Topological Ordering and Viscosity in the Glass-Forming Ge–Se System

What is it about?

Many of the properties of glassy materials are related to their underlying network structures. According to the mean field constraint counting theory of Phillips and Thorpe, these network structures should undergo an abrupt transformation from an elastically floppy to a stressed-rigid phase when the mean number of bonding constraints per atom is equal to number of degrees of freedom per atom. Here, the glassy GexSe1-x system has proved to be a test bed for exploring the structure-property relationships in network glass-forming materials, and the transformation is predicted to occur at x = 0.2 where the mean coordination number <n> = 2.4. More recently, temperature modulated differential scanning calorimetry experiments have been interpreted in terms of the existence of a third “intermediate phase,” i.e., a range of compositions that separate the floppy and stressed rigid regimes. The existence of this intermediate phase has, however, courted controversy, and a structural origin has not been discovered.

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Photo by JJ Ying on Unsplash

Photo by JJ Ying on Unsplash

Why is it important?

We look for a structural origin of the intermediate phase by making the first use of high-resolution neutron diffraction to explore the topological ordering in the glassy Ge-Se system. This technique gives a direct measure of the mean coordination number <n>, a core parameter in constraint counting theories. A structural origin of the intermediate phase could not, however, be detected, in contradiction to first-principles molecular dynamics simulations that predict a plateau in the composition dependence of the mean coordination number for Ge-Se glasses within the intermediate phase. We also looked for a dynamical origin of the intermediate phase by investigating the fragility index and the viscosity at the liquidus temperature. Here, a minimum in the fragility index and maximum in the viscosity are found at about x = 0.2. It appears, therefore, that the origin of the intermediate phase in the Ge−Se system, as found from the behaviour of the non-reversing enthalpy measured using temperature modulated differential scanning calorimetry at temperatures corresponding to the glass transition, is linked to the dynamical properties of this material in the liquid state.

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