The light alkali and alkaline-earth 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 initioanalysis using density functional theory and molecular dynamics simulation. While generally considered simple elements at ambient conditions which are well-described by the free-electron model, these simple 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, though this has not been observed experimentally.

Furthermore, electride-like phases constituting a localization of valence electrons into interstitial lattice sites and resulting in non-nuclear charge distribution maxima have been suggested by ab initio calculations in K and Mg. As such, there are a multitude of unanswered questions relating to these elements which we are trying to answer.

Here we present results from multi-megabar diamond anvil cell investigations into the properties and phases of Mg, K, and Rb, and discuss challenges associated with studying these elements. We also comment on development of toroidal diamond anvil cells which will enable static experiments above 500 GPa to routinely become possible.