The sharpness of thermally reversing windows as a measure of glass network homogeneity

Reversibility windows(RWs) have been observed\footnote{Boolchand et al., Phil. Mag 85, 3823 (2005)} in Chalcogenides, modified- and unmodified-oxides, and solid electrolytes. These are identified with isostaticaly rigid networks formed in narrow compositional windows between flexible and stressed-rigid elastic phases. Until recently, we found RWs in oxides and solid electrolytes to be sharper than in chalcogenides. Recently we introduced\footnote{Bhosle et al., Sol. St. Comm. 151, 1851 (2011)} a Raman profiling method to track homogeneity of melts during synthesis, and found that the kinetics of homogenization of chalcogenide melts are slow. The enthalpy of relaxation at T$_g$ measured in binary Ge$_x$Se$_{100-x}$ glasses show the RW to be intrinsically square-well like with sharp edges in homogeneous samples ($\Delta$x $\sim$ 0), and becomes trapezoidal ($\Delta$x = 1.5\%), then triangular ($\Delta$x = 3\%) and eventually disappears as glass heterogeneity increases. The heterogeneity deduced from Raman profiling experiments provides the Ge-stoichiometry variation ($\Delta$x) across a batch composition, which can be used to predict the observed RWs.