The development of 29Si magic angle spinning (MAS) NMR in oxynitride ceramics will be examined to include some historical perspective from the mid-1980s and some very recent work. Oxynitride phases provided a great expansion of the local aluminium and silicon coordination environments studied by NMR from the earlier work on silicates and aluminosilicates where oxygen dominated the local coordination units. Most initial NMR observations from oxynitrides were from crystalline samples with only a small number of well-defined local sites present in the structure. 29Si MAS NMR was usually able to distinguish different local environments SiOxN4−x (0 ≤ x ≤ 4) from the isotropic chemical shift. For 27Al, early NMR from oxynitrides showed changes in peak positions attributed to changes of x in for AlOxN4−x. However, with the relatively low magnetic fields then available, residual second-order quadrupolar broadening for 27Al under MAS caused significant overlap of the different peaks. β′-sialons (Si6–zAlzOzN8–z, where 0 ≤ z ≤ ~4.2) provide an excellent case study of such materials. The ability to probe and understand ordering in β′-sialons by solid-state NMR is presented. The significant methodological progress since the 1980s is discussed including the use of magnetic fields of up to 20 T, multiple quantum approaches and Density Functional Theory (DFT) calculations of both the structure and NMR parameters. Through detailed comparison of the observed variation with composition (z) of the unit cell parameters and the local coordination units present as suggested by the NMR data from the DFT calculations of the structure and the NMR parameters the four structural models that have been proposed for β′-sialons can be distinguished. It is shown that only one of these, the domain model is fully consistent with all the experimental data and is therefore a good structural model for β′-sialons.

29 Si and 27Al magic angle spinning NMR of oxynitride ceramics

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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