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.

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Pure Se glass and ternary AxBySe1−x−y glasses where A = Ge and B = P or As, in the low mean coordination number, ⟨r⟩, range of 2.00 < ⟨r⟩ < 2.12 display a 3‐ to 5‐fold reduction in the width of the glass transition when aged at room temperature over 4–8 months. Group IV (Ge) and group V (P, As) additives serve to crosslink the base Se glass polymeric chains, Sen, with n > 250 atoms and to reduce the length “n” between cross‐link points as x and y are increased. Herein, it is shown that in such weakly crosslinked glasses, the continued narrowing of the glass transition width Tg by a factor of 3–5, as in pure Se, stems from the fact that polymeric Sen chain segments between the crosslink points continue to have a length n of at least eight atoms or more. Such polymeric Sen chains are super‐flexible and reconstruct with each, as in pure Se, promoting structural ordering responsible for Tg narrowing. When n < 8, super flexibility is steadily lost. Along with the flexible, intermediate, and stressed‐rigid phases, a new super‐flexible phase of Se and Se‐rich glasses is obtained.