  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.
