Pressure Raman effects and internal stress in network glasses

Raman scattering from binary ${\mathrm{Ge}}_{x}{\mathrm{Se}}_{1\ensuremath{-}x}$ glasses under hydrostatic pressure shows onset of a steady increase in the frequency of modes of corner-sharing $\mathrm{Ge}{\mathrm{Se}}_{4}$ tetrahedral units when the external pressure $P$ exceeds a threshold value ${P}_{c}$. The threshold pressure ${P}_{c}(x)$ decreases with $x$ in the $0.15lxl0.20$ range, nearly vanishes in the $0.20lxl0.25$ range, and then increases in the $0.25lxl1∕3$ range. These ${P}_{c}(x)$ trends closely track those in the nonreversing enthalpy, $\ensuremath{\Delta}{H}_{\mathit{nr}}(x)$, near glass transitions $({T}_{g}s)$, and in particular, both $\ensuremath{\Delta}{H}_{\mathit{nr}}(x)$ and ${{P}_{c}}_{(x)}$ vanish in the reversibility window $(0.20lxl0.25)$. It is suggested that ${P}_{c}$ 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.