Sharp Rigid to Floppy Phase Transition Induced by Dangling Ends in a Network Glass

The count of Lagrangian bonding constraints nc(y) in ternary Ge₂₅S_75-yIy glasses $(0<y<0.30)$ is established from the molecular structure using Raman scattering and first-principles cluster calculations. The results show that nc(y) decreases to 3 as y increases close to yc0ex0ex=0ex0ex0.162(3), where a sharply defined global minimum in the nonreversing heat flow, ΔHₙᵣ(y) near Tg, is observed in scanning calorimetry. Here we have a rigidity transition induced by I dangling ends, with its sharpness resulting from the absence of self-organization in the random network.