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

The elastic behaviour of binary Gex Se1−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, r = 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 r = 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 × 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. (Some figures in this article are in colour only in the electronic version)