The influence of surface reconstruction on the collective motion of adsorbed atoms

Abstract

The effect of adsorbate-induced surface reconstruction on the motion of adsorbed particles is analyzed by means of Monte Carlo modeling. The reconstruction is assumed to be the order–disorder phase transition that is described by the two-position model introduced by A.V. Myshlyavtsev et al. [J. Chem. Phys. 92 (1990) 3909]. The lattice gas model is used to mimic the hydrogen adsorption on the (001) face of tungsten [H/W(001)], which presents a c(2×2) structure below the critical temperature, T c. The phase diagram of the real system is reproduced only at low coverages ( θ<0.25). One of the principal features of the model is that the critical temperature can either increase or decrease with coverage, depending on the relation of the parameters which describe the system. In addition, different ordered phases, such as c(2×2) and (2×1) can be present according to the characteristics of the metal–metal, J NN, adsorbate–adsorbate, ε NN, and adsorbate–metal, λ NN, interactions. Utilizing Monte Carlo simulations we characterize how the behavior of the chemical, D, the jump, D J, and the tracer, D *, diffusion coefficients depend on the different ordered phases predicted by the model. In order to calculate the chemical diffusion coefficient we apply the fluctuation and Kubo–Green methods. Surface diffusion is strongly anisotropic for the (2×1) ordered phase, indicating that it is facilitated in one direction and reduced in another.