ATOMIC SHORT-RANGE ORDER AND ALLOY ORDERING TENDENCY IN THE AG-AU SYSTEM

Accurate information of energetics is essential to map out the tempera ture versus composition phase diagram of a binary substitutional A(1-x )B(x) alloy. Since it is computationally prohibitive to calculate the total energies of all 2(N) configurations obtained by occupying N site s by A and B atoms, we map instead the ab initio calculated total ener gies of only O(10) simple structures (with less than or equal to 8 ato ms/cell) onto a generalized Ising model (including pair and many-body interactions)finding that for Ag-Au a close reproduction (within simil ar to 2 meV/atom) of LDA energies of arbitrary structures can be achie ved by including relatively short-ranged interactions. Subjecting thes e Ising interaction parameters to a Monte Carlo simulated annealing tr eatment, we obtain (i) the structures having T = 0 minimum energy ('gr ound 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 orderi ng temperatures for the ground state structures are too low to enable direct growth into the ordered phase, the calculated mixing enthalpy a nd the SRO parameters for Ag-Au agree quantitatively with experiment a nd clearly indicate a tendency for ordering, not phase separation.