Defects in spinel crystals grown by the verneuil process

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Abstract
Sections from a Verneuil grown crystal of the spinel MgAl2O4 were chemically thinned in ortho-phosphoric acid and examined by transmission electron microscopy. Dislocations in sections from the ‘as-grown’ boule and in specimens deformed at 1300°c are composed of two partial dislocations separated by a ribbon of stacking fault. Isolated dislocations within sub-grains have the appearance of super-lattice dislocations, as observed in ordered intermetallic compounds, and are separated by a fault in the cation lattice alone which has a fault vector of the type a4. A possible separation of dislocations into four partials when lying in {111} planes is not observed. Large extended defects nearly parallel to {100} planes and having fault vectors of the type a4 at 45° to the fault plane were observed in low density within sub-grains. They are interpreted as 'anti-phase boundaries' since they represent faults in cation stacking alone, frequently have folded or curved configurations and probably form by the impingement of 'domains' in which the partial filling of interstices by the cations is nucleated differently. The probable ionic configuration of such boundaries is discussed.

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