Unusual Role of P–P Bonds on Melt Dynamics and Topological Phases of the Equimolar Ge x P x Se100–2x Glass System

Equimolar Ge x P x Se100–2x 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, ⟨Δx⟩, 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 Ge x Se100‑x glasses, and P x Se100‑x glasses from earlier work, when combined with the presently measured TPs in the equimolar Ge x P x Se100–2x 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 P2(Se1/2)4 units are decoupled from the network backbone in the IP and in the SRP.