Adhesion, atomic structure, and bonding at the (formula presented) interface: A first principles study

We have performed a series of ab initio calculations to determine the atomic structure, ideal work of adhesion (formula presented) and bonding character of the (formula presented) 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 (formula presented) 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 (formula presented) [local density approximation (LDA)] and 1.06 (formula presented) [generalized gradient approximation (GGA)] for the optimal Al-terminated structure, which are in good agreement with the experimental value of 1.13 (formula presented) 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 (formula presented) (LDA) and 9.73 (formula presented) (GGA). Although cleavage preferentially occurs at the interface for the Atermination, strong bonding at the O-terminated interface favors cleavage within the metal.