Electron microscopy and diffraction of synthetic corundum crystals

Abstract

Boules of aluminium oxide containing various impurities, grown by the Verneuil technique, have been examined by transmission electron microscopy. Most of the samples have dislocation densities ∼5×10⁵ cm⁻², excluding grain boundaries. This is two orders of magnitude higher than that for comparable undoped crystals. There is a higher incidence of low angle boundaries and the characteristics of these are described. The samples rarely contain internal precipitates; furthermore, precipitates in sapphire do not normally create significant numbers of dislocations. The interactions of a singularity with a basal network of dislocations with ⅓〈1120〈 Burgers vectors are analysed. The configuration is not consistent with the singularity being a 〈1010〉 dislocation, but it can be explained by assigning the singularity a ⅓〈1011〉 vector.