The order or segregation properties of compounds or solid solutions are important ingredients of the phase diagrams. If the order can be described as an atomic distribution on an underlying lattice, and if the interactions can be expressed in terms of pairs and other multiplet potentials between the atomic species, phase diagrams should be deducible from these potentials, along with other properties, such as antiphase boundaries, core structures of the dislocations in ordered compounds&; This approach, i.e. the very existence of the potentials, is legitimated by electronic structure calculations for or alloys of normal [1] and transition metals [2]. The G.P.M. (General perturbation Method) allows indeed to develop the order energy in terms of interatomic potentials, the reference state, namely the random alloy, being calculated within the C.P.A. (Coherent Potential Approximation).