After successfully demonstrating the applicability of aXPCS to study atomic-scale diffusion in a binary metallic alloy (doi:10.1038/nmat2506), the next goal was investigate a solid solution. This seemed worthwhile, as it allowed crossing of several points on our agenda of advancing aXPCS. First of all, in a solid solution impurity diffusion of the solute atoms could be studied on the atomic scale and compared with atomistic models, e.g. the encounter model as introduced by Wolff, and could be directly compared with results from relatively simple Monte Carlo simulations. Secondly, as its preparation is relatively simple, single crystalline as well as polycrystalline samples could be manufactured and comprehensively studied. Thirdly, the feasibility of aXPCS for low concentrations of solute atoms could be investigated. 

Parallel to studies on crystalline phases we have undertaken feasibility tests to study amorphous matter. Metallic glasses were our first choice based on the expectation that they would behave very similar to simple model systems such as hard spheres. We measured diffusion properties of the Inoue metallic glass Zr65A7.5Ni10Cu17.5 and studied the influence of slow recrystallization on correlation times slightly below the glass temperature. It was, however, impossible to measure dynamics in a glassy state in close-to-equilibrium conditions. At temperatures below the glass temperature but high enough to be measurable by aXPCS with currently available X-ray brilliance, crystallization was already setting in, producing nanoscale, stress-related displacements. This could also be seen in the unusual, compressed shape of the autocorrelation function. Another paper on a different metallic glass (Mg65Cu25Y10) by Ruta et al. (Physical Review Letters 109, 165701 (2012)) published simultaneously yielded similar results. Concluding, it seems that the compressed autocorrelation function is universal and cannot be explained by the well-known theories for glassy dynamics, which all predict a stretched exponential shape in the liquid as well as in the glassy state. The existence of a universal dynamical behavior due to the scaling of the relaxation time to a single curve, independently of the microscopic details of the systems was suggested (Ruta et al., The Journal of Chemical Physics 138, 054508 (2013)) . It is a matter of dispute if the origin of the relaxation behavior lies in the internal stress reduction as suggested in the soft matter glassy systems, or in the crystallization process. It is our conviction that more brilliant X-ray sources than those currently available are required to settle this debate.

Understanding glasses on a fundamental level has been a long-standing challenge in many scientific disciplines. Structural random network formation is an important model for explaining the glass forming ability of many materials. The dynamics of basic network components play a key role in the quest to decode the peculiar properties of this material class. We utilized the new method of atomic-scale X-ray Photon Correlation Spectroscopy (aXPCS) for studying atomic dynamics in a network-forming glass and to shed light on the processes governing these substances on a fundamental level.