past year.

On the X-ray absorption beamlines BM29 and ID24, the simultaneous acquisition of X-ray diffraction and absorption spectra has been increasingly exploited to correlate changes in the local and electronic structure with long-range structural rearrangements occuring at phase transitions through the P-T phase diagram. In amorphous Ge, an abrupt change in both the local structure and electronic density of states near the Fermi level was observed to occur around 8 GPa, with no detectable long range order modification [1], indicating the occurrence of a polyamorphic phase transition. In the GeTe system, where a first phase transition transforms the crystal symmetry from rhombohedral to cubic, it was very surprising to see that upon decompression to ambient conditions the high pressure long range symmetry is preserved but the local structure recovers its initial rhombohedral arrangement [2].

The energy range of applications using the Diamond Anvil Cell (DAC) has increased considerably in the past year. Using drilled diamonds, it was possible to perform Ba L3 XANES studies (5.2 KeV) on Ba-doped silicon clathrates, showing that the displacement of Ba atoms from the center of the cages is accompanied by changes in the Ba-Si hydridisation [3]. At the other extreme, successful EXAFS data was collected on Rb-doped systems (15.2 KeV), including silicon clathrates [4]. Here again the availability of simultaneous XRD detection was essential to correctly follow the onset of cage-collapse instability.

In the field of liquids, recent studies on RbBr acqueous solutions up to 2.8 GPa and 430 K have shown that, upon increasing pressure, water undergoes a structural transformation which involves considerable molecular reorientation [5]. Efforts are also being dedicated to implement additional techniques aimed at the study of the physical properties of liquids under extreme conditions, such as radiometric measurements of viscosity [6].

A clear interest towards m-XAS applications is emerging. Using a 10 x 10 µm beam, mapping of the oxidation state of Fe in (Fe,Mg) silicates and in Mg-Fe-O-compounds subject to high pressure in a laser irradiated or externally heated DAC has allowed to shed light on the possible chemical reactions occuring in the Earth's lower mantle [7].

Finally, the recent feasibility of RMXS on beamline ID20 has opened the way to a wealth of information, both novel and complementary to neutron scattering, as the exploitation of resonance properties becomes possible and the phase diagram is enlarged for synchrotron X-rays (smaller samples become accessible). RMXS has been successfully applied on the classical mixed-valent system, SmS, to study the quantum critical point [8]. Equally, the application of pressure on doped CeFe2, Ce(Co0.07Fe0.93)2, has led to a greater understanding of the magnetic ground state of CeFe2 since it was possible to directly influence the hybridisation of the itinerant electrons.

[1] E. Principi, A. Di Cicco, F. Decremps, A. Polian, S. De Panfilis, and A. Filipponi, submitted (2003)
[2] M.V. Coulet, G. Aquilanti, J.Y. Raty, W. Crichton, C. Bichara, S. Pascarelli, and J.P. Gaspard (in preparation).
[3] A. San Miguel, A. Merlen, P. Toulemonde, A. Aouizerat, B. Massenelli, F. Tournus, S. Pascarelli, G. Aquilanti, T. Le Bihan, J. P. Itié and S. Yamanaka (in preparation)
[4] N. Bendiab, A. Merlen, P. Toulemonde, A. San Miguel, P. MacMillan, G. Aquilanti, S. Pascarelli (in preparation)
[5] A. Filipponi, S. De Panfilis, C. Oliva, M.A. Ricci, P. D'Angelo, and D. Bowron, Phys. Rev. Lett. 91, 165505-1 (2003)
[6] W. Crichton, S. De Panfilis, S. Pasternak, M. Mezouar and G. Monaco (in preparation).
[7] L. Dubrovinsky, I.Yu. Kantor, A. Kantor, C.A. McCammon, S. Pascarelli, G. Aquilanti, W. Crichton (in preparation).
[8] P. Deen, N. Kernavanois, D. Braithwaite and L. Paolasini (in preparation).