Returning to close packed structures and core bonding in Cs metal under high-pressure

Numerous experimental and theoretical studies showed 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, d atomic and quasi-atomic orbitals. However, little is known to the behavior of these metals beyond this pressure region. Using first principles calculations and crystal structure search methods, we show that Cs returns to close packed structures under ultrahigh pressure, including the transition from DHCP to FCC structure at 183 GPa and a further transition into HCP at 655 GPa. More significantly, in these structures, the 5p electrons are activated and form strong chemical bonds between neighboring Cs atoms that governs the properties of Cs under very high pressure, including the transition from FCC to HCP.