Metallic Character of the Al₂O₃(0001)-(√31 × √31)R ± 9° Surface Reconstruction

The basal (0001) surface plane of α-Al₂O₃ has been extensively studied both by experimental techniques and applications of theory. Although this surface is not easily created upon cleavage, it grows in during aluminum oxidation and has been shown to be one of the lowest energy surfaces of alumina. Upon heating under vacuum, this (0001) surface of α-Al₂O₃ undergoes a series of reconstructions. The final structure obtained at ∼1700 K is an air-stable termination known as the (√31 × √31)R ± 9° reconstruction. Alternatively, this reconstructed surface can also be obtained at lower temperatures by aluminum deposition on a clean Al₂O₃ (1 × 1) basal plane surface. Applying density functional theory within the generalized gradient approximation to a model surface, we show that the transformation from the (1 × 1) termination to (√31 × √31)R ± 9° is accompanied by a dramatic change in the band gap associated with the ceramic's surface. Specifically, the (1 × 1) termination is insulating for both surface and deeper "bulk-like" atoms, while the (√31 × √31)R ± 9° termination results in a metallic surface aluminum coating that behaves very much like an aluminum (111) surface. Although some experimental evidence for the (√31 × √31) R ± 9° metallic surface states exists, it has generally been ignored, and both the (1 × 1) and (√31 × √31)R ± 9° terminations are commonly treated as insulating. The metallic surface character of the reconstructed basal plane of α-Al₂O₃ may present interesting implications for future applications of heat-treated alumina surfaces.