Atomic resolution electron microscopy of NiO grain boudaries

Atomic-scale detail in 〈100〉 tilt grain boundaries (GB) of high-purity bicrystals of NiO has been studied with a 400 kV high-resolution electron microscope. Crystallinity is always maintained right up to the grain boundary and there is a strong tendency for coherent matching of atomic planes across the GB in symmetric and asymmetric GBs, and for all misorientations. Low-angle boundaries are characterized by their primary dislocation structure which is, depending on the GB plane, composed of a〈100〉 and 1 2a〈110〉 type dislocations. High-angle boundaries, even those close to σ = 5, often take on asymmetric configurations with the boundary close to a (100) plane in one of the crystals. The observation of symmetric as well as highly asymmetric facets at high angles suggests that both configurations correspond to local minima in the free energy. These results are discussed in terms of current models of tilt GBs of NiO.