Adhesion, atomic structure, and bonding at the Al(111)/α-Al2O3(0001) interface: A first principles study

We have performed a series of ab initio calculations to determine the atomic structure, ideal work of adhesion (W_ad ), and bonding character of the Al(111)/α-Al₂O₃(0001) interface. Six candidate interface geometries were considered, including Al and O terminations of the oxide. Minimization of the Hellman-Feynman forces resulted in substantial changes to the atomic structure of the metal near the interface, wherein some atoms adopted positions consistent with a continuation of the oxide's Al-sublattice crystal structure across the interface. Consequently, the lowest-energy structures (i.e., having the largest W_ad ) are those that facilitate this ``oxide extension'' mechanism. By applying several methods of analysis we have thoroughly characterized the electronic structure and have determined that Al-O bonds constitute the primary interfacial bonding interaction. These bonds are very similar to the cation-anion bonds found in the oxide bulk and are mainly ionic, yet maintain a small amount of covalent character. In addition, there is evidence of metal-cation bonding at the optimal Al-terminated interface. Taking into account recent theoretical and experimental evidence suggesting an Al termination of the clean oxide surface, our calculations predict W_ad =1.36 J/m² [local density approximation (LDA)] and 1.06 J/m² [generalized gradient approximation (GGA)] for the optimal Al-terminated structure, which are in good agreement with the experimental value of 1.13 J/m² as scaled to 0 K. These values are approximately an order of magnitude smaller than what is found for the optimal O-terminated interface: 10.70 J/m² (LDA) and 9.73 J/m² (GGA). Although cleavage preferentially occurs at the interface for the Al termination, strong bonding at the O-terminated interface favors cleavage within the metal.