Synthesis, Homogenization and Molecular Structure of Chalcogenide glasses*

Over the years, bulk glasses have been synthesized by reacting starting materials in evacuated (10$^{-5}$ Torr to 10$^{-7}$Torr) quartz tubings for various periods at suitable elevated temperatures. The lack of a non-invasive structural probe to track spatial heterogeneity of samples during synthesis has been an impediment to tune synthesis conditions, and obtain a homogeneous product. We have developed a Raman profiling technique to understand the homogenization kinetics of Ge$_x$Se$_{100-x}$ melts, and find dry samples (2 gram size) take 7 days of reaction at 950$^{\circ}$C to homogenize on a scale of 50 microns, while wet ones homogenize quicker ($\sim$3 days), but possess physical properties measurably different from their dry counterparts. Rotating sample tubes during synthesis assists in homogenization of samples incrementally but not dramatically. A score of compositions were homogenized across the 10\% $<$ x $<$ 33.3\% range, and calorimetric, Raman scattering, and molar volume data accumulated. These data provide clear evidence for three distinct regimes of behavior as a function of Ge content, which are identified with the three elastic phases discussed earlier.\footnote{P.Boolchand et al. J.Non-Cryst. Solids 293, 348 (2001).}