  We present a two-dimensional lattice gas with anisotropic interactions which
model the known properties of the surface reconstructions of CdTe and ZnSe. In
constrast to an earlier publication [12], the formation of anion dimers is
considered. This alters the behaviour of the model considerably. We determine
the phase diagram of this model by means of transfer matrix calculations and
Monte Carlo simulations. We find qualitative agreement with the results of
various experimental investigations.
