Short-range order and pair potentials in Au-Ag

Au-Ag single crystals with 25, 48, and 75 at.% Ag in a state of thermodynamic equilibrium frozen in by quenching were investigated by diffuse x-ray scattering. The elastic-scattering contributions due to short-range order and static atomic displacements were separated using the method of Borie and Sparks and of Georgopoulos and Cohen. Weak short-range-order maxima were always found at 100 positions. The effective pair potentials determined by an inverse Monte Carlo method as well as by the high-temperature approximation of Clapp and Moss are dominated by the nearest-neighbor interaction of 10--16 meV. Based on these pair potentials, the superstructures L1₀ and L1₂ are found as stable low-temperature phases. For the stoichiometric compositions Monte Carlo simulations give order-disorder transitions at 100--200 K.