Melt fragility index, configurational entropy, and topological phases in ternary Ge_xAs_xSe_100-2x Chalcogenides

The melt fragility index, m, of titled chalcogenides display a global minimum with m < 20, in the narrow connectivity range 2.28(3) < <r> < 2.51(3) defining a fragility window. Separately, the non-reversing enthalpy of relaxation (ΔH_nr) at T_g in corresponding glasses shows a square-well like global minimum, with the ΔH_nr term nearly vanishing in the same 2.28(3) < <r> < 2.51(3) connectivity range, defining the reversibility window, which represents the isostatically rigid Intermediate Phase (IP). The near vanishing ΔH_nr for IP glasses is signature for the change in excess configurational entropy (ΔS_ex) between melt and glass to be minuscule, and leads to super-strong melts as expected. The order of magnitude increases in ΔH_nr for non-IP glass compositions as one goes away from the IP glass compositions, lead both ΔS_ex and m to steadily increase for non-IP melts as observed in experiments.