Topological Ordering and Viscosity in the Glass-Forming Ge–Se System: The Search for a Structural or Dynamical Signature of the Intermediate Phase

Abstract

The topological ordering of the network structure in vitreous Ge x Se 1−x was investigated across most of the glass-forming region (0 ≤ x ≤ 0.4) by using high-resolution neutron diffraction to measure the Bhatia-Thornton number-number partial structure factor. This approach gives access to the composition dependence of the mean coordination number n and correlation lengths associated with the network ordering. The thermal properties of the samples were also measured by using temperature-modulated differential scanning calorimetry. The results do not point to a structural origin of the so-called intermediate phase, which in our work is indicated for the composition range 0.175(8) ≤ x ≤ 0.235(8) by a vanishingly small non-reversing enthalpy near the glass transition. The midpoint of this range coincides with the mean-field expectation of a floppy-to-rigid transition at x = 0.20. The composition dependence of the liquid viscosity, as taken from the literature, was also investigated to look for a dynamical origin of the intermediate phase, using the Mauro-YueEllison-Gupta-Allan (MYEGA) model to estimate the viscosity at the liquidus temperature. The evidence points to a maximum in the viscosity at the liquidus temperature, and a minimum in the fragility index, for the range 0.20 ≤ x ≤ 0.22. The utility of the intermediate phase as a predictor of the material properties in network glass-forming systems is discussed.

Topological Phases of Chalcogenide Glasses Encoded in the Melt Dynamics

“…The alloying time to achieve complete homogenization, we would estimate would be closer to 12 days, given that it took Bhosle et al typically 7–9 days to homogenize 2 gram sized batch as determined from Raman profiling experiments (Figure ). These findings of Zeidler et al confirm, in our view, that melt viscosity of compositions in the IP are rather high and serve as a barrier to atomic scale diffusion. It is also for this reason, in our view, that the reported molar volumes of their glasses are lower than those of Bhosle et al…”

“…The extended range molecular structure of these glasses, clearly does not emerge till the variance in Ge stoichiometry across the batch is less than “ x ” < ½% as the IP evolves with sharp stress and rigidity transitions (Figure ). For the 3.6 gram batch size used by Zeidler et al, that are nearly 1.8 times greater in weight than the ones used by Feng et al (2 gram) and Bhosle et al (2 gram), alloying for 47 h at 975 °C in the same sized (5 mm ID) quarts tube would appear to be clearly insufficient time for melts to homogenize. The alloying time to achieve complete homogenization, we would estimate would be closer to 12 days, given that it took Bhosle et al typically 7–9 days to homogenize 2 gram sized batch as determined from Raman profiling experiments (Figure ).…”

“…In (b), the labels F, A 1 , and A 2 represent Δ H nr ( x ) values in fresh glasses (), glasses aged at room temperature (), and aged at 240 °C (). c) Trends in T g ( x ) on Ge–Se glasses from Zeidler et al () compared to those of Bhosle et al (). d) Enthalpy of relaxation Δ H nr ( x ) reported by Zeidler et al () compared to those of Bhosle et al ().…”

“…Zeidler et al have recently reported on calorimetric and neutron scattering results on Ge x Se 100– x glasses. The T g ( x ) and Δ H nr ( x ) trends of their samples acquired by MDSC (Figure c and d) are compared to those of Bhosle et al and Feng et al An examination of the results of Zeidler et al shows that these results bear close similarities to those of Feng et al The T g ( x ) trends reported Feng et al, Bhosle et al, and Zeidler et al are remarkably similar, reflecting that the global connectivity of the glasses is quite similar.…”

Topological Constraint Theory and Rigidity of Glasses

“…Glasses belonging to the intermediate phase have been shown to exhibit unusual properties, e.g., an optimal space-filling tendency [42], a low propensity for relaxation [44], maximum resistance to creep [12], maximum fracture toughness [45], and maximum resistance to irradiation [46]. Note that the existence of the intermediate phase as well as that of potential structural signatures remains debated [14,47,48].…”