Atomic View of Surface Self-Diffusion: Tungsten on Tungsten

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), ${\text{D=3×10}}^{\text{−2}}\exp \text{(−22 000/RT)}{\mathrm{cm}}^2/\sec$ ; (321), ${\text{1×10}}^{\text{−3}}\exp \text{(−20 000/RT)}$ ; (211), ${\text{2×10}}^{\text{−7}}\exp \text{(−13 000/RT)}$ . Differences in diffusion on the (211) and (321), planes of very similar structure, suggest a weakening of interatomic forces at lattice edges.