Structures of Insulating Phases of Dense Lithium

Two high-pressure insulating phases of lithium were predicted using random search and evolutionary algorithm methods with first-principles electronic structure calculations. It is shown that lithium will transform from the metallic cubic $cI16$ phase to an insulating monoclinic $C2$ structure at 74 GPa. The $C2$ structure is the most stable phase up to 91 GPa, where it transforms to a second insulating orthorhombic $Aba2$ structure. The $C2$ and $Aba2$ structures are the first theoretical models explicitly showing the band gap opening in compressed lithium. The theoretical findings are supported by recent experimental evidence from electrical resistance and x-ray diffraction measurements.