Predicted complex lithium phases at terapascal pressures

We investigate the pressure-temperature (p-T) phase diagram of elemental lithium (Li) up to multiterapascal (TPa) pressures using ab initio random structure search (AIRSS) and density functional theory (DFT). At zero temperature, beyond the high-pressure Fd3¯m diamond structure already predicted in previous studies, we find 11 solid-state phase transitions to structures of greatly varying complexity. The full p-T dependence of the phase boundaries are computed within the vibrational quasi-harmonic approximation (QHA), and the solid-liquid melting line is calculated using two different ab initio molecular dynamics simulation methods (heat-until-melt and the Z method). Notably, between 39.1 and 55.7 TPa, Li adopts an elaborate monoclinic structure with 46 atoms in the primitive unit cell, and between 71.9 and 103 TPa, an incommensurate host-guest phase of the Ba-IV type. We find that Li, hitherto predicted to be an electride at TPa pressures, abruptly loses its electride character above 16 TPa, reverting back to normal metallic behavior with a corresponding rise in the Fermi-level electronic density of states (eDOS) and broadening of the electronic bands.