(29)Si NMR Chemical Shifts in Crystalline and Amorphous Silicon Nitrides

We investigate (29)Si nuclear magnetic resonance (NMR) chemical shifts, δ(iso), 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 (29)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 Si(3)N(4), as well as amorphous Si(3)N(4) structures. We find good agreement with available experimental (29)Si NMR data for tetrahedral Si([4]) and octahedral Si([6]) in crystalline Si(3)N(4), predict the chemical shift of a trigonal-bipyramidal Si([5]) to be about −120 ppm, and quantify the impact of Si-N bond lengths on (29)Si δ(iso). We show through computations that experimental (29)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 (29)Si δ(iso) in hydrogenated silicon nitride structure, silicon diimide Si(NH)(2).