Rotation domains in α-silicon nitride

A domain structure is abundant in as-grown chemically vapour-deposited α -Si3N4 crystals. Transmission electron microscopy has been employed to examine the nature of the domains and domain boundaries. Selected-area electron diffraction patterns taken from either side of the domain walls are essentially the same, but sometimes different convergent-beam electron diffraction patterns and image contrast are observed. They are not simple stacking faults or antiphase domain boundaries or inversion boundaries. Analysis shows that they are 60° rotation domains which can occur because the structure has threefold rotational symmetry instead of sixfold along the c axis of the trigonal unit cell. The domain boundaries often lie on low-index planes but can be curved and occur on any plane. Sometimes they give a stacking-fault-like contrast.