129I and 119Sn Mossbauer spectroscopy, reversibility window and nanoscale phase separation in binary GexSe1-x glasses

The molecular structure of the prototypical chalcogenide glass system-GexSe1-x in the 0 < x 1/3 range, has received much scrutiny over the years. These glasses have been probed by modulated DSC, Raman scattering, Sn-119 absorption and I-129 emission Mossbauer spectroscopy, and neutron scattering. The I-129 measurements utilize Te-129m parent as a dopant in glasses, and reveal a bimodal (A, B) distribution of sites, with the site intensity ratio, I-B/I-A (x), tracking changes in glass structure as a function of x. At low x (< 0.15) Se-n-chains are stochastically cross-linked by Ge additive, and IB/IA (x) sharply declines with x. But at x > 0.15, rigid regions nucleate at the expense of floppy ones, and the ratio IB/IA (x) reverses slope to display a global maximum in the 0.20 < x < 0.25 range. The latter coincides with the reversibility window usually taken as signature of self-organization of these networks. At x > 0.26, these glasses enter a stressed-rigid elastic phase and in the 0.31 < x < 1/3 range nanoscale phase separate into Se-rich and Ge-rich regions. The signature of the latter is saturation of I-B/I-A (x) at a high value of 1.5 at x = 1/3. Sn-119 Mossbauer spectroscopy measurements independently support the picture of broken chemical order of the stoichiometric glass inferred from the I-129 Mossbauer experiments. These observations using local probes are well correlated to Raman scattering, modulated differential scanning calorimetric and diffraction measurements. (c) 2006 Elsevier B.V. All rights reserved.