Structural analysis of the two c(2 × 2) phases of Na adsorbed on Al(100)

Abstract

We have studied the adsorption of Na on Al(100) at different substrate temperatures using low-energy electron diffraction (LEED) and density-functional theory (DFT) calculations. Below 180 K and above 260 K well-ordered c(2 × 2) overlayers develop which are completely different in structure. As determined by the LEED intensity analysis the Na atoms in the phase formed at low temperature occupy fourfold hollow sites where the nearest-neighbour NaAl bond length is 3.27 ± 0.01 A ̊ . The Na atoms in the phase formed above 260 K occupy substitutional sites. For this structure the LEED intensity analysis shows a contraction of the topmost Al interlayer spacing of 9.1% ± 0.5% and a nearest-neighbour NaAl bond length of 3.07 ± 0.01 A ̊ . On heating, the low-temperature c(2 × 2) structure is irreversibly transformed into the room-temperature structure. The adsorption sites and accompanying substrate relaxations and/or reconstructions agree very closely with those predicted from DFT calculations.