Abrupt boundaries of intermediate phases and space filling in oxide glasses.

Modulated differential scanning calorimetry measurements on bulk (Na(2)O)(x)(GeO(2))(1-x) glasses show a sharp reversibility window in the 14%<x<19% soda range, which correlates well with a broad global minimum in molar volumes. Raman and IR reflectance transverse- and longitudinal-optic mode frequencies exhibit anomalies between x(c)(1) = 14% (stress transition) and x(c)(2) = 19% (rigidity transition), with optical elasticity power laws confirming the nature of the transitions. Birefringence measurements dramatize the macroscopically stress-free nature of the intermediate phase in the reversibility window.