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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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.
Perspectives
There is a drive to understand the structure-related properties of glass-forming systems, which is held-back by the intrinsic structural disorder of these materials. Here, we investigate directly the topological nature of this disorder in the Ge-Se system by applying neutron diffraction, and explore the structural origin of glasses in and around the so-called intermediate phase. We also investigate the dynamical properties of the glass-forming melt as expressed by the viscosity and fragility index. No structural origin to the intermediate phase was found. There is a relation, however, between intermediate phase compositions and the dynamical properties of the melt.
Professor Philip S Salmon
University of Bath
Read the Original
This page is a summary of: Topological Ordering and Viscosity in the Glass-Forming Ge–Se System: The Search for a Structural or Dynamical Signature of the Intermediate Phase, Frontiers in Materials, November 2017, Frontiers,
DOI: 10.3389/fmats.2017.00032.
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