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 calcul ations. In particular we study the Fermi surface nesting vectors as functio ns of the concentration and along a tetragonal Bain path. It turns out that the nesting vectors that can be associated with the experimentally observe d ordering transitions towards the B2 phase and L1(2) phases are commensura te with the lattice only at non-stoichiometric concentrations. The competit ion between this frustration effect and the electrostatic gain obtained by chemical ordering determines the critical concentrations for the transition s. We argue that the inclusion of these Fermi surface frustration effects i s necessary to obtain agreement between theory and experiment on the determ ination of the phase diagram.