ATOMIC-STRUCTURE OF SIGMA-7(01(1)OVER-BAR2) SYMMETRICAL TILT GRAIN-BOUNDARIES IN ALPHA-AL2O3

High resolution transmission electron microscopy is used to study the atomic structure of a Sigma 7 grain boundary (Sigma is equal to the reciprocal of the fraction of lattice sites in coincidence) in Al2O3. This grain boundary in Al2O3 has misorientation [0 (1) over bar 11]180 degrees. The grain boundary plane is (01 (1) over bar 2)(1) parallel to(01 (1) over bar 2)(2). Four possible unrelaxed grain boundary structures can be constructed from the constrained coincidence-site-lattice (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 Sigma 7 grain boundary was identified. The Burgers vector of this dislocation is 1/14[05 (5) over bar 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 alpha-domain. There are no unsaturated Al-O bonds in the boundary core of the alpha domain, but the Al-O bonds are slightly elongated. Coexistence of two interfacial domains, alpha and beta, is observed. The beta domain has the same atomic structure as that of alpha 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 Sigma 7(01 (1) over bar 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 Sigma 7(01 (1) over bar 2) grain boundary always appear as a pair which resembles a stacking fault in the bulk of the crystal.