Flexible-Rigid elastic transition in sodium germanate glasses: A Brillouin light scattering study.

Global structures in network glasses are characterized by their connectedness or mean co-ordination number, $r$, and have been classified as being flexible ($r\sim $2), intermediate($r\sim $2.4) or stressed rigid ($r\sim $2.7) . Recently these ideas have been extended from covalent networks to ionically bonded ones, as evidence of a reversibility window in (Na$_{2}$O)$_{x}$(GeO$_{2})_{1-x }$ glasses has emerged$^{2}$ in the 14{\%} $<$ x $<$ 19{\%} range. Glasses at x $<$ 14{\%} are viewed as stressed-rigid and those at x $>$ 19{\%} to be flexible. We have performed Brillouin light scattering measurements on this glass system, and will present results on the dependence of the longitudinal (LA) and transverse (TA) acoustic phonon velocities across a broad range of compositions, 0 $<$ x $<$ 30{\%}. Variations in C$_{11}$(x) and C$_{44}$(x) elastic constants across the three elastic phases will be discussed. $^{2}$ K.Rompicharla et al. (unpublished)