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

The elastic behaviour of binary Ge_xSe_1-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 Ge_1/4Se_3/4-yI_y 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-mI_m 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 Ge_1/4Se _3/4-yI_y glasses, when compared to those on Ge _1/4S_3/4-yI_y glasses, reveal crucial differences in the way the reversibility window collapse occurs. Raman scattering examined as a function of the exciting light (647nm) power in Ge_1/4Se _3/4-yI_y glasses shows the IP to systematically collapse and to vanish once increases to 1.5 × 10⁶Wcm^-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.