Diffusion anisotropy of oxygen and of tungsten on the tungsten (211) plane

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Abstract
The diffusion of oxygen and also of W on W(211) has been measured along (∥) and across (⊥) channels which exist on this plane. For W E ∥ =13.2 kcal, D 0(∥)=7.1×10− 7 cm2 s− 1; E ⊥ =6.2 kcal, D 0(⊥) =1.8×10− 9 cm2 s− 1. The cross channel values suggest diffusion by displacement of a channel wall atom; this is also supported by prediffusive flip–flop measurements, which show almost the same activation energy and frequency. For O/W (211) complicated low activation energy, low D 0 regimes are seen at temperatures below ‘‘normal’’ diffusion and are tentatively attributed to low probability cooperative motions of substrate and adsorbate atoms. In the normal regimes E ∥ ≊E ⊥ but D 0(∥)>D 0(⊥). Activation energies are very coverage dependent, and this can be accounted for semiquantitatively in terms of repulsive nearest neighbor and attractive next nearest neighbor interactions in a channel, as well as attractive interactions between atoms in equivalent positions in adjacent channels, if the energies used by Wang and Lu to account for the phase diagram are used. With these assumptions values of E ∥ ≂E ⊥ =10–14 kcal at θ=0 are deduced. In addition to diffusion prediffusive flip–flop is also seen. This is activated on the clean W surface but shows virtually no activation for oxygen. Anisotropies in mean square fluctuations are seen not only in the diffusion but also in the flip–flop temperature ranges, suggesting that flip–flop may be a cooperative long wavelength process, akin to phonon modes of the adsorbate, in which the displacements of more than a single atom from the stablest to less stable binding sites and back occur in phase.