Silicon-29 NMR characterization of .alpha.- and .beta.-silicon nitride

Fine structure in the {sup 29}Si MAS NMR spectra of Si{sub 3}N{sub 4} has been observed, prompting a reevaluation of the current interpretation of Si{sub 3}N{sub 4} spectra. On the basis of variable field studies, this fine structure indicates the presence of quadrupole-perturbed residual dipolar coupling to {sup 14}N. This is apparently the first such observation in {sup 29}Si NMR. The peaks in the spectra of {alpha}- and {beta}-Si{sub 3}N{sub 4} are shown to arise from differences in the non-nearest-neighbor environments. Arguments based on a sphere of influence approach appear to be an attractive interpretive tool for these and similar ceramic materials. Some speculation is also presented concerning the relationship between the {sup 29}Si-{sup 14}N splitting and the structure of Si{sub 3}N{sub 4}.