Atomic short-range order and alloy ordering tendency in the Ag-Au system

Accurate information of energetics is essential to map out the temperature versus composition phase diagram of a binary substitutional A_1-xB_x alloy. Since it is computationally prohibitive to calculate the total energies of all 2^N configurations obtained by occupying N sites by A and B atoms, we map instead the ab initio calculated total energies of only O(10) simple structures (with <or=8 atoms/cell) onto a generalized Ising model (including pair and many-body interactions) finding that for Ag-Au a close reproduction (within approximately 2 meV/atom) of LDA energies of arbitrary structures can be achieved by including relatively short-ranged interactions. Subjecting these Ising interaction parameters to a Monte Carlo simulated annealing treatment, we obtain (i) the structures having T=0 minimum energy ('ground states'); (ii) the order-disorder phase transition temperatures; (iii) the mixing enthalpy for the disordered alloy; and (iv) the high-temperature atomic short-range order (SRO). While the predicted ordering temperatures for the ground state structures are too low to enable direct growth into the ordered phase, the calculated mixing enthalpy and the SRO parameters for Ag-Au agree quantitatively with experiment and dearly indicate a tendency for ordering, not phase separation.