SYNTHESIS AND MAGIC-ANGLE SPINNING NUCLEAR-MAGNETIC-RESONANCE OF 15N-ENRICHED SILICON NITRIDES

The synthesis of α-Si315N4 by nitridation of silicon powder using a 15N2/H2 mix and the conversion to β-Si315N4 are described. The 15N NMR spectra of these two solid phases have been obtained, and they broadly correlate with the crystal structures. The 15N shift range for the α-polymorph covers nearly 25 ppm, suggesting that the sensitivity of the 15N chemical shift to structural factors may be greater than that of 29Si shifts. The 15N spin-lattice relaxation times for β-Si315N4 were also measured, and were shown to be very long (~3000 s), with implications for optimization of experimental conditions. Silicon-29 spectra show that, contrary to expectations, unaveraged (29Si,14N) coupling is not the primary cause of line broadening in these phases. © 1990, American Chemical Society. All rights reserved.