Simulations of hydrogen diffusion on BCC metal (110) surfaces; coverage and temperature dependence

Abstract

We study the coverage and temperature dependences of the tracer diffusion coefficient for a chemisorbed layer of interacting hydrogen atoms on a model of a bcc metal (110) surface. The surface is rigid, and the short bridge barriers between adjacent chemisorption wells are sufficiently low that hydrogen atoms diffuse actively across the surface, on a time scale compatible with our molecular dynamics simulations. We deduce the coverage dependence of the effective activation barrier and the prefactor. We also examine, as a function of coverage, the percentage of jumps from singly occupied to either empty or occupied chemisorption wells, and from doubly occupied wells to empty or singly occupied wells. Although the effective activation barrier deduced from the numerical data exhibits a weak dependence on coverage, as found in data on H diffusion on the W(110) surface, the percentage of jumps of the types mentioned varies dramatically. The prefactor in the diffusion constant extracted from the simulations agrees well with elementary expectations for the rigid surface, but is much larger than that found experimentally. Finally, the low coverage tracer diffusivity is found to be appreciably anisotropic. The anisotropy decreases substantially as coverage increases.