Confirmation of an exception to the ‘‘general rule’’ of surface relaxations

Low-energy electron diffraction (LEED) I–V spectra for Al(111) have been remeasured and analyzed to resolve a conflict concerning interlayer relaxations that are induced by the creation of the surface. Previous measurements of the Al(111) surface relaxations generally report a small expansion of the first interplanar spacing with respect to the bulk spacing. Several recent theories not only predict a contraction, but also that an expansion is not even possible with the formalism used. A large database of I–V measurements at room temperature and at 160 K has been compared to a very extensive set of calculated spectra. It was determined that the surface was expanded, Δd12=+1.7±0.3%, Δd23=+0.5±0.7%. Importantly, the analysis has also identified surface-enhanced thermal motions and no temperature dependence for the static interplanar relaxation within an accuracy of 0.007 Å.