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 bas ed upon first-principles total energy calculations and scanning-tunneling-m icroscopy imaging of atomic structure. Pd-rich alloys form pseudomorphic so lid-solution phases with pronounced chemical short-range order (SRO), consi stent with the prediction of a low-temperature order-disorder transition to a (root 3 x root 3)-R30 degrees ground state structure. Calculated and mea sured SRO parameters are found to be in excellent agreement and are quantit atively described by a lattice-model Hamiltonian dominated by short-ranged interactions.