The structural changes in glasses and liquids induced by high-pressure and/or high-temperature conditions can alter substantially their dynamical and transport properties. A notable example is pro- vided by so-called polyamorphic transitions where the variation of a state parameter such as pressure or temperature leads to an abrupt transformation between two phases having the same composition but different densities. Unravelling the mechanisms by which these transformations occur is, however, a formidable task owing to the nature of structural disorder and the experimental difficulties associated with the investigation of materials under extreme conditions.
This talk will focus on recent progress in measuring the structure of network-forming glasses using in situ neutron diffraction with a Paris-Edinburgh press at pressures up to 17.5(5) GPa [1–5]. In particular, the mechanisms of density-driven structural collapse in glasses such as B2O3 , SiO2 , GeO2 [2–4] and GeSe2  will be considered, where the debate is informed by results obtained from the first applications of the method of high-pressure neutron diffraction with isotope substitution. The diffraction data are compared to new molecular dynamics simulations, made using theoretical schemes that suit the materials under investigation.
In the case of oxide materials, we will show that the coordination number of network-forming structural motifs, which plays a key role in defining the topological ordering, can be rationalized in terms of the oxygen packing-fraction over an extensive pressure and temperature range . The result is a structural map for predicting the likely regimes of topological change. This information can be used to forecast when transformations may occur to the transport properties and compressibility of e.g. fluids in planetary interiors, and is a prerequisite for the preparation of new materials following the principles of rational design.
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