Surface diffusion of Pd and Au on W single crystal planes: I. Spreading behaviour of Pd and Au layers

In two subsequent publications we report on the diffusion behaviour of Pd and Au on W(110) and stepped W(110) vicinals. The first part deals with the concentration profiles obtained by Pd(Au) spreading from linear sources and recorded by a scanning Auger microscope (SAM). The diffusion experiments were carried out in the temperature range 600–900°C and revealed the formation of Pd(Au) layers of distinct coverages ($\text{1}\text{2}$,1,$\text{3}\text{2}$, 2 monolayers) during the spreading process. Whereas on the W(110) plane essentially a Pd monolayer phase proceeds with a sharp boundary and a square root of time dependence, the concentration profiles observed on the stepped W(10 9 0), W(650) and W(750) surfaces exhibit somewhat rounded shapes. The spreading behaviour can be qualitatively rationalized by considering the gain in binding energy upon the formation of Pd-W bonds and strong lateral Pd(Au)-Pd(Au) attractive interactions. The activation energy for Pd diffusion deduced from the temperature dependence of the spreading rate amounts to 40–45 kcal/mole. In the case of the stepped surfaces this energy applies to both spreading directions parallel and perpendicular to the steps. The preexponential factor however differs by one order of magnitude favouring diffusion along the steps. Combined diffusion experiments involving Pd and Au diffusion simultaneously reveal that Au is the faster moving species. The following paper reports the eminent role of step edges as diffusion paths and dwells on the anisotropic diffusion behaviour caused by regular step structures.