Liquid structure under extreme conditions: High-pressure x-ray diffraction studies

Under extreme conditions of high pressure and temperature, liquids
can undergo substantial structural transformations as their atoms rearrange to
minimise energy within a more conned 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.