A picture showing a phase diagram of potassium from MLMD simulations

Simulations supported by machine learning have shown an unusual phase transition in liquid high pressure potassium. At around 10 GPa, the element gradually changes from a free-electron metal to an electride – a strange, salt-like structure in which electrons take over the role of anions.

Little is known about the behavior of liquid potassium – and other alkali metals – at high pressure. This is not surprising, as their high reactivity makes them difficult to process even in the solid state and at atmospheric pressure. Ab initio molecular dynamics (AIMD) has shown a possible liquid-liquid phase transition in several high-pressure alkali metals, which is indicated by a decrease in the atomic coordination number. However, AIMD calculations can only investigate short-range interactions, not the behavior of bulk solids, so it remained unclear what exactly happened.

A team of researchers in China, Great Britain and Italy has now added machine learning to AIMD to simulate long-range order in high pressure potassium. They discovered two different liquid states: a low-pressure atomic form and a high-pressure electrode. In the latter, the electrons act like anions in an ionic salt and are deposited in the spaces between positively charged potassium nuclei.

The transition from the free electron metal to the electride takes place gradually, as the calculations showed. From around 10 GPa, potassium becomes a two-component liquid in which the atomic and the electride form coexist, until the latter takes over at around 20 GPa.

The discovery has also solved the mystery of why the high pressure liquid phase of potassium is denser than its solid one, a face-centered cubic crystal lattice that is the most efficient packing for spheres. Since the electride’s ions are smaller than atoms, they can sit closer together, with the tiny electrons fitting into the remaining spaces.

The team suggests that other liquid alkali metals should exhibit a similar electron-electron free transition when exposed to enough pressure.

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