Revealing low-temperature atomic ordering in bulk Co-Pt with the high-throughput ab-initio method

The low-temperature phase diagram of bulk Co-Pt is studied with a high-throughput ab-initio method. Global, hcp-, and fcc-restricted convex hulls are constructed to evaluate stable and metastable phases. It is found that fcc-L10 is energetically degenerate with hcp-B19. Both structures are unstable with respect to phase decomposition into hcp-D019↔fcc-β2 at low temperature. Furthermore, L10 is an adaptive structure on the fcc-restricted convex hull which relates to the low energies of antiphase boundaries. Fcc-L12 is energetically degenerate with fcc-D023 for both Co3Pt and CoPt3. L12-Co3Pt and L10-CoPt belong to the fcc-restricted convex hull. They might stabilize above the Co hcp/fcc transition and remain kinetically frozen below. L12-CoPt3 is energetically well above the convex hull. Its experimental observation may result from yet unexplained finite-temperature effects.