Single elements are able to form incommensurate crystal structures under pressure, where a zeolite-type “host" sublattice surrounds a “guest" sublattice comprising 1D chains of atoms. On “chain melting", diffraction peaks from the guest sublattice vanish, while those from the host remain. Diffusion of the guest atoms is expected to be confined to the channels in the host sublattice, which suggests 1D melting. Here, we present atomistic simulations of Potassium to investigate this phenomenon, and demonstrate that the chain-melted phase has no long-ranged order either along or between the chains. This 3D disorder provides the extensive entropy necessary to make the chain melt a true thermodynamic phase of matter, yet with the unique property that diffusion remains confined to 1D only.

Calculations necessitated the development of an interatomic forcefield using machine learning (MLMD), which we show fully reproduces Potassium’s phase diagram (fig. 1), including the chain-melted state and 14 known phase transitions. Under pressure the alkali metals enter complex solid crystal phases with changing coordination and electron structure such as off-atom site localized electron density. These changes from the simple metal picture are likely to occur on the liquid under pressure in a similar manner. Here we investigate the high pressure liquid and transitions around the melting lines in the more massive alkali metals.