Abrupt boundaries of intermediate phases and space filling in oxide   glasses
  Modulated DSC measurements on bulk (Na2O)x(GeO2)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 TO and LO mode frequencies exhibit anomalies between xc(1) = 14% (stress transition) and xc(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. 
K. Rompicharla, D. I. Novita, Ping Chen, P. Boolchand, M. Micoulaut,   W. Huff