Cu-Au, Ag-Au, Cu-Ag, and Ni-Au intermetallics: First-principles study of temperature-composition phase diagrams and structures

The classic metallurgical systems—noble-metal alloys—that have formed the benchmark for various alloy theories are revisited. First-principles fully relaxed general-potential linearized augmented plane-wave ~LAPW! total energies of a few ordered structures are used as input to a mixed-space cluster expansion calculation to study the phase stability, thermodynamic properties, and bond lengths in Cu-Au, Ag-Au, Cu-Ag, and Ni-Au alloys. ~i! Our theoretical calculations correctly reproduce the tendencies of Ag-Au and Cu-Au to form compounds and Ni-Au and Cu-Ag to phase separate at T50 K. ~ii! Of all possible structures, Cu 3Au (L12) and CuAu (L10) are found to be the most stable low-temperature phases of Cu12xAu x with transition temperatures of 530 K and 660 K, respectively, compared to the experimental values 663 K and '670 K. The significant improvement over previous first-principles studies is attributed to the more accurate treatment of atomic relaxations in the present work. ~iii! LAPW formation enthalpies demonstrate that L12, the commonly assumed stable phase of CuAu 3, is not the ground state for Au-rich alloys, but rather that ordered ~100! superlattices are stabilized. ~iv! We extract the nonconfigurational ~e.g., vibrational! entropies of formation and obtain large values for the size-mismatched systems: 0.48 kB/atom in Ni 0.5Au 0.5 (T51100 K!, 0.37 kB/atom in Cu 0.141Ag 0.859 (T51052 K!, and 0.16 kB/atom in Cu 0.5Au 0.5 (T5800 K!. ~v! Using 8 atom/cell special quasirandom structures we study the bond lengths in disordered Cu-Au and Ni-Au alloys and obtain good qualitative agreement with recent extended x-ray-absorption fine-structure measurements. @S0163-1829~98!08009-6#