Rigidity transitions in binary Ge-Se glasses and the intermediate phase

Raman scattering measurements, undertaken on bulk GexSe1-x glasses at 0 < x < 1/3, show evidence of two rigidity transitions as monitored by compositi onal trends in corner-sharing (nu (CS)) and edge-sharing (nu (ES)) Ge(Se-1/ 2)(4) mode frequencies. A second-order transition from a floppy to an unstr essed rigid phase occurs near x(c)(1) = 0.20(1) where both nu (CS)(x) and n u (ES)(x) show a kink. A first-order transition from an unstressed rigid to a stressed rigid phase occurs near x(c)(2) = 0.26(1), where nu (2)(CS)(x) displays a step-like discontinuity between x = 0.25 and 0.26 and a power-la w behavior at x > x(c) (2). In sharp contrast, earlier micro-Raman measurem ents that use at least three orders of magnitude larger photon flux to exci te the scattering, showed only one rigidity transition near x(c) = 0.23, th e mid-point or the intermediate phase (x(c)(1) < x < x(c)(2)). Taken togeth er, these results suggest that the intermediate phase, observed in the low- intensity Raman measurements, undergoes a light-induced melting to a random network in the micro-Raman measurements. (C) 2001 Elsevier Science B.V. Al l rights reserved.