Predicted complex lithium phases at terapascal pressures

We investigate the pressure-temperature <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mo>(</a:mo><a:mi>p</a:mi><a:mtext>−</a:mtext><a:mi>T</a:mi><a:mo>)</a:mo></a:mrow></a:math> phase diagram of elemental lithium (Li) up to multiterapascal (TPa) pressures using random structure search (AIRSS) and density functional theory (DFT). At zero temperature, beyond the high-pressure <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mrow><b:mi>F</b:mi><b:mi>d</b:mi><b:mover accent="true"><b:mn>3</b:mn><b:mo>¯</b:mo></b:mover><b:mi>m</b:mi></b:mrow></b:math> diamond structure already predicted in previous studies, we find 11 solid-state phase transitions to structures of greatly varying complexity. The full <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:mrow><d:mi>p</d:mi><d:mtext>−</d:mtext><d:mi>T</d:mi></d:mrow></d:math> 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 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.