Atomistic Simulation of Surface Structures and Energies of Alkaline Earth Hexa-Aluminates
Date
2004-04
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
New York State College of Ceramics at Alfred University. Kazuo Inamori School of Engineering.
Abstract
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.
Description
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
Type
Thesis