Leonardo Properzi Ph.D. student (joint University of Camerino-Soleil synchrotron France) will give a seminar at the Physics Division, room H, tuesday 5 May, 15pm. 

  Short-range order of chalcogenide glasses under pressure probed by X-ray absorption spectroscopy (XAS)

The physics of amorphous materials is complicated by the absence of long-range order, which makes both the prediction and the measurement of structural properties difficult, as epitomized by the long-standing glass transition problem. Concerning chalcogenide (S, Se and Te) glasses, the existence of an 'intermediate phase' has been proposed as a finite composition region between under- and over-constrained network regimes, but the question is still debated. The nature of the networks is also an open question, and the existence of a continuous (perfect chemical order) network is opposed to a broken (with 'wrong bonds') one. In this work, the short- and intermediate-range order of amorphous GeSe₂ and As₂Se₃ have been studied by means of X‑ray absorption spectroscopy at both Ge(As) and Se K‑edges. A double-edge data analysis allowed to extract the Ge(As)‑Se average distances, together with the interatomic mean square relative displacement, up to a pressure of about 30 GPa. A gradual elongation of the first-shell average distance indicates a progressive coordination increase, possibly connected to the metallization process. The possibility of extracting the percentage of chemical disorder by XAS at ambient conditions is finally discussed.