Fragility and slow kinetics of melt homogenization in the As-Se binary

Two gram sized As$_{x}$Se$_{100-x}$ batches at various As content x were synthesized using pure Se and As$_{2}$Se$_{3}$ as starting materials that were reacted at 700$^{o}$C. Such melts typically took 3-12 days to homogenize, as monitored in punctuated, off-line FT-Raman line profiling\footnote{S. Bhosle et al., Sol. St. Commun.151, 1851-1855 (2011)} experiments. We have now undertaken mDSC experiments as a function of modulation frequency to establish the compositional dependence of complex C$_{p}$(x), and deduce the variation of fragility m(x). We find the fragility to be rather low, m $<$ 20, across the 22\% $<$ x $<$ 38\% range, and to rapidly increase at x $<$ 22\% to acquire a value of 43 near x = 3\%. We show that the slow melt homogenization is a direct consequence of the ``strong'' character of melts that serves as a bottleneck in melt-mixing at high temperatures. Once homogenized, physical properties of glasses, such as density, glass transition temperature T$_{g}$(x), the Intermediate phase, and variation of enthalpy of relaxation at T$_{g}$(x) differ significantly from their inhomogeneous counterparts.