THE DIFFUSION OF OXYGEN ON W(110) - THE INFLUENCE OF THE P(2X1) ORDERING

Tracer and chemical surface diffusion coefficients for oxygen chemisor bed on W(110) have been investigated by means of Monte Carlo modeling, taking full advantage of the numerical power of a supermassive parall el supercomputer. The simulations were performed for a lattice gas wit h up to fourth nearest neighbor interactions. These were chosen to rep roduce the experimentally determined O/W(110) phase diagram for theta less than or equal to 0.5 which shows a dominating p(2 x 1) phase arou nd half coverage. It was found that the p(2 x 1) ordering strongly inf luences surface diffusion at low temperatures. The tracer diffusion co efficient shows a strong minimum slightly below half coverage and a ve ry small maximum above half coverage. This behavior is attributed to t he change of the dominating lattice defects of the p(2 x 1) phase from vacancies in filled rows below theta = 0.5 to interstitials in empty rows above theta = 0.5. For the p(2 x 1) ordered lattice gas phase the chemical diffusion coefficient exhibits a strong maximum which become s more pronounced as the temperature is lowered. This is attributed to the behavior of mean square fluctuations <(delta N)(2) >/< N >, i.e. the inverse of the thermodynamic factor, which dominates the chemical diffusion coefficient under these circumstances. The p(2 x 1) ordering causes the surface diffusion to become highly anisotropic. Attempts t o simulate the increase in activation energy with coverage found exper imentally showed that this was not possible without invoking adsorbate induced changes in the saddle point energies for diffusion. (C) 1997 Elsevier Science B.V.