The grain boundary segregation of Y and Zr in α-alumina, and the structural environment of the Y and Zr atoms, were investigated by using high-resolution STEM and EXAFS techniques. At low concentrations, the Y ions in the grain boundaries, were coordinated with - on average – four O atoms; at a distance of 0.230nm. This corresponded closely to the Y-O bond length in cubic Y2O3. The Zr ions were coordinated with five O atoms at a distance of 0.214nm. This was approximately the same as the average Zr-O bond length in monoclinic ZrO2. However, the Y-cation and Zr-cation next-nearest neighbor shells could not be clearly identified in the EXAFS radial distribution function. The results suggested that Y and Zr, at low concentrations, occupied grain-boundary sites having well-defined nearest-neighbor cation-O bond-lengths which were similar to those in the parent oxides; but with the next-nearest neighbor cation-cation distances varying considerably from site to site. Grain growth could cause the grain boundaries to become supersaturated with Y. In this case, both the Y-O nearest-neighbor coordination number and the ordering of Y with respect to Al ions beyond nearest-neighbor O were increased. This Y-Al distance was the same as that expected for the Y-Al distance when Y replaced Al while relaxing the Y-O distance to that in Y2O3. This was suggested to indicate that, as the Y concentration increased, the Y began to occupy near-boundary sites in planes on each side of the geometrical boundary. In these near-boundary planes, the nearest-neighbor ordering extended at least to nearest-neighbor cations. Nucleation of a YAG phase led to the depletion of these partially ordered layers.

Structure of Y and Zr Segregated Grain Boundaries in Alumina. C.M.Wang, G.S.Cargill, H.M.Chan, M.P.Harmer: Interface Science, 2000, 8[2-3], 243-55