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

The non-reversing heat flow, Δ H nr ( x ) near T g in ternary Ge x As x Se 1 − 2 x glasses is examined by temperature-modulated differential scanning calorimetry. The Δ H nr ( 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, = 2 + 3 x , the large width, Δ = r c (1) − r c (2) = 0.15, of the intermediate phase and its low onset value r c (1) = 2.27 are shown to be consistent with the presence of Se = As(Se 1/2 ) 3 units in addition to pyramidal As(Se 1/2 ) 3 and tetrahedral Ge(Se 1/2 ) 4 units in the stress-free backbone. The vanishing of Δ H nr ( 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 .