Model for the surface reconstruction phase transition of clean and hydrogen-adsorbed W(001) surfaces: Low-coverage solution

We summarize the theoretical indications and experimental evidence justifying a shortrange structural model for the clean and H-adsorbed W(001) surface reconstruction phase transition. A detailed study of the effect of the adsorbate treated as a lattice gas within the context of such a model is presented. The results, though based on mean field theory, are accurate in the low-cover-age region within the context of the model and succeed in reproducing the low-coverage features of the phase diagram, thus enabling a determination of some of the interaction constants in the model. The energy difference between distortions along the 〈11〉 and 〈10〉 directions is 3.0 × 10^?3 eV per atom. The force exerted on a surface W atom by an adsorbed H atom in an adjacent bridge site is $\text{0.04}\text{eV}\text{u}{}_0$, where u₀ is the magnitude of the clean surface W atom displacement at low temperature.