Pressure-induced structural changes in the network-forming isostatic glass GeSe 4 : An investigation by neutron diffraction and first-principles molecular dynamics

What is it about?

Chalcogenide glasses are materials that contain one or more of the elements S, Se, Te, and are exploited widely in infra-red optics and as phase-change memory alloys. They are also testbeds for examining the properties of atomic network structures using the likes of constraint-counting theory. Here, glasses such as GeSe4 correspond to compositions for which the bonding is optimally constrained to avoid network stress, and the glass-forming tendency is optimised. But how will this type of “isostatic” material respond to pressure? Can a network adapt in order to maintain a stress-free network as the material is squeezed?

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Why is it important?

We combined in situ high-pressure neutron diffraction with first principles molecular dynamics simulations to investigate the structure and related properties of GeSe4. At pressures up to ∼8 GPa, corresponding to a density increase of ∼37 %, the mean coordination number does not change from its ambient pressure value. This indicates a reorganization of the network at constant topology, i.e., isostatic networks may remain optimally constrained to avoid stress and retain their favourable glass-forming ability over a large density range. However, as the pressure is increased to ∼13 GPa, corresponding to a density increase of ∼49 %, the mean coordination number starts to increase as the electronic band gap decreases, i.e., the results are consistent with the onset of a semiconductor to metal transition.

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