The diffusion of oxygen on W(110) The influence of the p(2 × 1) ordering

Abstract

Tracer and chemical surface diffusion coefficients for oxygen chemisorbed on W(110) have been investigated by means of Monte Carlo modeling, taking full advantage of the numerical power of a supermassive parallel supercomputer. The simulations were performed for a lattice gas with up to fourth nearest neighbor interactions. These were chosen to reproduce the experimentally determined O W(110) phase diagram for θ ≤ 0.5 which shows a dominating p(2 × 1) phase around half coverage. It was found that the p(2 × 1) ordering strongly influences surface diffusion at low temperatures. The tracer diffusion coefficient shows a strong minimum slightly below half coverage and a very small maximum above half coverage. This behavior is attributed to the change of the dominating lattice defects of the p(2 × 1) phase from vacancies in filled rows below θ = 0.5 to interstitials in empty rows above θ = 0.5. For the p(2 × 1) ordered lattice gas phase the chemical diffusion coefficient exhibits a strong maximum which becomes more pronounced as the temperature is lowered. This is attributed to the behavior of mean square fluctuations 〈(δN) 2〉 〈N〉 , i.e. the inverse of the thermodynamic factor, which dominates the chemical diffusion coefficient under these circumstances. The p(2 × 1) ordering causes the surface diffusion to become highly anisotropic. Attempts to simulate the increase in activation energy with coverage found experimentally showed that this was not possible without invoking adsorbate induced changes in the saddle point energies for diffusion.