Si NMR Chemical Shifts in Crystalline and Amorphous Silicon Nitrides.

We investigate Si nuclear magnetic resonance (NMR) chemical shifts, δ, of silicon nitride. Our goal is to relate the local structure to the NMR signal and, thus, provide the means to extract more information from the experimental Si NMR spectra in this family of compounds. We apply structural modeling and the gauge-included projector augmented wave (GIPAW) method within density functional theory (DFT) calculations. Our models comprise known and hypothetical crystalline Si₃N₄, as well as amorphous Si₃N₄ structures. We find good agreement with available experimental Si NMR data for tetrahedral Si and octahedral Si in crystalline Si₃N₄, predict the chemical shift of a trigonal-bipyramidal Si to be about -120 ppm, and quantify the impact of Si-N bond lengths on Si δ. We show through computations that experimental Si NMR data indicates that silicon dicarbodiimide, Si(NCN)₂ exhibits bent Si-N-C units with angles of about 143° in its structure. A detailed investigation of amorphous silicon nitride shows that an observed peak asymmetry relates to the proximity of a fifth N neighbor in non-bonding distance between 2.5 and 2.8 Å to Si. We reveal the impact of both Si-N(H)-Si bond angle and Si-N bond length on Si δ in hydrogenated silicon nitride structure, silicon diimide Si(NH)₂.