Abstract
Under extreme conditions of high pressure and temperature, liquids can undergo substantial structural transformations as their atoms rearrange to minimise energy within a more confined volume. Understanding the structural response of liquids under extreme conditions is important across a variety of disciplines, from fundamental physics and exotic chemistry to materials and planetary science. In situ experiments and atomistic simulations can provide crucial insight into the nature of liquid–liquid phase transitions and the complex phase diagrams and melting relations of high-pressure materials. Structural changes in natural magmas at the high-pressures experienced in deep planetary interiors can have a profound impact on their physical properties, knowledge of which is important to inform geochemical models of magmatic processes. Generating the extreme conditions required to melt samples at high-pressure, whilst simultaneously measuring their liquid structure, is a considerable challenge. The measurement, analysis, and interpretation of structural data is further complicated by the inherent disordered nature of liquids at the atomic-scale. However, recent advances in high-pressure technology mean that liquid diffraction measurements are becoming more routinely feasible at synchrotron facilities around the world. This topical review examines methods for high pressure synchrotron x-ray diffraction of liquids and the wide variety of systems which have been studied by them, from simple liquid metals and their remarkable complex behaviour at high-pressure, to molecular-polymeric liquid–liquid transitions in pnicogen and chalcogen liquids, and density-driven structural transformations in water and silicate melts.
Highlights
IntroductionUndergoing continuous diffusive motions, like a compressed gas, resulting in an absence of long-range structural order
Liquids are an intriguing state of condensed matter with a density close to that of the solid-state but with all atoms undergoing continuous diffusive motions, like a compressed gas, resulting in an absence of long-range structural order
The results reveal a disappearance of a pre-peak observed in S(Q) indicative of a loss of intermediate range order associated with correlations between As4 molecules [334, 336]
Summary
Undergoing continuous diffusive motions, like a compressed gas, resulting in an absence of long-range structural order. Synchrotron x-ray and neutron diffraction are both well established techniques for high-p solid-state crystallography [77, 78], diffraction studies of liquids at high-p remain challenging This is due to the technically demanding task of generating the simultaneous high-p–T conditions necessary for melting combined with the requirement to accurately isolate the diffuse liquid scattering signal from the large background contributions arising from the apparatus containing the sample. Classical potentials can be of limited accuracy and in situ measurements of liquid structure are essential to provide an effective test of theoretical predictions from classical or ab initio MD and, enable their extension to p–T conditions that are difficult to achieve experimentally [122] This topical review is primarily focussed on in situ XRD studies of liquid structure at high-p. The review concludes with a future perspective on in situ liquid diffraction studies
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