On the point group of stoichiometric spinel, MgO·Al2O3

The Structure of Surface Steps on Low-Index Planes of Oxides

Surface steps have been observed on low-index planes of alpha-alumina and spinel. The steps were produced by annealing both bulk and perforated thin foil single crystals. A comparative study of bright-field TEM and dark-field REM of the same crystals makes it possible to construct models for the surface structure. Many faceted steps can be observed on the (0001), {0112}, {1010} and {1120} surfaces of alpha-alumina and on the {100} surface of spinel. A detailed study of step faceting and step facet movement during reheating is presented.

Applications of convergent beam electron diffraction in materials science

The methods by which Convergent Beam Electron Diffraction (CBED) is used to solve practical problems in materials science are reviewed. Particular emphasis is placed on those applications where a consensus has developed on the potential difficulties, but these problems have never been explicitly treated in the literature. Point-group determination using CBED is a standard technique, but there are several different experimental methods which are outlined here; the different approaches and the information obtained are compared. These point-symmetry determinations are shown to rely heavily on elimating the large number of potential artefacts: the possible sources of symmetry-breaking are summarised, and their salient features are identified. This should give a prescription for demonstrating whether symmetry-breaking arises through crystal structure, defects, strain, or specimen morphology. In space-group determinations the practical approach is now fairly standard, but the article draws attention to some anomalies. Strain and lattice parameter determinations are also standard, but a consensus is beginning to emerge that the interpretive difficulties involved in allowing for thin film relaxation are extremely severe: a few of these difficulties are outlined. Finally, the article also addresses some of the techniques which have been used to extract structural or compositional information from CBED: the experimental approach depends on the specific problem, and the article aims to provide some examples to illustrate what is possible.