Atomistic Simulation of Surface Structures and Energies of Alkaline Earth Hexa-Aluminates

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New York State College of Ceramics at Alfred University. Kazuo Inamori School of Engineering.
Atomistic computer simulation techniques were used to model the surfaces of five crystal structures in the hexa-aluminate family. Calcium, strontium, and the theoretical barium hexa-alumunates with the magnetoplumbite structure and two barium β-alumina crystals were investigated. It was found in all the crystal structures that the [001] surface had the lowest surface energy. Each system modeled resulted in plate-like crystals. Coordination of the exposed surface ions and the density of ions on the surface was found to be the dominant factor in the energy of the surfaces. The relaxation of ions to positions above the original surface (thus a low coordination with the other ions) was found to increase the energy of the surfaces for all the crystal systems. The surface energy increased with increasing divalent cation size in the calcium and strontium magnetoplumbite surfaces. The theoretical barium magnetoplumbite had the lowest calculated overall surface energy value of the magnetoplumbite crystals. The lower values for barium magnetoplumbite were due to the rumpling of the oxygen layers above and below the mirror plane in the bulk crystal structure. The relaxed positions and the number of exposed divalent cations also had a large influence on the surface energy for a given orientation in these structures. The location of the Ba2+ ion plays only a minor role in the lowering of surface energies in the β-aluminas. The coordination of the surface ions, mostly the number of dangling O2- ions, and the reduction of polarization in the surface structure have the greatest impact on the surface energy of a given orientation. It was concluded that surface energy stabilization of barium magnetoplumbite was not possible. The overall energy reduction caused by the formation of the two barium β- alumina crystals cannot be overcome by the lower surface energy of adopting the theoretical magnetoplumbite structure. The effect of isovalent cation substitution defects on the stability of the theorectical barium magnetoplumbite was also investigated. Calculation of isovalent substitution defects of the divalent cations and the aluminum ion on the surface was also examined. It was found that the addition of such surface defects did not stabilize the magnetoplumbite structure.
Advisory committee members: Robert Condrate, Paul Johnson, Doreen Edwards. Dissertation completed in partial fulfillment of the requirements for the degree of Doctorate of Philosophy in Ceramics at the Kazuo Inamori School of Engineering, New York State College of Ceramics at Alfred University