Fermi surface origin of non-stoichiometric ordering in CuPd alloys

We discuss the peculiarities of the CuPd phase diagram in relation with the Fermi surface of the random alloys, on the basis of first-principle calculations. In particular we study the Fermi surface nesting vectors as functions of the concentration and along a tetragonal Bain path. It turns out that the nesting vectors that can be associated with the experimentally observed ordering transitions towards the B2 phase and L1(2) phases are commensurate with the lattice only at non-stoichiometric concentrations. The competition between this frustration effect and the electrostatic gain obtained by chemical ordering determines the critical concentrations for the transitions. We argue that the inclusion of these Fermi surface frustration effects is necessary to obtain agreement between theory and experiment on the determination of the phase diagram.