Composition dependence of the glass forming ability in binary mixtures: The role of demixing entropy

Abstract

We present a comparative study of the glass forming ability of binary systems with varying composition, where the systems have similar global crystalline structure (CsCl+fcc). Biased Monte Carlo simulations using umbrella sampling technique show that the free energy cost to create a CsCl nucleus increases as the composition of the smaller particles is decreased. We find that systems with comparatively lower free energy cost to form CsCl nucleus exhibit more pronounced pre-crystalline demixing near the liquid/crystal interface. The structural frustration between the CsCl and fcc crystal demands this demixing. We show that closer to the equimolar mixture, the entropic penalty for demixing is lower and a glass forming system may crystallize when seeded with a nucleus. This entropic penalty as a function of composition shows a non-monotonic behaviour with a maximum at a composition similar to the well known Kob-Anderson (KA) model. Although the KA model shows the maximum entropic penalty and thus maximum frustration against CsCl formation, it also shows a strong tendency towards crystallization into fcc lattice of the larger "A" particles which can be explained from the study of the energetics. Thus for systems closer to the equimolar mixture although it is the requirement of demixing which provides their stability against crystallization, for KA model it is not demixing but slow dynamics and the presence of the "B" particles make it a good glass former. The locally favoured structure around "B" particles is quite similar to the CsCl structure and the incompatibility of CsCl and fcc hinders the fcc structure growth in the KA model. Although the glass forming binary systems studied here are quite similar, differing only in composition, we find that their glass forming ability cannot be attributed to a single phenomenon.

Crystallization Instability in Glass-Forming Mixtures

“…Here, of course, we have a binary system, but the predominant crystal structures we find are fcc and hcp of the large A species only, and very little mixed AB bcc. The lack of bcc is consistent with predictions that the crystal nucleation barrier is much higher relative to fcc [28] and with the equilibrium KA phase diagram [29]. For the KA model, we therefore neglect the bcc structure and focus on the hcp and fcc crystals in the following.…”

Unveiling the medium-range order in glass models and its role in glass formation

“…Several factors have been proposed to be responsible for the stabilities of KA glass in the literature, including the structure difference of the dominant SRO in liquid and crystal [41], the order competition between bcc and fcc [43], and the high demixing entropy [44]. Here we give some insight from the MRO perspective.…”

“…Here we give some insight from the MRO perspective. The abundant pentagon-rich MRO in KA glass poses a high-energy barrier for a bcc (B2) seed to grow, as reported by Nandi et al [44]. As a result, the crystallization in the KA system can only happen with the help of other types of nuclei.…”

Evaporating laminar microjets for studies of rapidly evolving structural transformations in supercooled liquids

“…Mixing pure pH 2 with the J = 0 ortho-deuterium (oD 2 ) isotope allows moving over the next stage of complexity in the study of supercooled quantum liquids. From a very general point of view, experiments and theory show that supercooled binary liquid mixtures display a much richer behaviour than the single components [44][45][46][47][48][49][50][51][52]. For instance, mixing two species is a well-established route to increase the glass-forming ability of supercooled liquids [46,51,52].…”

“…From a very general point of view, experiments and theory show that supercooled binary liquid mixtures display a much richer behaviour than the single components [44][45][46][47][48][49][50][51][52]. For instance, mixing two species is a well-established route to increase the glass-forming ability of supercooled liquids [46,51,52]. The study of isotopic supercooled pH 2 -oD 2 mixtures can be further motivated by recent simulation results on a model binary liquid, which suggest that quantum effects can stabilize the metastable liquid phase to produce a quantum glass state, whereas the classical analogue would rapidly crystallize [36].…”