Structural imperfection of beta-Si3N4 crystals associated with nucleation

Some kinds of dense strain contrast in beta -Si3N4 have been considered as dislocation tangles formed during mechanical milling. However, our present electron microscopic analyses indicate that the strain contrast with radial contours is not in the least attributed to the dislocations but rather to the subgrain boundaries associated with the initial growth of the crystal, i.e. the nucleation. A number of growth variants, independently nucleated o n the surface of a single Si-N cluster, coarsen and form closure mistakes w hen they approach, giving rise to the formation of subgrain boundaries exte nded from the core. The subgrain boundaries are found to locate only on {10 0}, {110} and {101} planes of the beta -Si3N4 crystal structure.