Structural characterization of Li α-sialon ceramics by high-resolution 27Al and 29Si NMR spectroscopy

Abstract

High-resolution 29Si and 27Al NMR spectra have been obtained from seven crystalline samples of lithium α-sialon with different compositions. The 29Si isotropic chemical shift is found to be constant within the experimental error across the whole substitution range and its value, δSi ≈ −48 ppm, suggests that silicon remains largely in an SiN4 coordination for all compositions. The 27Al 1D MAS NMR spectra exhibit a range of tetrahedral coordinations AlOxN4 − x (x = 0–4), which vary as the aluminium and oxygen contents increase. The results are consistent with previous studies of sialons, which indicate that silicon prefers to be coordinated by nitrogen whereas aluminium prefers coordination by oxygen. Some NMR parameters are derived from two-dimensional 27Al MQMAS spectra, including those for the resolved AlO4 peak. However, it is shown that this technique does not allow the different aluminium tetrahedral mixed environments (AlO3N, AlO2N2 and AlON3) to be resolved.