  The ground state of a two-dimensional ionic mixture composed of oppositely
charged spheres is determined as a function of the size asymmetry by using a
penalty method. The cascade of stable structures includes square, triangular
and rhombic crystals as well as a dipolar pair gas and a gas of one-dimensional
crystalline chains. Thereby we confirm the square structure, found
experimentally on charged granulates, and predict new phases detectable in
experiments on granular and colloidal matter.
