The properties of materials depend on the way the constituent atoms combine with each other, for example to form molecules, the aggregation of molecules and atoms into crystalline or disordered solids, liquids or glasses, and the microstructural arrangement of the grains of a material to make up the bulk. Synchrotron X-ray radiation provides some of the most powerful tools to study these structures and thereby gain an understanding of why materials behave as they do, and how properties can be tailored and improved for specific applications. Many experiments at the ESRF and CRG beamlines fall into the broad category of materials science, exploiting the techniques of diffraction, scattering or imaging. Choosing just a few examples from such a broad expanse of work is difficult. The accounts presented in this chapter are only the very tip of the iceberg as far as materials research at the ESRF is concerned.

Synchrotron radiation can be used to study the evolution of structure in systems. The first example involves the dissociation of an organometallic molecule induced by a flash of laser light, where the various dissociation products are identified, and the pathways by which these revert to the starting structure are followed on the timescale of nanoseconds, thus enabling fundamental chemical processes of bond breaking and making to be studied in detail. On a more human timescale, experiments have revealed the recrystallisation and growth of nanoparticles of palladium on exposure to hydrogen gas, with implications perhaps for future solid hydrogen-storage systems, and of grain growth in an aluminium-based alloy on annealing. The mechanical properties of polymers depend intimately on the conditions of temperature and flow under which they are formed and processed. In situ studies of the crystallisation of high-density polyethylene give new insights into how morphology and structure can be linked to the thermo-mechanical history of the sample.

The interactions between liquids and surfaces can have profound effects on the properties of a system: everyone knows that wet sand makes a better sandcastle, and understanding how is of great importance for more serious issues concerning landslides, or for the processing of powdered substances in the food and pharmaceutical industries. Micro-tomographic imaging was used to visualise the intricate networks of liquid that form between small glass spheres (a model system) with different liquid contents. In another study, high energy X-ray reflectivity probed the interaction between a sapphire surface and liquids composed of bulky charged ions (so called ionic liquids, with potential applications including solvents for advanced chemical syntheses), revealing the extent to which the ions order or disorder with varying chemical composition and temperature.

The chemical elements, which might be expected to be the simplest of systems, continue to surprise at temperatures or pressures away from those of everyday life. Thus seven distinct phases for sodium were discovered within a narrow pressure and temperature range close to 118 GPa, some of extraordinary structural complexity. Sulphur at low temperatures and high pressures is found to undergo a number of structural changes, including a change from a high density to low-density amorphous phase, or at higher pressures to an incommensurately modulated structure via a charge density wave instability. For iron at high temperatures and pressures, X-ray emission spectroscopy, a local probe of the 3d spin magnetic moment, demonstrates the collapse of the short-range magnetic state under these conditions.

The final examples in the chapter involve crystal structure analysis via high-resolution-powder or single-crystal techniques, giving insight into the electronic behaviour and superconductivity in hole-doped

Nd1-xSrxFeAsO; the solution of the crystal structure of novel organometallic complexes of copper with peptides, which reveals a pleasing network of hydrogen bonds; and metal-organic framework materials containing rare-earth ions that combine high thermal stability and other multifunctional properties (magnetism, luminescence, microporosity, hydrophobicity). The combination of a luminescent Ln3+ centre and the chosen ligands results in materials that can sense ethanol in air even in the presence of water.

Preparations are underway for the experiments that will yield the articles of future editions of the ESRF highlights. Ideas and plans for improved experimental approaches are always being sought, and, with the start of the ESRF Upgrade Programme, two beamlines relevant to this chapter are under consideration. The first is for high-energy diffraction using a microfocussed beam for penetrating through or into samples with high spatial resolution, and the second is a fully-dedicated beamline for pump-probe time-resolved diffraction. These would each greatly enhance the ESRF's capabilities in these exciting areas.

A. Fitch