Ab initio study of long-period superstructures in close-packed A3B compounds

We have performed ab initio calculations of the stability of one-dimensional long-period superstructures in Cu3Pd, Cu3Al, and Ag3Mg by means of an interface Green's function technique based on the linear-muffin-tin-orbitals method within the tight-binding and atomic-sphere approximations. The energies of the superstructures relative to the L1(2) structure are found by an expansion based on the calculated energy of a single (001) antiphase boundary and the calculated interaction energy between two and three antiphase boundaries of varying distance. The expansion agrees with standard band-structure calculations of the structural energy differences for the two short-period superstructures DO22 and DO23. We find that at zero temperature the ground states of Cu3Pd, Cu3Al, and Ag3Mg are one-dimensional superstructures with antiphase boundary separations of 2-5 unit cells of the underlying L1(2) structure.