We have shown that the lattice-gas model with four repulsive pair interacti
on constants (corresponding to the four nearest coordination shells) on a s
imple cubic lattice is a sufficient model to describe the main types of cat
ion ordering in relaxors. The phase diagram, obtained by the cluster variat
ion method, shows the sequences of transitions between the phases 1:2 -->1:
1 --> disordered and 1:2 --> disordered.