The crystal structure, crystal chemistry, and defects of this material were reviewed, with emphasis being placed upon the structure of the  phase. High concentrations of vacancies which developed in connection with the release of lattice strain energy and with the replacement of N by O, in order to satisfy electroneutrality requirements, appeared to be a normal structural feature of this phase. It was deduced that the concentrations and configurations of O atoms and vacancies in the  phase were therefore likely to depend upon the specific fabrication process, environment, availability of O, reaction rate, annealing time and cooling rate. It was noted that -phase grains could be distinguished from -phase grains, in transmission electron microscopic images, by the presence of vacancy clusters; which seemed to be unique to the  phase. Wide variations in the unit-cell dimensions and densities of -phase nitride were attributed to differences in the concentrations and configurations of these lattice defects and lattice O concentrations. It was suggested that superlattices, nano-cracks and strain centers which had been reported to have been detected in this phase were related to differences in the configurations of the vacancy clusters. It was thought that any possible contribution of dislocation movement to the high-temperature deformation behavior of the nitride was likely to be masked by the effect of amorphous intergranular films, via the release of stress concentrations and the prevention of the initiation and movement of dislocations. It was concluded that pre-existing vacancies and related defects in -phase powder particles had to be taken into account when considering the finer details of processes such as the -phase to -phase transformation, the formation of ’-sialons, and the high-temperature mechanical behavior of the  phase and its nano-composites.

C.M.Wang, X.Pan, M.Rühle, F.L.Riley, M.Mitomo: Journal of Materials Science, 1996, 31[20], 5281-98