Stiffness transitions in SixSe1Àx glasses from Raman scattering and temperature-modulated differential scanning calorimetry

Temperature-modulated differential scanning calorimetry ~MDSC! measurements on SixSe12x 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.