Direct Evidence for Stiffness Threshold in Chalcogenide Glasses

Abstract

Raman scattering in ${\mathrm{Ge}}_x{X}_{1-x}$ glasses, $X=\mathrm{S}$ or Se, reveals that the frequency of $A_1$ modes of corner-sharing $\mathrm{Ge}(X_{1/2}{)}_4$ tetrahedra displays a discontinuous jump between $x=0.225\mathrm{}$ and $x=0.230\mathrm{}$, which coincides with a minimum in the nonreversing heat flow at the glass transition $T_g$ established from modulated differential scanning calorimetry. These results constitute direct evidence for a stiffness threshold at a mean coordination $〈r{〉}_c=2.46\mathrm{}\left(1\right)$, which is well described by mean-field constraint counting procedures.