Glass structure , rigidity transitions and the intermediate phase in the Ge - As - Se ternary

– The non-reversing heat flow, ∆Hnr(x) near Tg in ternary GexAsxSe1−2x glasses is examined by temperature-modulated differential scanning calorimetry. The ∆Hnr(x) term shows a deep minimum (which is almost zero) in the 0.09 < x < 0.14 range, identified with the intermediate phase, and an increase, both at low x (< 0.09) in the floppy phase and at high x (> 0.14) in the stressed rigid phase. Expressed in terms of mean coordination number, r̄ = 2 + 3x, the large width, ∆r̄ = rc(1) − rc(2) = 0.15, of the intermediate phase and its low onset value rc(1) = 2.27 are shown to be consistent with the presence of Se = As(Se1/2)3 units in addition to pyramidal As(Se1/2)3 and tetrahedral Ge(Se1/2)4 units in the stress-free backbone. The vanishing of ∆Hnr(x) in the intermediate phase is in harmony with the notion that the number of Lagrangian constraints/atom exhausts the three available degrees of freedom, and leaves the backbone in a mechanically stress-free state. Introduction. – A floppy-to-rigid transition in network glasses was predicted [1, 2] in the early 1980’s. The transition has enjoyed widespread interest in science, not only because the glass forming tendency is optimized near this transition, but also because it serves as a paradigm of percolative transitions in condensed matter [3], and computational complexity [4] in computer science (NP — complete problem). In random networks numerical calculations [2] have shown that a solitary floppy-to-rigid transition occurs when the connectivity or mean coordination number r̄ increases to 2.385, quite close to the predicted mean-field value of 2.40. Experimentally, new details of the transition have recently emerged [5–8] in binary (Ge or Si)xSe1−x glasses by Raman scattering and T -modulated Differential Scanning Calorimetry (MDSC). In contrast to numerical simulations on random networks [2], Raman optical elasticities provide evidence of two transitions, a second-order transition near rc(1) = 2.40 from a floppy to an unstressed rigid phase, and a first-order transition near rc(2) = 2.52 from an unstressed rigid to a stressed rigid phase. The unstressed nature of the intermediate