So far, diffusion in solids has been investigated by profiling the depth dependence of tracer atoms diffused into the sample. Although one can obtain the diffusion constant from this, the question is how diffusion takes place on the atomic scale, rather than on the micron scale. Sometimes quasielastic neutron scattering as well as Mobauer spectroscopy can be used in a very limited number of fortunate cases. A research group led by Professor G. Vogl (University of Vienna, Austria) recently reported the use of X-ray photon correlation spectroscopy (XPCS) to observe the dynamics of diffusing atoms. The research was done for intermetallic alloy Cu90Au10, at temperatures of around 540 K, where the system is a substitutional solid solution, that is, the Au atoms statistically occupy sites in the Cu fcc lattice. The research gives the dynamical behavior of single atoms as a function of their neighborhood, and confirms quantitatively that Au atoms have a tendency to locally order on a certain set of sites in the crystal. Photon correlation spectroscopy is based on analysis of 'speckle' patterns, which are fine-scale diffraction patterns that appear in the scattering of coherent light from a disordered system. Speckle patterns are sensitive to the exact spatial arrangement of the disorder. By observing the intensity fluctuations in the speckle pattern, the characteristic times of fluctuations in the system can be determined. For more information, see the paper, "Atomic diffusion studied with coherent X-rays", M. Leitner et al., Nature Materials,8, 717 (2009).
