Chemical alloying and light-induced collapse of intermediate phases in chalcohalide glasses

The elastic behaviour of binary GexSe1-x glasses, examined in Raman scattering experiments earlier, has shown glasses at x<0.20 to be in the flexible phase, those at x>0.25 to be in the stressed-rigid phase and those in the 0.20<x<0.25 range to be in the intermediate phase (IP). The IP width in mean-coordination-number space, Δr = 0.10. We have now examined ternary Ge1/4Se3/4-yIy glasses in Raman scattering and modulated DSC experiments, and find that the IP width dramatically collapses by an order of magnitude to Δr = 0.009(2). Alloying iodine for Se serves to scission the network backbone progressively as mixed Ge(Se)4-mIm tetrahedra (m-units) emerge with 1<m<4. The concentrations of various m-units are quantitatively tracked in Raman scattering, and this shows the m = 1 units to be rather special because they are isostatic. The present results on Ge1/4Se3/4-yIy glasses, when compared to those on Ge1/4S3/4-yIy glasses, reveal crucial differences in the way the reversibility window collapse occurs. Raman scattering examined as a function of the exciting light (647 nm) power P in Ge1/4Se3/4-yIy glasses shows the IP to systematically collapse and to vanish once P increases to 1.5 x 106 W cm-2. Here, an intense beam of near-bandgap light serves to optically pump the delicate intermediate range order prevailing in the IP and reversibly destroy it