Distinguishing between Structural Models of β′-Sialons Using a Combined Solid-State NMR, Powder XRD, and Computational Approach

Abstract

βʹ-sialons (Si6-zAlzOzN8-z, where 0 ≤ z ≤ ~4.2) are studied using a combination of 29 Si and 27 Al solid-state NMR, including using magnetic fields of up to 20 T, powder X-ray diffraction and Density Functional Theory (DFT) calculations of both the structure and NMR parameters. Four different structural models have been proposed in the literature for the replacement of silicon and nitrogen by aluminium and oxygen within a β-Si3N4-structured lattice. Experimental data is presented of the variation with composition (z) of the unit cell parameters from diffraction and the local coordination units present suggested by NMR data. The experimental data is compared to the changes with composition in the DFT calculations of the structure and the NMR parameters according to the four models, allowing the models to be distinguished. It is shown that only one of these, the domain model is fully consistent with all of the experimental data and is therefore a good structural model for βʹ-sialons. More speculatively it is suggested that for the domain model, 27 Al NMR data might provide a constraint on the thickness of its aluminium-rich layers.

Improved Understanding of Atomic Ordering in Y4SixAl2–xO9–xNx Materials Using a Combined Solid-State NMR and Computational Approach

“…This is well outside of the range calculated for aluminum tetrahedra within the Si/Al N-YAM structure and also for such tetrahedra within β′-sialons. 34 However, ref 34 showed the shift could come from fivecoordinated species. It is also interesting to compare to medium field data from lanthanum sialon glasses where a mixture of four-, fiveand six-fold aluminum coordinations are present where a peak in this shift region occurs.…”

“…This is well outside of the range calculated for aluminium tetrahedra within the Si/Al N-YAM structure and also for such tetrahedra within β´-sialons. 34 However ref. 34 showed the shift could come from fivecoordinated species.…”

“…34 However ref. 34 showed the shift could come from fivecoordinated species. It is also interesting to compare to medium field data from lanthanum sialon glasses where a mixture of four-, five-and six-fold aluminium coordinations are present where a peak in this shift region occurs.…”

“…Furthermore, in the (Y/La)−Si−Al− O−N phases, although clear changes in the resonance positions were again apparent for AlO x N 4−x, components, the discrete aluminum signals could not be fully resolved or characterized. 29,31,32 The NMR methodology combining high applied magnetic fields with DFT calculations to probe Si/Al and O/N ordering in β′-sialons 33,34 is applied here to crystalline Y 4 Si x Al 2−x O 9−x N x phases.…”

“…Given the importance of Y 2 O 3 as an additive for improved sintering of Si 3 N 4 the crystalline phases of the Y–Si–O–N system provided a range of local environments SiO x N 4– x (0 ≤ x ≤ 4), being studied in detail by 29 Si MAS NMR. − The structural similarity of lanthanum La–Si–O–N phases meant that they were also reported on and for the N-YAM phase will be referred to below. , For 27 Al more detailed NMR studies exist for the Y 2 O 3 –Al 2 O 3 system where locally aluminum is only coordinated by oxygen. , In these materials, some aluminum positions are characterized by large quadrupolar interactions which thus limits the effectiveness of midrange external magnetic field strengths and MAS frequencies for the study of these systems. Furthermore, in the (Y/La)–Si–Al–O–N phases, although clear changes in the resonance positions were again apparent for AlO x N 4– x , components, the discrete aluminum signals could not be fully resolved or characterized. ,, The NMR methodology combining high applied magnetic fields with DFT calculations to probe Si/Al and O/N ordering in β′-sialons , is applied here to crystalline Y 4 Si x Al 2– x O 9– x N x phases.…”