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

Abstract

The count of Lagrangian bonding constraints ${n}_{c}(y)$ in ternary ${\mathrm{Ge}}_{25}{\mathrm{S}}_{75\ensuremath{-}y}{\mathrm{I}}_{y}$ 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 ${y}_{c}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.162(3)$, where a sharply defined global minimum in the nonreversing heat flow, $\ensuremath{\Delta}{H}_{\mathrm{nr}}(y)$ 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.