Jeudi 22 octobre 2009, à 14h00
Disorder is common in a large class of real materials, and usually is more complex than random deviations from an average order called crystal structure. While well-ordered structures extend to micrometer size, structural disorder usually correlates at the nanoscale. Many mechanical, magnetic, electronic and optical properties depend not only on the average crystal structure, but also show a crucial link to short-range static or dynamic correlations.
Structural disorder and vibrational motion of atoms in a crystal give rise to diffuse scattering located on and between Bragg reflections. Static diffuse scattering is found in many materials, ranging from disordered alloys to proteins, but, at variance with Bragg diffraction, the number of disordered systems studied so far is much less than that for average structures. The use of thermal diffuse scattering remains also quite limited despite of its great potential. To optimise properties and develop new and better materials we need to understand the physics and chemistry of structural and vibrational fluctuations, i.e. to measure and interpret diffuse scattering and make a distinction between the elastic (quasielastic) and inelastic scattering contributions.
Diffuse intensities are usually much weaker than Bragg scattering and one needs a bright source of radiation to identify the diffuse signal. Thanks to the advent of bright synchrotron radiation and fast area detectors, diffuse scattering experiments are now possible for most materials. However, there is still no unique protocol allowing the routine characterisation of structural and thermal fluctuations from diffuse scattering data. Coupling of diffuse scattering measurements with energy-resolving spectroscopy is indispensable in numerous cases and has already shown to be of mutual benefit for both methods.