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

Under extreme conditions of high pressure and temperature, liquidscan undergo substantial structural transformations as their atoms rearrange tominimise energy within a more conned volume. Understanding the structuralresponse of liquids under extreme conditions is important across a variety ofdisciplines, from fundamental physics and exotic chemistry to materials andplanetary science. In situ experiments and atomistic simulations can providecrucial insight into the nature of liquid-liquid phase transitions and the complexphase diagrams and melting relations of high-pressure materials. Structuralchanges in natural magmas at the high-pressures experienced in deep planetaryinteriors can have a profound impact on their physical properties, knowledge ofwhich 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-raydiffraction of liquids and the wide variety of systems which have been studiedby them, from simple liquid metals and their remarkable complex behaviour athigh-pressure, to molecular-polymeric liquid-liquid transitions in pnicogen andchalcogen liquids, and density-driven structural transformations in water andsilicate melts