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

The non-reversing heat flow, ΔH_nr(x) near T_g in ternary Ge_xAs_xSe_{1 − 2x} glasses is examined by temperature-modulated differential scanning calorimetry. The ΔH_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 + 3x, 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 ΔH_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.