We report an ab initio thermodynamic analysis of the a-Al2O3 (1-102) surface aimed at understanding the experimentally observed terminations over a range of surface preparation conditions, as well as a novel stoichiometric model for the (2x1) surface reconstruction observed after high temperature annealing. As temperature is increased under both ultra-high vacuum and ambient hydrated conditions, the predicted minimum energy structural model goes through the same series of changes: from the hydroxylated ``missing-Al'' surface model (or half layer model in which the topmost Al site of the stoichiometric surface has zero occupancy), to the hydroxylated stoichiometric model, to another hydroxylated missing-Al surface model with tetrahedral coordinated surface Al, and finally to the clean (1x1) stoichiometric model. These results are in agreement with observations of both missing-Al and bulk-like stoichiometries under wet conditions and in agreement with similar trends reported for isostructural hematite. However, we observe that the models with excess oxygen have a relatively higher surface free energy and distinct surface relaxations in the case of alumina as compared to hematite. At very high temperatures where oxygen defects are generated, we find that a novel stoichiometric, charge-neutral (2x1) structure becomes the most thermodynamically stable. This is consistent with the observation of a (2x1) electron diffraction pattern when the surface is annealed at 2000 K, while a (1x1) pattern persists at lower annealing temperatures. A general rule that emerges from our modeling results is that while the full phase space of hydrated and defective surfaces is expansive, model stoichiometries that can be made charge-neutral through either hydration or defects offer the greatest thermodynamic stability.
Citation: Physical Review B
Pub Type: Journals
interfaces, oxide surfaces, thermodynamic properties