It was noted that an idealized crystal structure for sapphire (α-Al2O3) (perfect

oxygen hcp packing, flat cation planes perpendicular to [00•1]) had been used by

Kronberg (1957) and many others over the past 50 years in order to describe basal

slip and basal twinning at the atomic level. However, it had been recognized over a

decade ago that the actual structure of sapphire allows much simpler atomic

mechanisms to be postulated for basal slip and basal twinning. These models were

supported by convincing arguments derived from chemical and structural

considerations. Recently, a climb-dissociated basal dislocation in the boundary of a

manufactured bicrystal was observed by atomic resolution transmission electron

microscopy. The images were interpreted as indicating non-stoichiometric charged

dislocation cores and it was inferred that, during dislocation motion on the basal

plane, the basal dislocations had to move according to a variant of Kronberg's

mechanism. This conclusion was difficult to reconcile, with (i) models based upon

the actual structure, (ii) weak-beam TEM images, which contradicted important

implications of this variant of Kronberg's model, (iii) implications concerning

dislocation motion in ionic materials, and (iv) the possibility that interface

dislocations can be subject to entirely different constraints than apply to gliding

lattice dislocations.

Do Moving Basal Dislocations in Sapphire (α-Al2O3) Have Non-Stoichiometric

Cores? K.P.D.Lagerlöf, J.Castaing, A.H.Heuer: Philosophical Magazine, 2009,

89[5], 489-99