The structure of the [formula omitted] superlattice phase on (111) α-Cu−16 at % Al; A leed intensity analysis

Abstract

A dynamical LEED analysis is presented of the ( 3 × 3 ) R30° superlattice on the (111) surface of an α-CuAl alloy single crystal with a bulk Al concentration of 16 at %. This surface ordering contrasts with the lack of long-range order in the bulk. In the ordered surface the Al atoms are found to be subtitutionally arranged within the topmost layer with the topmost layer with (√3×√3)R30° symmetry. No evidence for the ordering of Al in deeper layers is detected by this analysis. The fcc-(111) stacking sequence is maintained at the surface, and the surface atoms occupy positions within ±0.05 A ̊ of bulk Cu lattice positions, i.e. no appreciable top-layer spacing change and no mixed-layer buckling are detected.