Ordering and segregation in XPt (X=V, Cu, and Au) random alloys

We examine the phase stability and the ordering tendencies of some Pt-based fcc random alloys using the generalized perturbation method (GPM) implemented in the linear muffin-tin orbitals (LMTO) basis. The reference medium for the GPM is chosen as the completely disordered state of the alloy and its electronic structure is described in the coherent potential approximation (CPA). Ordering tendencies and phase stability are examined via effective pair interactions and their lattice Fourier transforms. Relativistic effects on the ground state cohesive properties and the ordering tendencies are determined by carrying out nonrelativistic and fully relativistic (in some cases also scalar-relativistic) LMTO-CPA calculations. In all cases considered, namely $X\mathrm{Pt}$ with $X=\mathrm{V},$ Cu, and Au, the correct ordering tendency is obtained. The ordering tendency is found to be somewhat overestimated in the GPM. Relativistic effects are found to be most prominent in AuPt, where the nonrelativistic LMTO-CPA-GPM description shows a tendency towards ${L1\mathrm{}}_1$ ordering and the correct result, i.e., phase segregation, is obtained only in the fully relativistic description. The sensitivity of the ordering tendency to factors such as lattice relaxation and volume per atom is examined briefly. Finally, the effect on the phase stability of adding a third component, such as V or Au to CuPt alloy, is studied by extending the formalism to the case of a ternary alloy.