Structural transitions in ultra-thin nickel films on Rh{111}

Abstract

For the first three monolayers, ultra-thin nickel films on Rh{111} grow in a close to layer-by-layer fashion. A nickel monolayer film is “pseudomorphic” with the underlying substrate. An automated tensor LEED I( V) analysis indicates the clean Rh{111} surface to be laterally unreconstructed with a small amplitude damped oscillatory relaxation of the outermost two atomic layer spacings of Δd z 12 = −2.7 ± 1.4% and Δd z 23 = + 0.4 ± 1.4%, resulting in a Pendry R-factor of 0.16. For the nickel adsorption site is found to be the fcc threefold hollow, with a nickel to rhodium interlayer spacing of 2.06 Å, slightly shorter than the sum of metallic hard sphere radii, with the outermost rhodium interlayer spacing relaxed to Δd z 12 = −1.4 ± 3.2%. The minimum Pendry R-factor of 0.28 was obtained. Spot profile analysis indicated significant beam broadening upon initiation of second layer growth with asymmetry at high parallel momentum transfer consistent with formation of small domains of incommensurate nickel with a nearest-neighbour separation intermediate between rhodium and nickel, finally yielding a complex pattern due to multiple scattering between the pseudomorphic p(1 × 1) slab and the incommensurate film for thicknesses beyond 1 ML. Model pair potential calculations indicate a layer dependent gradual relaxation of the nickelickel interlayer spacing towards that of bulk nickel in the outermost three atomic layers. Alloying occurs on an experimental time scale at temperatures above 600 K leading to formation of a substitutionally disordered RhNi alloy of variable composition.