EVOLUTION OF THE SI ENVIRONMENT IN MULLITE SOLID-SOLUTION BY SI-29 MAS-NMR SPECTROSCOPY

The evolution of the silicon environment has been investigated in a mu llite solid solution, Al4+2xSi2-2xO10-x, using Si-29 magic angle spinn ing nuclear magnetic resonance (MAS-NMR) spectroscopy and X-ray diffra ction experiments. Various single phase precursors with compositions r anging from 50 to 71.4 mol% Al2O3 (2 less than or equal to Al/Si less than or equal to 5) were synthesized, and the chemical compositions of the crystallized samples were determined from their lattice parameter s. Four sites have been observed by Si-29 NMR spectroscopy, whatever t he composition in the solid solution, at -86 ppm, -90 ppm, -94 ppm and a shoulder at -80 ppm. Their occupancies have been obtained by simula ting the spectra with Gaussian lines. They evolve linearly with the al umina content and with the number, x, of oxygen vacancies. The main co ntribution, corresponding to the shoulder at -80 ppm, is in fact a bro ad line centered at -88 ppm. It increases with a slope of 2x in the so lid solution and is extrapolated to zero for sillimanite (x = 0). It h as been attributed to a silicon site in triclusters or near oxygen vac ancies. The occupancies of the sites at -86 ppm and -90 ppm decrease w ith a slope of is with the increase of Al2O3 content. They are assigne d to sillimanite-type sites: the original one (-86 ppm) or one modifie d by replacement of an Al by a Si atom in the second sphere of coordin ation (-90 ppm). The relation between the structures of sillimanite an d mullite is clearly shown. The occupancy of the minor fourth site (-9 4 ppm) slightly increases with x. This site may result from another re placement of Al by Si around the silicon atom.