Complex metallic alloys with large unit cells can be formed in Al-transition metal systems. Some of them are structurally related to quasicrystals, with similar local order. The Al-Cu phase diagram contains two such phases, one being the γ-Al4Cu9, which is an electronic compound stabilized by a Hume-Rothery effect like Al-based quasicrystals. We report on the formation of this complex metallic alloy upon annealing of an Al thin film deposited on the Cu(111) surface.
The growth of Al on Cu(111) surface has been investigated for a coverage (θ) ranging from 0.6 monolayer (ML) up to 35 ML, deposited at room temperature. Surface alloy formation is observed in situ during the annealing by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED) and photoemission spectroscopies (XPS and UPS).
At submonolayer coverage, islands with monoatomic height attach preferentially at step edges. A layer-by-layer growth is observed up to a critical coverage of θc~3ML, then 3D islands are formed. The LEED patterns reveal an epitaxial growth for θ≤ θc, despite a rather large lattice mismatch (aAl/aCu ~11 %). For θc ≤ θ ≤ 12 ML, the LEED shows a (2x2) reconstruction. Then for θ > 12 ML, a (1x1) pattern is recovered, with reciprocal lattice dimensions consistent with Al(111). The 3D growth of Al(111) islands can be observed with the STM, together with a high density of screw dislocations.
Upon annealing, Cu diffusion is activated and different phase transformations are observed at the surface, in a sequence that depends on the initial film thickness. The various structures observed by LEED and atomically resolved STM images are (i) a (2x2) reconstruction on a solid solution of Cu in Al, (ii) the Al2Cu tetragonal phase, (iii) the Al4Cu9 gamma-brass phase and finally (iv) a (√30x√30)R30° reconstruction on the (111) surface of a solid solution of Al in Cu. The two compounds have interfacial relationships with the substrate, resulting in three-fold twinning of nanocrystalline domains separated by grain boundaries.
The European network of Excellence on 'Complex Metallic Alloys', contract NMP3-CT-2005-500145, is acknowledged for its financial support.
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