Direct Evidence for Stiffness Threshold in Chalcogenide Glasses

Raman scattering in GeₓX_1-x glasses, X0ex0ex=0ex0exS or Se, reveals that the frequency of A₁ modes of corner-sharing Ge(X1/2)₄ tetrahedra displays a discontinuous jump between x0ex0ex=0ex0ex0.225 and x0ex0ex=0ex0ex0.230, which coincides with a minimum in the nonreversing heat flow at the glass transition Tg established from modulated differential scanning calorimetry. These results constitute direct evidence for a stiffness threshold at a mean coordination 〈r〉c0ex0ex=0ex0ex2.46(1), which is well described by mean-field constraint counting procedures.