Stiffness transitions in SixSe1-x glasses from Raman scattering and temperature-modulated differential scanning calorimetry

Temperature-modulated differential scanning calorimetry (MDSC) measurements on SixSe1-x glasses show glass transitions to be thermally reversing in character in the composition window 0.20<x<0.27. Raman scattering shows a trimodal distribution of vibrational modes, identified with corner-sharing (CS), edge-sharing (ES), and Se-chain modes (CM). In the floppy region, 0<x<0.19, these modes are symmetric and are characteristic of a random network. In the transition region, 0.20<x<0.27, the ES and CM split into doublets suggesting appearance of extended range structural correlations. In the percolatively rigid region, x > 0.27, the CS mode in addition to the ES and CM also splits into a doublet indicating growth of substantial medium range structure. The large compositional width (0.20<x<0.27) associated with Raman elastic thresholds coincides with the thermally reversing window from MDSC.