Electron core ionization in compressed alkali metal cesium

Abstract: Elements of group I in the Periodic table have valence electrons of s-type and are usually considered as simple metals. Crystal structures of these elements at ambient pressure are close-packed and high-symmetry of bcc and fcc types, defined by electrostatic (Madelung) energy. Diverse structures were found under high pressure with decrease of the coordination number, packing fraction and symmetry. Formation of complex structures can be understood within the model of Fermi sphere - Brillouin zone interactions and supported by Hume-Rothery arguments. With the volume decrease there is a gain in the band structure energy accompanied by a formation of many-faced Brillouin zone polyhedrons. Under compression to less than a half of the initial volume the interatomic distances become close to or smaller than the ionic radius which should lead to the electron core ionization. At strong compression it is necessary to assume that for alkali metals the valence electron band overlaps with the upper core electrons which increases the valence electron count under compression.