Observations of Dislocations and Surface Features in Corundum Crystals by Electron Transmission Microscopy

Crystals of corundum, grown by the flame-fusion method, have been chemically thinned and examined by electron transmission microscopy. Some specimens were etch-pitted after thinning, and the relationship between etch pits and dislocations was investigated. Within subgrains there is a close agreement between dislocation and etch pit densities; this agreement breaks down along many boundaries. The etching behavior of crystals containing Cr₂O₃, TiO₂ and other impurities has been examined. Etch-tunneling along grown-in dislocations is commonly observed in the impurity-doped materials: it is particularly pronounced in ruby. In heat-treated crystals containing TiO₂, local differences in solid solubility can lead to the formation of etch hillocks. Thermally etched surfaces have also been characterized, and gas atmospheres are shown to affect the surface topology. Evaporation experiments in the electron microscope are reported.