The compositional variation of the non‐reversing enthalpy at Tg, ΔHnr(x), in GexSe100−x glasses decreases abruptly by an order of magnitude as x increases to xc(1) = 19.5(5)%, the rigidity transition, and then remains minuscule till x increases to xc(2) = 26.0(5)%, when the term abruptly increases by an order of magnitude as glasses become stressed‐rigid. The rigid but unstressed networks formed in between these two transitions represent the Intermediate Phase (IP). The square‐well like variation of ΔHnr(x), also known as the reversibility window develops sloping walls, then a triangular shape and eventually disappears as glasses of increasing heterogeneity are studied. The ΔHnr term ages over weeks outside the IP but not inside the IP. Raman line shapes of as‐quenched melts are quite similar to those of Tg‐cycled glasses for compositions in the IP, but not outside the IP– an optical analog of the thermal reversibility window. Variations of Molar volumes, display a global minimum in the IP and a pronounced increase outside that phase. Physical behavior of dry and homogeneous chalcogenide glasses that leads to sharp elastic and chemical phase transitions remains to be understood theoretically. The physics of network may be even more interesting than hitherto recognized.

Variation of fragility (m) of specially homogenized Ge x Se 100−x melts are established from complex specific heat measurements, and show m(x) has a global minimum at an extremely low value (m=14.8(0.5)) in the 21.5% < x < 23% range of Ge. Outside of that compositional range, m(x) then increases at first rapidly then slowly to about m=25-30. By directly mapping melt stoichiometry as a function of reaction time at a fixed temperature T>T g , we observe a slowdown of melt-homogenization by the super-strong melt compositions, 21.5% < x < 23%. This range furthermore appears to be correlated to the one observed between the flexible and stressed rigid phase in network glasses. These spectacular features underscore the crucial role played by topology and rigidity in the properties of network-forming liquids and glasses which are highlighted when fragility is represented as a function of variables tracking the effect of rigidity.Finally, we investigate the fragility-glass transition temperature relationship, and find that reported scaling laws do not apply in the flexible phase, while being valid for intermediate and stressed rigid compositions.

An FT‐Raman profiling method is developed to monitor growth of structural homogeneity of binary GexSe100‐x melts in real time (tR) non‐invasively as starting materials are reacted over days. Raman spectra of quenched melts were acquired along a one inch long column of a sample in a quartz tube. In the first step of reaction, tR < 2 days, Ge‐rich crystalline‐ and glassy‐ phases form and coexist with Se‐rich glasses. In the second step, tR extending up to 7 days, local structures characteristic of melts/glasses form, and steadily homogenize as Ge and Se atoms diffuse. The process terminates when all Raman lineshapes taken along the length of a sample coalesce into unique profile. Several factors contribute to the long reaction time tR for melts to homogenize, including liquid density difference of Ge and Se, diffusion controlled nanoscale mixing of melts, batch dryness, batch sizes, and laser spot size. Physical properties of such homogeneous GexSe100‐x glasses are found to be quite different from their inhomogeneous counterparts realized after the first step of reaction. Slow homogenization of chalcogenides melts may occur generally. Variations in physical properties of chalcogenide glasses possessing nominally the same composition may have their origin in structural heterogeneity and purity.

We introduce a Raman profiling method to track homogenization of Ge x Se 100-x melts in real time, and show that 2 gram melts reacted at 950°C in high vacuum homogenize in 168 hours on a scale of 10μm. Homogenization of melts is precursive to self-organization of glasses. In the present glasses, compositional variation of Raman active corner-sharing mode frequency of GeSe 4 units, molar volumes, and the enthalpy of relaxation at T g , reveal the rigidity ( x c (1)= 19.5(3)%) and the stress (x c (2) = 26.0(3)%) transitions to be rather sharp (Δx < 0.6%). These abrupt elastic phase transitions are intrinsic to these materials and have a direct bearing on physics of glasses.Bulk glasses have been synthesized by reacting starting materials to produce melts, which are then quenched to produce disordered solids[1]. One generally assumes that holding melts at several hundred degrees above the liquidus for several hours would homogenize them. In equilibrium phase diagrams, glass forming compositions are usually bordered by congruently melting crystalline phases [2]. These crystalline phases, in principle, can nucleate as melts are 2 quenched to produce microscopic heterogeneities (MH). In practice, such equilibrium thermodynamic effects can be suppressed in pure melts as heterogeneous nucleation sites become minuscule as shown recently in metallic glasses [3]. We introduce here a Raman profiling method to track homogenization of chalcogenides in real time, and find that the process consists of two steps, an initial step 1 as starting materials react to produce MH consisting of crystalline phases embedded in a glass. Continued reaction leads the crystalline phases to dissolve and local structures characteristic of melts/glasses to emerge. In step 2, intermediate and extended range structures evolve as melt stoichiometry across a batch composition equalizes by a process described as Melt-Nanoscale Mixing (MNM). We illustrate these ideas for the case of the well studied Ge x Se 100-x binary [4][5][6][7][8][9][10][11][12][13][14], and find that slow MNM and not MH due to thermodynamic phase separation [15] is the determinative factor that contributes to glass heterogeneity in chalcogenides.The finding has a direct bearing on the sharpness of the rigidity and stress transitions in the Ge x Se 100-x binary system. Rigidity theory [16][17][18] has been the corner stone to understanding network glasses in terms of their topology. An intermediate phase (IP) forms between these transitions [19]and has attracted widespread interest because of its exceptional functionalities, including the stress-free, non-aging, dynamically reversible nature of networks formed in this nanostructured phase [8,9,18,20,21]. In the present homogenized glasses, we find the rigidity and stress transitions to occur near x = 19.5(3)% (rigidity) and 26.0(3)% (stress) and to be remarkably sharp (width Δx < 0.6%) . Our findings illustrate that the intrinsic stress-driven behavior of these chalcogenide glasses may be far richer than hitherto recognized. The Ra...

The kinetics of homogenization of binary As x Se 100−x melts in the As concentration range 0% < x < 50% are followed in FT-Raman profiling experiments, and show that 2 gram sized melts in the middle concentration range 20% < x < 30% take nearly two weeks to homogenize when starting materials are reacted at 700 o C. In glasses of proven homogeneity, we find molar volumes to vary non-monotonically with composition, and the fragility index M displays a broad global minimum in the 20% < x < 30% range of x wherein M < 20.We show that properly homogenized samples have a lower measured fragility when compared to larger underreacted melts. The enthalpy of relaxation at T g , ∆H nr (x) shows a minimum in the 27% < x < 37% range. The super-strong nature of melt compositions in the 20% < x < 30% range suppresses melt diffusion at high temperatures leading to the slow kinetics of melt homogenization.