A NEW APPROACH ON THE COORDINATION OF AL IN NONCRYSTALLINE GELS AND GLASSES OF THE SYSTEM AL2O3 - SIO2

In the present study the idea of five-fold coordinated aluminium in Al 2O3-rich aluminiumsilicate gels and glasses with compositions near to that of mullite (approximate to 72 wt.% Al2O3, 28 wt.% SiO2) is re-axi mated. Analyses of Al-27 MAS NMR spectroscopy and pair correlation fun ctions on the basis of X-ray diffraction data together with a critical review of crystallization mechanisms provide evidence for short-range -order similarities of mullite and corresponding gels and glasses. The study provides evidence for an occurrence of (Al/Si)O-4-tetrahedra an d of AlO6-octahedra in the gels and glasses. The (Al/Si)O-4 to AlO6 ra tios of non-crystalline aluminium silicates are similar to that of mul lite and comparable tricluster units of tetrahedra may occur in both, mullite and its noncrystalline counterparts. There is less evidence th at 5-fold coordinated Al exists in aluminium-silicate gels and glasses as was reported frequently in the literature.