Stoichiometric reconstruction of the Al₂O₃(0001) surface

Macroscopic properties of materials stem from fundamental atomic-scale details, yet for insulators, resolving surface structures remains a challenge. The basal (0001) plane of α-Al₂O₃ was imaged with noncontact atomic force microscopy with an atomically-defined tip apex. The surface forms a complex (√31 × √31)R±9 reconstruction. The lateral positions of the individual O and Al surface atoms come directly from experiment; how these connect to the underlying crystal bulk was determined based on computational modeling. 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, Al₂O₃.