gS = 99 and Σ = 41 Grain boundaries

The structure and crystallography of two special high-angle grain boundaries which have been observed in spinel are discussed in terms of coincident-site lattice concepts. The two grain boundaries correspond to relatively high-Σ values; the value of Σ = 99 is higher than that which is usually considered to be significant for grain boundaries. It is proposed that it is not the actual value of Σ which is important for these interfaces but rather the fact that the misorientations which are responsible for producing the grain boundaries cause several pairs of low-index crystal lattice planes to be nearly parallel to one another at the interface. The relationship between this interpretation and the frequent observation of both low-Σ interfaces and asymmetric grain boundaries in this and other materials is emphasized. © 1988.