Lawrence Alexander Victor Naden Robinson
(The University of Edinburgh)

Abstract:
High pressure research has led to many exciting discoveries, such as the highest critical temperature superconductors currently found [1], although not without recent controversy. Many other unexpected phenomena have been observed and predicted at high pressure, and here I will go through some of them. Molecular mixtures at planetary conditions, such as water+ammonia and water+methane, are expected to be found in many celestial bodies such as Uranus and Neptune. Here we find new ionic phases and mixtures of ammonia and water were predicted up to 500 GPa, and now recently measured [2], using crystal structure searching methods. These solid mixtures were then heated to reveal the expected superionic transition before melting, but also found a plastic regime at lower temperatures associated with various forms of molecular movements. At lower pressures, liquid methane and water have been observed to mix and here we perform long AIMD simulations, as well as many body decomposition (ML) potentials, between 0 and 3.0 GPa combined with neutron scattering data to examine the structure and increased solubility of methane in water. The second half of this talk examines the effect of high pressure on the previously considered simple alkali metals. These elements enter complex phases, such as host-guest and electride structures, following the typical bcc and fcc once compressed. Here we investigated the ideal host-guest atom ratio and found that these phases have moderate electride character, shown by a build of electron localization in non-nuclear sites. These host-guests structures also show an order-disorder transition, known as "chain-melting", and here we trained a machine-learned interatomic potential to reveal how these guest chains disorder with temperature [3]. Making further use of ML potentials and AIMD for liquids, we show that alkali metals localize electrons on non-nuclear sites as "pseudoanions" under sufficient pressure during a continuous liquid-liquid transition[4]. Finally, I may discuss recent work on mixing water+hydrogen at extreme conditions, the oxidation state of early earth from a material science perspective, and recent observations of a post ice X phase of ice above 230 GPa.