Behavior of rubidium at over eightfold static compression

The behavior and structure of Rb are explored up to 264 GPa, a pressure at which the density is increased more than eightfold. The previously predicted double-hexagonal close packed structure is stable above 200 GPa, while at lower pressures a change in compressibility at 30 GPa arises from $s$-$d$ charge transfer. Above 30 GPa, the authors find that Rb has the regular compression of a $d$-type transition metal.