Arrangement of multiple structural units in a [0001] Σ49 tilt grain boundary in ZnO

Arrangement of structural units in a ZnO [0001] Σ=49 symmetric tilt boundary was investigated using a combination of high-resolution electron microscopy and atomistic calculation in detail. The boundary is found to be described by the combination of two different dislocationlike structural units and a bulklike structural unit. The two dislocationlike units were very similar to structural units found in a Σ=7 boundary. One of the dislocationlike units contains threefold-coordinated atoms and the other one has fivefold-coordinated atoms in contrast to fourfold coordination in ZnO bulk. Interestingly, the Σ=49 boundary dominantly consists of a straight alternate array of dislocationlike units and a bulklike unit. A zigzag array of the units partly appeared, which can be related to the straight array via atom flipping at the boundary core. It is considered that the alternate array of dislocationlike units effectively relaxes local strain in the vicinity and, hence, minimizes the boundary energy.