Past exam of the chemistry course of the year 3 of the natural sciences course of the University of Cambridge / 2022 / Paper 4 / 65 / d / ii / Solution

Close-packed layers occupy one of three lateral registries, conventionally A, B and C. Hexagonal close packing has ABAB stacking, while face-centered cubic stacking is ABCABC. On a bulk-terminated (111) terrace the two kinds of threefold hollow differ by subsurface registry: an hcp hollow places an atom above a second-layer atom, while an fcc hollow continues the ABC sequence.

Compressing the top Au layer makes its registry drift relative to the unreconstructed layers. Parts of the cell occupy fcc hollows, others hcp hollows, and transition strips pass through less favorable bridge-like positions. Those registry changes are associated with partial-dislocation/domain boundaries. Varying local coordination and repulsion from the substrate cause vertical relaxation, producing the observed corrugation rather than an entirely flat shifted sheet.