Introduction by S. de Panfilis, ESRF

X-ray Absorption Fine Structure (XAFS) spectroscopy is a structural probe for the local atomic environment around selected chemical species. This unique and powerful technique has recently been used in combination with other structural or chemical methods, providing an improved comprehension of the observed phenomena. The scientific issues addressed by these studies cover many different domains and it is well beyond the scope of this text to cite all of them. A few of these are reported in the following pages. Here we would like to present the general trend which has emerged in the last year.

On the X-ray absorption beamlines of the ESRF, the simultaneous usage of X-ray diffraction (XRD) and absorption spectroscopy (XAS) has become more and more routine. Thanks to the availability of novel experimental apparatus[1] designed to collect both XAS and XRD, XAFS measurements of systems under extreme conditions of high pressure and high temperature provide novel insight into the local atomic interactions as a function of thermodynamic parameters. This is very clearly demonstrated in the following articles from A. Filipponi et al. and D. Testemale et al. on aqueous solutions, and from A. San Miguel et al. on open clathrate Si crystals. Along the same lines, other interesting experiments have also been carried out in recent months. Among these, a study on the structural investigation of liquid Ga droplets dispersed in an epoxy matrix [2] revealed an extraordinary extension of metastable liquid Ga up to 2.3 GPa at room temperature. These measurements also showed the possible appearance of different Ga liquid polymorphs as a function of pressure. In another study on the structure of cubic ReO3 by means of simultaneous XAFS and XRD over a wide range of temperatures [3], it was possible to reveal anomalies in the Re local environment, known to be responsible for the negative thermal expansion observed in this system. The high quality of the data allowed the scientists to determine the thermal evolution of Re-O distances with subpicometer sensitivity.

The usage of conventional laboratory techniques (UV-Vis, IR, FT-IR, DRIFTS, mass spectroscopy, galvanostatic electrochemistry) applied in combination with XAS in chemical experiments is also becoming an asset for the understanding of complex chemical reactions. Indeed, in situ time-resolved UV-Vis spectroscopy and dispersive XAFS were simultaneously applied to investigate homogeneous catalytic reaction mechanisms, as it is clearly shown in the following article from M. Tromp et al.. Analogously, synchronous, time-resolved, infra-red, XAFS, and mass spectroscopies were used to investigate the dynamic behaviour of Rh/Al2O3 catalysts during NO reduction by CO [4]. As a result, it has been found that the NO conversion, and its kinetic character are closely correlated to the conversion of Rh(I) to Rh(0). In a study on the intercalation of oxygen into the SrCoO(2.5) system, the covalence from the starting compound to the final SrCoO3 reaction product was measured by XAFS during charge transfer under galvanostatic controlled conditions [5]. The Co oxidation was found to occur following a double step process, and an ordered intermediate phase was identified by X-ray and neutron diffraction at the corresponding charge transfer.

Another novel issue that deserves to be highlighted is the usage of very high photon energies for recording XAS spectra at high Z element K-edges. Exploiting the high photon flux from the source at very high energies, XAFS data collected at the lanthanides K-edges[6] were able to shed new light on the so-called "gadolinium brake" in the coordination of water molecules around rare-earth ions in aqueous solutions. In a different study, new results on the local coordination of trace rare earth cations in garnets were obtained by means of Dy K-edge XAS[7]. Different local environments were observed in distinct garnet compositions. This result, in contrast to what was previously believed on the basis of atomistic simulations of garnet structure, is of relevance for modelling petrologic and geochemical processes.

References
[1] For example, A. Filipponi et al., Rev. Sci. Instrum. 74, 2654 (2003).
[2] R. Poloni et al., submitted (2004).
[3] J. Purans et al., in preparation.
[4] M.A. Newton et al., Chem. Comm., 21, 2382 (2004).
[5] R. Le Toquin et al., submitted (2004).
[6] A. Abassi et al., in preparation.
[7] S. Quartieri et al., Phys. Chem. Minerals 31, 162 (2004).