Pressure Raman effects and internal stress in network glasses

Raman scattering from binary GeₓSe_1-x glasses under hydrostatic pressure shows onset of a steady increase in the frequency of modes of corner-sharing GeSe₄ tetrahedral units when the external pressure P exceeds a threshold value Pc. The threshold pressure Pc(x) decreases with x in the $0.15<x<0.20$ range, nearly vanishes in the $0.20<x<0.25$ range, and then increases in the $0.25<x<1∕3$ range. These Pc(x) trends closely track those in the nonreversing enthalpy, ΔHₙᵣ(x), near glass transitions (Tgs), and in particular, both ΔHₙᵣ(x) and Pc₍ₓ₎ vanish in the reversibility window $(0.20<x<0.25)$. It is suggested that Pc provides a measure of stress at the Raman-active units, and its vanishing in the reversibility window suggests that these units are part of an isostatically rigid backbone. Isostaticity also accounts for the nonaging behavior of glasses observed in the reversibility window.