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 ch aracter in the composition window 0.20<x<0.27. Raman scattering shows a tri modal 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. I n the transition region, 0.20<x<0.27, the ES and CM split into doublets sug gesting appearance of extended range structural correlations. In the percol atively rigid region, x > 0.27, the CS mode in addition to the ES and CM al so splits into a doublet indicating growth of substantial medium range stru cture. The large compositional width (0.20<x<0.27) associated with Raman el astic thresholds coincides with the thermally reversing window from MDSC.