  The analysis of Coulomb crystallization is extended from one-component to
two-component plasmas. Critical parameters for the existence of Coulomb
crystals are derived for both classical and quantum crystals. In the latter
case, a critical mass ratio of the two charged components is found which is of
the order of 80. Thus, holes in semiconductors with sufficiently flat valence
bands are predicted to spontaneously order into a regular lattice. Such hole
crystals are intimately related to ion Coulomb crystals in white dwarf and
neutron stars as well as to ion crystals produced in the laboratory. A unified
phase diagram of two-component Coulomb crystals is presented and is verified by
first-principle computer simulations.
