Complex structures of dense lithium: electronic origin

Abstract: Lithium - the lightest alkali metal - exhibits unexpected structures and electronic behaviour at high pressures. As the heavier alkalis, Li is bcc at ambient pressure and transforms first to fcc (at 7.5 GPa). The post-fcc high-pressure form Li-cI16 (at 40-60 GPa) is similar to Na-cI16 and related to more complex structures of heavy alkalis Rb-oC52 and Cs-oC84. The other high pressure phases for Li (oC88, oC40, oC24) found at pressures up to 130 GPa are specific the only to Li. The different route of Li high-pressure structures correlates with its special electronic configuration containing the only 3 electrons (at 1s and 2s levels). Crystal structures for Li are analyzed within the model of Fermi sphere - Brillouin zone interactions. Stability of post-fcc structures for Li can be supported by Hume-Rothery arguments when new Brillouin zone plains appear close to the Fermi level producing pseudogaps near the Fermi level and decreasing the crystal energy. The filling of Brillouin-Jones zones by electron states for a given structure defines the physical properties as optical reflectivity, electrical resistivity and superconductivity. To understand complexity of structural and physical properties of Li above 60 GPa is necessary to assume the valence electrons band overlap with the upper core electrons and increase the valence electron count under compression.