Alkali metal incommensurate phases under pressure: phase trasisitions, stability and electronic structure

The free-electron model fails when "free-electron" metals are compressed, leading to the appearance of open and complex structures under pressure. A remarkable and intriguing example are the incommensurate host-guest phases in sodium, potassium, and rubidium under pressure. The "guest" substructure forms chains of atoms interpenetrating the host so that the axial ratio (c_h/c_g) is irrational. For computational studies, commensurate unit cells are used as approximants with c_h/c_g ratios close to those seen in nature. Using commensurate structures with different c_h/c_g ratios, we assess the ability of density functional theory to describe the phase sequence in each alkali metals, and study the pressure evolution of the guest content. Topological analysis of the electron density and the electron localization function (ELF) provides insight into why nature chooses these strange phases: in addition to the guest atoms, tubes of electrons are also found in channels through the guest structure. Which channels contain electrons and which contain guest atoms depends on the material. Finally, we use molecular dynamics to simulate the phase transformation into the host-guest structure, despite its complexity, it is remarkably easy to nucleate and grow.