Atomic View of Surface Self-Diffusion: Tungsten on Tungsten

TLDR

Surface diffusion of tungsten adatoms on low‑index tungsten planes has been directly observed for the first time using a field‑ion microscope. Diffusion coefficients were quantified from mean‑square displacement rates, yielding D = 3×10⁻² exp(−22 000/RT) cm²/s for (110), 1×10⁻³ exp(−20 000/RT) for (321), and 2×10⁻⁷ exp(−13 000/RT) for (211). At room temperature, mobility increases in the order (211) > (321) ≈ (110) > (310) ≈ (111); atoms are reflected at (110), (211), and (321) boundaries, favoring motion along atomic rows on (211) and (321), and the similar structures of (211) and (321) imply weakened interatomic forces at lattice edges.

Abstract

Surface diffusion of tungsten adatoms on several smooth, low-index planes of the tungsten lattice has for the first time been followed by direct observation of individual atoms in the field-ion microscope. Contrary to expectation, the mobility at room temperature is found to increase in the order (211) > (321) ∼ (110) > (310) ∼ (111). Migrating atoms are reflected at the boundaries of the (110), (211), and (321) planes; on the latter two, motion along atomic rows is favored over diffusion across lattice steps. From quantitative determinations of the rate of change of the mean-square displacement, diffusion coefficients are obtained as follows: (110), D=3×10−2exp(−22 000/RT)cm2/sec; (321), 1×10−3exp(−20 000/RT); (211), 2×10−7exp(−13 000/RT). Differences in diffusion on the (211) and (321), planes of very similar structure, suggest a weakening of interatomic forces at lattice edges.