Midgap states, Raman scattering, glass homogeneity, percolative rigidity and stress transitions in chalcogenides

Raman scattering from binary GexSe100 x glasses excited using 1064 nm radiation display vibrational modes whose linewidths significantly exceed those observed using 647 nm radiation. In these glasses, 1064 nm radiation excites midgap states while 647 nm radiation excites conduction band tail states. Presence of midgap states in glasses, ascribed to coordination defects, is responsible for vibrational mode broadening that smears glass compositional variation of mode frequency using 1064 nm radiation but not with the 647 nm radiation. In the latter, mode-frequency variation of Corner(CS) and Edge-sharing (ES) tetrahedral units, in specially prepared homogeneous glasses, display thresholds near the rigidity (x1⁄4 19.5%) and stress (x1⁄4 26.0%) transitions, opening an intermediate phase (IP) that correlates well with the reversibility window (blue region) observed in calorimetric measurements (figure on the right). Compositional trends in the non-reversing enthalpy of relaxation at Tg, DHnr(x), in binary GexSe100 x glasses examined in 1997, 2009, and 2011. The reversibility window (blue region) progressively sharpens to acquire a square-well like behaviour as melts/glasses homogenize. Figure taken from Ref. [1].