  The classic metallurgical systems -- noble metal alloys -- that have formed
the benchmark for various alloy theories, are revisited. First-principles fully
relaxed general potential 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 T=0 K. (ii) Of all possible structures, Cu/sub 3/Au (L1/sub 2/) and
CuAu (L1/sub 0/) are found to be the most stable low-temperature phases of
Cu/sub 1-x/Au/sub 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 L1/sub 2/, the commonly assumed
stable phase of CuAu/sub 3/, is not the ground state for Au-rich alloys, but
rather that ordered <100> superlattices are stabilized. (iv) We extract the
non-configurational (e.g., vibrational) entropies of formation and obtain large
values for the size mismatched systems: 0.48 k/sub B//atom in Ni/sub 0.5/Au/sub
0.5/ (T=1100 K), 0.37 k/sub B//atom in Cu/sub 0.14/Ag/sub 0.86/ (T=1052 K), and
0.16 k/sub B//atom in Cu/sub 0.5/Au/sub 0.5/ (T=800 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 EXAFS
measurements.
