Structure and stability of metastable transition metal clusters on W(011)

The structure of clusters of rhodium and palladium atoms formed on W(011) b y deposition below and above the adatom migration temperature, as well as u pon subsequent annealing of the films in the range T-an = 100-1000 K, is st udied by conventional and video low-energy electron diffraction (LEED) for various submonolayer coverages. Films deposited below the single-adatom mig ration temperature show a predominantly random distribution of adatoms over the adsorption sites with some signs of order, evidencing mobility anomalo us for single adatoms. Upon annealing, the structure of the films acquires long-range order and undergoes a set of irreversible phase transitions. Two types of adatom cluster are found: one-dimensional densely packed chains a ligned along the substrate [111] directions and two-dimensional densely pac ked islands. The parameters of the cluster structure, such as interatomic a nd interchain spacing, chain length and temperature range of stability, are derived from LEED intensity profiles. The cluster structures and phase tra nsitions are discussed in terms of indirect interactions between adatoms. T he work function changes are measured with the contact potential difference method as a function of coverage and annealing temperature. The correlatio n between work function changes and phase transitions is determined. (C) 19 98 Elsevier Science B.V. All rights reserved.