Stoichiometric reconstruction of the Al ₂ O ₃ (0001) surface

Macroscopic properties of materials stem from fundamental atomic-scale details, yet for insulators, resolving surface structures remains a challenge. We imaged the basal (0001) plane of α–aluminum oxide (α-Al 2 O 3 ) using noncontact atomic force microscopy with an atomically defined tip apex. The surface formed a complex ( 31 × 31 ) R ±9° reconstruction. The lateral positions of the individual oxygen and aluminum surface atoms come directly from experiment; we determined with computational modeling how these connect to the underlying crystal bulk. Before the restructuring, the surface Al atoms assume an unfavorable, threefold planar coordination; the reconstruction allows a rehybridization with subsurface O that leads to a substantial energy gain. The reconstructed surface remains stoichiometric, Al 2 O 3 .