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-principl es fully relaxed general-potential linearized augmented plane-wave (LA PW) 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, a nd 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-A g to phase separate at T=0 K. (ii) Of all possible structures, Cu3Au ( L1(2)) and CuAu (L1(0)) are found to be the most stable low-temperatur e phases of Cu1-xAux with transition temperatures of 530 K and 660 K, respectively, compared to the experimental values 663 K and approximat e to 670 K. The significant improvement over previous first-principles studies is attributed to the more accurate treatment of atomic relaxa tions in the present work. (iii) LAPW formation enthalpies demonstrate that L1(2), the commonly assumed stable phase of CuAu,, is not the gr ound state for Au-rich alloys, but rather that ordered (100) superlatt ices are stabilized. (iv) We extract the nonconfigurational (e.g., vib rational) entropies of formation and obtain large values for the size- mismatched systems: 0.48 k(B)/atom in Ni0.5Au0.5 (T=1100 K), 0.37 k(B) /atom in Cu0.141Ag0.859 (T=1052 K), and 0.16 k(B)/atom in Cu0.5Au0.5 ( T=800 K). (v) Using 8 atom/cell special quasirandom structures we stud y the bond lengths in disordered Cu-Au and Ni-Au alloys and obtain goo d qualitative agreement with recent extended x-ray-absorption fine-str ucture measurements.