Structure of ZnCl₂ Melt. Part II: Fragile-to-Strong Transition in a Tetrahedral Liquid

Abstract

The fraction of edge- and corner-sharing tetrahedra in liquid ZnCl is quantified as a function of temperature using Raman spectroscopy and ab initio molecular dynamic simulations. Two distinct regimes are found in the temperature dependence of the change in these structural units. This behavior is consistent with the existence of a fragile-to-strong transition in liquid ZnCl as suggested by calorimetric and viscosity measurements. The structural origin of this transition is rationalized in terms of a constraint counting formalism. It is suggested that the ratio of edge- to corner-sharing tetrahedra controls the configurational entropy and in turn the viscosity of the melt. The temperature dependence of this ratio above the melting point is also found to be qualitatively consistent with neutron diffraction data. The observation of a similar fragile-to-strong transition in the isostructural GeSe melt indicates that it may be a common feature of tetrahedral liquids.

Liquid‐liquid phase transitions in glass‐forming systems and their implications for memory technology

“…For Ge 15 Te 85 and tellurium, the density increase is more pronounced, in accordance with the sharper F‐S transition. Density data for the Ge‐Se system indicate that such transitions are prevalent in the Ge‐rich portion of the binary system, including GeSe 2 as previously suggested . This is consistent with viscosity data indicating F‐S transitions in all these Ge‐rich compositions .…”

“…Density data for the Ge-Se system indicate that such transitions are prevalent in the Ge-rich portion of the binary system, including GeSe 2 as previously suggested. 8,60 This is consistent with viscosity data indicating F-S transitions in all F I G U R E 2 Maxima in thermodynamic response functions indicative of liquid-liquid phase transitions in chalcogenide systems. Arrows represent the melting point or liquidus temperature for each system.…”

Uncovering β-relaxations in amorphous phase-change materials

“…Non-PCMs are characterized by slow crystallization kinetics (13) and good glass-forming abilities in terms of the Turnbull parameter T g /T l ≥ 0.6, where T g and T l are the glass transition temperature at a conventional rate (e.g., 20 K/min) and the liquidus temperature, respectively. Turnbull (37) showed that T g /T l close to 2/3 is the criterion for good glass formers [T g /T l = 0.6 for GeSe (38), 0.61 for Ge 15 Te 85 (39), and 0.68 for GeSe 2 (40)]. On the contrary, PCMs have T g /T l ~ 0.5, indicative of poor glass-forming T (K) abilities (35,38).…”

“…For PCM applications, GeTe, on July 4, 2020 http://advances.sciencemag.org/ Downloaded from Ge 2 Sb 2 Te 5 , and AIST crystallize on a time scale of nanoseconds (~10 to 100 ns) at elevated temperatures (5,10). By contrast, non-PCMs like Ge 15 Te 85 , GeSe, and GeSe 2 crystallize much more slowly (36,40), rendering them unsuitable for phase-change memory applications. According to the classical theory of nucleation and growth, the nucleation rate I s and crystal growth velocity u can be respectively expressed as (50-52)…”

Topological Phases of Chalcogenide Glasses Encoded in the Melt Dynamics

“…We have also examined the fragility index of the homogenized Ge–Se melts using the Moynihan approach . In this approach one examines the scan rate ( q = d T /d t ) dependence of T g in a DSC experiment.…”