Solid-state NMR and XRD study of α-SiAlON powders prepared by combustion synthesis

27Al and 29Si solid-state NMR spectra and X-ray diffraction (XRD) patterns were obtained for α-SiAlON powders prepared by combustion synthesis, according to which the phase transformation and structure evolution of α-SiAlON were studied. It was found that in α-SiAlON 29Si chemical shift values (−48 < δSi < −47) were close to those in β-Si3N4 and α-Si3N4, indicating that Si atoms kept SiN4 coordination to a large extent in α-SiAlON despite the presence of O atoms. Dissimilarly, 27Al chemical shift values in α-SiAlON deviated clearly from that corresponding to AlN4 coordination (δAl ∼ 112) and occurred in a range from δAl 95.5 to 99.9, which should be assigned to tetrahedral AlOxN4−x (0 ≤ x ≤ 4) coordination. The broadening effect of AlOxN4−x peaks was noticed, which was suggested to induced by slight dispersion of α-SiAlON compositions. 27Al resonances assigned to AlO4 or AlO6 were also observed, revealing the existence of glassy phase in combustion products. By comparing the intensities of AlO4 and AlO6 resonance peaks, it was found that the composition and structure of the co-existing liquid assisting α-SiAlON formation were not constant but varied with temperature in combustion synthesis. In addition, with CaF2 used as Ca-source α-SiAlON with low oxygen contents (n < m/2) were prepared, and it was found that there was a low threshold of oxygen content only beyond which α-SiAlON could be formed.