²⁹Si magic-angle-spinning nuclear-magnetic-resonance study of spinel-type Si₃N₄

²⁹Si magic-angle-spinning nuclear-magnetic resonance has been measured on spinel-type cubic silicon nitride (c-Si₃N₄). c-Si₃N₄ shows two ²⁹Si resonances at -50.0±0.2 and -225.0 ±0.2 ppm, corresponding to the tetrahedron SN₄ and the octahedron SiN₆, respectively. Integration of the spectrum gives SN₄/SiN₆ about one half that of the spinel structure. Ab initio self-consistent field Hartree-Fock molecular orbital calculations also indicate that the chemical shift for octahedral Si is more negative in nitride than in oxides.