Short-Range Order and Phase Stability of Surface Alloys: PdAu on Ru(0001)

Structural and thermodynamic properties of thermally equilibrated Pd-Au/Ru(0001) surface alloys are studied within the framework of lattice models based upon first-principles total energy calculations and scanning-tunneling-microscopy imaging of atomic structure. Pd-rich alloys form pseudomorphic solid-solution phases with pronounced chemical short-range order (SRO), consistent with the prediction of a low-temperature order-disorder transition to a ({radical}(3){times}{radical}(3) )-R30{degree} ground state structure. Calculated and measured SRO parameters are found to be in excellent agreement and are quantitatively described by a lattice-model Hamiltonian dominated by short-ranged interactions. {copyright} {ital 1999} {ital The American Physical Society}