Core bonding in Cs metal under high pressure and its return to close-packed structures

Numerous experimental and theoretical studies have shown that alkali and alkaline metals transform into various structures with large open interstitials under pressure, an intricate behavior that is caused by the electron transfer among $s, p$, and $d$ atomic and quasiatomic orbitals. However, little is known about the behavior of these metals beyond this pressure region. Using first-principles calculations and crystal structure search methods, we show that Cs returns to simple close-packed structures under ultrahigh pressure, including the transition from double hcp to fcc structure at 183 GPa and a further transition into hcp at 655 GPa. More significantly, under ultrahigh pressure, the $5p$ electrons are activated and form strong chemical bonds between neighboring Cs atoms that govern the properties of Cs under very high pressure, including the transition from fcc to hcp.