Complementary but independent medium-energy and low-energy ion scattering studies of the (0001) surfaces of V 2 O 3 films grown on Pd(111), Au(111) and Cu 3 Au(100) reveal a reconstructed full O 3 -layer termination creating a VO 2 surface trilayer. This structure is fully consistent with previous calculations based on thermodynamic equilibrium at the surface during growth, but contrasts with previous suggestions that the surface termination comprises a complete monolayer of vanadyl (V=O) species.

Density Functional Theory (DFT) calculations have been applied to investigate the known difference in behaviour of S adsorption on Cu(100) and Ni(100). Both surfaces form a 0.25 ML (2x2) adsorption phase, but while at higher coverage a 0.5 ML c(2x2) phase forms on Ni(100), on Cu(100) only a reconstructed 0.47 ML (17x17)R14°s tructure occurs. Calculations of the energy, structure, and surface stress of (2x2) and c(2x2) phases on both substrates show there is an energy advantage on both surfaces to form the higher coverage phase, but that both surfaces show local surface strain around the S atoms in the (2x2) phase, a phenomenon previously investigated only on Cu(100).More than forty different structural models of the Cu(100)(17x17)R14°-S phase have been investigated. The pseudo-(100)c(2x2) structure previously proposed, containing 16Cu adatoms per unit mesh in the reconstructed layer, is found to be less energetically favourable than many other possible structures, even after taking account of local structural relaxations. Significantly more favourable is a structure with 12 Cu adatoms per (17x17)R14°unit mesh, previously proposed on the basis of scanning tunnelling microscopy (STM), and found to yield simulated STM images in good agreement with experiment. This model has all S atoms in local 4-fold coordinated hollows relative to the Cu atoms below, half being located above Cu adatoms with the remainder lying above the underlying outermost substrate layer. However, an alternative model with only 4 Cu adatoms and with half the S atoms at 3-fold coordinated sites on the periphery of the Cu adatom cluster, has an even lower energy and gives simulated STM images in excellent agreement with experiment.

Medium energy ion scattering (MEIS), using 100 keV H