Structure and formation of the Al(1 0 0)-(5×5)R27°-Na phase: a LEED, DFT and HRCLS study

Adsorption of 0.2 ML Na on Al(1 0 0) at room temperature yields a disordered Al(1 0 0)-(1×1)-Na phase, which transforms reversibly to a well-ordered Al(1 0 0)-(5×5)R27°-Na phase on cooling below 250 K. Based on low energy electron diffraction (LEED) and high resolution core-level spectroscopy (HRCLS) measurements, and on ab initio calculations, it is concluded that the structure of the Al(1 0 0)-(5×5)R27°-Na phase consists of Na atoms occupying substitutional sites. The structural parameters obtained from LEED and density functional theory analyses are in quantitative agreement. Adsorption of 0.2 ML Na at 100 K yields an Al(1 0 0)-c(2×2)-Na island structure, which transforms irreversibly into the (5×5)R27°-Na structure by annealing above 190 K. The nature of the reversible and irreversible phase transformations to the (5×5)R27° structure is investigated by HRCLS and LEED.