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

The non-reversing heat flow, DeltaH(nr)(x) near T-g in ternary GexAsxSe1-2x glasses is examined by temperature-modulated differential scanning calorimetry. The DeltaH(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, (r) over bar = 2 + 3x, the large width, Delta(r) over bar = r(c)(1) - r(c)(2) = 0.15, of the intermediate phase and its low onset value r,(l) = 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 DeltaH(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.