Résumé
SignificanceUpon X-ray irradiation, a number of glasses undergo a fluidization process: The atoms move from their original positions while keeping a similar distribution of interatomic distances, as for two snapshots of a liquid. This process has been studied looking at the atomic displacements over interatomic distances. We here extend these investigations to much longer length scales in the few nanometer range. For silica irradiated at doses of ∼5 GGy, our experiments clarify that this process is not characterized by the common atomic diffusion typical of liquids: It is rather the outcome of an atomic acceleration due to X-ray-induced local stresses random in both time and space, as in the famous example of stochastic acceleration of charged particles by interstellar fields.
The X-ray-induced, nonthermal fluidization of the prototypical SiO2 glass is investigated by X-ray photon correlation spectroscopy in the small-angle scattering range. This process is initiated by the absorption of X-rays and leads to overall atomic displacements which reach at least few nanometers at temperatures well below the glass transition. At absorbed doses of ∼5 GGy typical of many modern X-ray-based experiments, the atomic displacements display a hyperdiffusive behavior and are distributed according to a heavy-tailed, Lévy stable distribution. This is attributed to the stochastic generation of X-ray-induced point defects which give rise to a dynamically fluctuating potential landscape, thus providing a microscopic picture of the fluidization process.