Direct Observation of Multilayer Adsorption on Alumina Grain Boundaries

Grain-boundary films 0.6 nm in size have been observed on the grain boundaries of neodymia (Nd₂O₃)-doped alumina (α-Al₂O₃) sintered at 1800°C. Direct observation by high-angle annular dark-field imaging in the aberration-corrected scanning transmission electron microscope shows that this type of grain-boundary structure is the result of multilayer adsorption. Neodymium cations adsorb onto the faces of each of the two grains that comprise the grain boundary by substituting for aluminum cations. The positions of these cations are slightly distorted relative to the perfect lattice, and a third atomic layer in the core of the grain-boundary resides between these two layers. The measurements also confirm that the thickness deduced from high-resolution transmission electron microscopy lattice images are accurate.