Low-energy electron diffraction analysis of the buried-heteroatom type Pd(100)-p(2×2)-p4g-Al surface

The structure of the p(2×2)-p4g ordered bimetallic phase formed on aluminum-deposited Pd(100) is determined by dynamical low-energy electron diffraction analysis. The best-fitted structure among 29 different model types examined consists of the Pd top layer clock-reconstructed to the hard sphere limit, and a buckled c(2×2)-PdAl second layer. The buried-heteroatom scheme, a new mechanism of in-plane atom movements, is proposed to interpret the yielded structure; Al atoms migrate beneath the topmost Pd layer and replace half of the second-layer Pd atoms to demand optimum coordination. Pd–Al bonding interaction forces top-layer Pd atoms to move systematically towards the buried Al. Half of the fourfold hollows of top layer are opened and the other half are deformed as a result of the movements. The top and second layers of the reconstructed surface can be regarded as a surface-limited compound of Pd3Al composition supported on the Pd(100) substrate, because a reduced d-state overlap between the top two layers and the Pd substrate is expected.