Ab initio phase diagram of oxygen adsorption on W(110)

The phase diagram of oxygen adsorption on the W(110) surface is derived without any empirical parameters by a combination of density functional theory (DFT) calculations, the cluster expansion (CE) technique and Monte Carlo (MC) applications. Coverages up to 1 monolayer are considered corresponding to the range of oxygen concentrations, 0≤xO≤1. DFT results for single-site adsorption and in particular for full coverage reveal that adsorption at threefold hollow (H3) sites is by far the most stable one. Therefore, the CE is done for an atomic layer with the two H3 sublattices of the W(110) surface. Based on 60 DFT calculations with fully relaxed atomic geometries of lateral unit cells containing 12 atoms, and a ground state search for 80 394 structures, four ground state structures are found with the lateral unit cells (2 x 5) for xO = 0.20, (2 x 2) (a) for xO = 0.25, (2 x 1) for xO = 0.50 and (2 x 2) (b) for xO = 0.75. In agreement with experiments the most stable structures are (2 x 1) and (2 x 2) (b), which correspond to higher coverages. The thermodynamical stability of the two ground states at lower coverages is very weak. Detailed analysis of the relaxation of the (2 x 1) structure reveals sizeable lateral stresses acting on the surface tungsten atoms. On the basis of the effective cluster interactions MC simulations are performed in order to derive the critical temperatures by which the phase diagram is finally constructed.