Evolution of the Si environment in mullite solid solution by 29Si MAS-NMR spectroscopy

Abstract

The evolution of the silicon environment has been investigated in a mullite solid solution, Al4 + 2xSi2 − 2xO10 − x, using 29Si magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy and X-ray diffraction experiments. Various single phase precursors with compositions ranging from 50 to 71.4 mol% Al2O3 (2 ≤ AlSi ≤ 5) were synthesized, and the chemical compositions of the crystallized samples were determined from their lattice parameters. Four sites have been observed by 29Si NMR spectroscopy, whatever the composition in the solid solution, at −86 ppm, −90 ppm, −94 ppm and a shoulder at −80 ppm. Their occupancies have been obtained by simulating the spectra with Gaussian lines. They evolve linearly with the alumina content and with the number, x, of oxygen vacancies. The main contribution, corresponding to the shoulder at −80 ppm, is in fact a broad line centered at −88 ppm. It increases with a slope of 2x in the solid solution and is extrapolated to zero for sillimanite (x = 0). It has been attributed to a silicon site in triclusters or near oxygen vacancies. The occupancies of the sites at −86 ppm and −90 ppm decrease with a slope of 1x with the increase of Al2O3 content. They are assigned to sillimanite-type sites: the original one (−86 ppm) or one modified by replacement of an Al by a Si atom in the second sphere of coordination (−90 ppm). The relation between the structures of sillimanite and mullite is clearly shown. The occupancy of the minor fourth site (−94 ppm) slightly increases with x. This site may result from another replacement of Al by Si around the silicon atom.