Compression of Heavy Alkali Metals at Multi-Megabar Pressures

The light alkali metals are of particular interest to both experimental and theoretical investigations due to their exotic behaviour at extreme conditions, and relative atomic simplicity facilitating ab initio analysis using DFT and MD simulation. While considered simple elements at ambient conditions, well-described by the free-electron model, these metals become increasingly complex at extreme conditions;
incommensurate host-guest structures become favourable in K at 20 GPa and Rb at 16 GPa. These two elements are also predicted to mirror the transitions of Cs at increasing pressures, I41 / amd → Cmca → dhcp.
We present an exploration Rb at pressures up to 250 GPa, discussing the challenges involved in studying these highly reactive elements in diamond anvil cells. We further discuss the transition to dhcp phase, and comment on the bizarre compressive behaviour caused by the plethora of phase changes in these ‘simple’ metals.