Atomic structure of Σ7(01(formula presented)2) symmetrical tilt grain boundaries in α-Al₂O₃

High resolution transmission electron microscopy is used to study the atomic structure of a Σ7 grain boundary (Σ is equal to the reciprocal of the fraction of lattice sites in coincidence) in Al₂O₃. This grain boundary in Al₂O₃ has misorientation [0(formula presented)11]/180°. The grain boundary plane is (01(formula presented)2)1∥(01(formula presented)2)₂. Four possible unrelaxed grain boundary structures can be constructed from the constrained coincidence-sitelattice (CCSL) patterns. The atomic structure of the boundary is determined from a through-focal series of high resolution images and computer image simulation. A DSC dislocation in the Σ7 grain boundary was identified. The Burgers vector of this dislocation is 1/14[05(formula presented)2] and it is a pure edge dislocation. Most of the boundary has a structure comprising oxygen terminating layers with an identical width of 0·135 nm, known as the α-domain. There are no unsaturated Al-O bonds in the boundary core of the a domain, but the Al-O bonds are slightly elongated. Coexistence of two interfacial domains, α and β, is observed. The domain has the same atomic structure as that of α domain, but the width of the interfacial layers is 0·17 nm. These two interfacial structures show different contrast in the high resolution images and are separated by partial-DSC dislocations in the Σ7(01(formula presented)2) grain boundary. The Burgers vector of the partial-DSC dislocations can be determined with reference to the interfacial structures, CCSL patterns, step vector and close failure of the Burgers circuit. The partial-DSC dislocations in the Σ7(01(formula presented)2) grain boundary always appear as a pair which resembles a stacking fault in the bulk of the crystal.