Sharp rigid to floppy phase transition induced by dangling ends in a network glass

The count of Lagrangian bonding constraints n_c(y) in ternary Ge₃₅S_75-yI_y glasses (0 < y < 0.30) is established from the molecular structure using Raman scattering and first-principles cluster calculations. The results show that n_c(y) decreases to 3 as y increases close to y_c = 0.162(3), where a sharply defined global minimum in the nonreversing heat flow, ΔH_nr(y) near T_g, 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.