STRUCTURAL-ANALYSIS OF THE 2 C(2X2) PHASES OF NA ADSORBED ON AL(100)

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 wel l-ordered c(2 x 2) overlayers develop which are completely different i n structure. As determined by the LEED intensity analysis the Na atoms in the phase formed at low temperature occupy fourfold hollow sites w here the nearest-neighbour Na-Al bond length is 3.27 +/- 0.01 angstrom . The Na atoms in the phase formed above 260 K occupy substitutional s ites. For this structure the LEED intensity analysis shows a contracti on of the topmost Al interlayer spacing of 9.1% +/- 0.5% and a nearest -neighbour Na-Al bond length of 3.07 +/- 0.01 ansgstrom. On heating, t he low-temperature c(2 x 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.