Crucial effect of glass processing and melt homogenization on the fragility of non-stoichiometric chalcogenides

The kinetics of homogenization of binary As$_x$Se$_{100-x}$ melts in the As concentration range $0\%$ $<$ $x$ $<$ $50\%$ are followed in FT-Raman profiling experiments, and show that 2 gram sized melts in the middle concentration range $20\%$ $<$ $x$ $<$ $30\%$ take nearly two weeks to homogenize when starting materials are reacted at 700$^o$C. In glasses of proven homogeneity, we find molar volumes to vary non-monotonically with composition, and the fragility index ${\cal M}$ displays a broad global minimum in the $20\%$ $<$ $x$ $<$ $30\%$ range of $x$ wherein ${\cal M}<$ 20. We show that properly homogenized samples have a lower measured fragility when compared to larger under-reacted melts. The enthalpy of relaxation at T$_g$, $\Delta$H$_{nr}$($x$) shows a minimum in the $27\%$ $<$ $x$ $<$ $37\%$ range. The super-strong nature of melt compositions in the $20\%$ $<$ $x$ $<$ $30\%$ range suppresses melt diffusion at high temperatures leading to the slow kinetics of melt homogenization.