THEORETICAL IDENTIFICATION OF A (2X2) COMPOSITE DOUBLE-LAYER ORDERED SURFACE ALLOY OF NA ON AL(111)

We performed density-functional theory calculations to investigate the (2 X 2) surface structure of Na on Al(111). This structure forms at r oom temperature and contains two Na atoms in the surface unit cell. Ex periments have surprisingly indicated a strong intermixing of Na and A l in the surface region. From the calculations we predict that the sta ble geometry is an unusual composite double layer ordered surface allo y where the first Na atom in the surface unit cell occupies a substitu tional site and the second Na atom and an Al atom are situated in thre efold fcc- and hcp-hollow sites respectively.