Abstract
Equimolar GexPxSe100-2x ternary glasses have been synthesized over a wide composition range, 4% < x < 25%, and examined in Raman scattering, modulated DSC, and volumetric experiments. Modulated DSC experiments show the enthalpy of relaxation at T-g to display a square-well-like reversibility window with an onset (end) of 9.0% (18.0%) respectively, thus, fixing the onset of the rigidity transition, near x(r) = 9.0% and the stress transition, near x(s) = 18.0%. These findings show that the Intermediate-Phase (IP) resides in the 9% < x < 18% range. Melt fragility index, m(x) display a Gaussian-like minimum with m(x) < 20 for IP compositions and with m > 20 for non-IP ones. Fragility index results show a global minimum of m = 14 near the center of the window, x = 14, underscoring IP melts to be superstrong. Molar volumes, V-m(x), of glasses reveal a Gaussian-like minimum, for IP compositions in relation to the non-IP ones. Melt fragility indices are closely correlated to the glass enthalpy of relaxation, and they show that superstrong melts yield IP glasses, while fragile melts yield flexible phase (FP) or stressed-rigid phase (SRP) glasses upon cooling. Special synthesis of glasses permitted the variance of Ge or P content, <Delta x >, across 1.5-g-sized batch compositions could be reduced to less than 0.1%. The homogeneous nature of glasses led to abrupt rigidity- and stress-elastic phase transitions in harmony with the percolative nature of these transitions as predicted by theory. Availability of the topological phases (TPs) in GexSe100-x glasses, and PxSe100-x glasses from earlier work, when combined with the presently measured TPs in the equimolar GexPxSe100-2x glasses, has permitted constructing a global TP diagram for the Ge-P-Se composition triangle. The global plot will assist in the choice of appropriate glass compositions for select applications of these materials. In spite of homogenization of these melts/glasses, compositional trends in T-g(x) and Raman vibrational modes show that the P-P bearing local structural units of P4Se3 monomers and ethylene-like P-2(Se-1/2)(4) units are decoupled from the network backbone in the IP and in the SRP.