A new approach on the coordination of Al in non-crystalline gels and glasses of the system Al2O3 - SiO2

In the present study the idea of five-fold coordinated aluminium in Al2O3-rich aluminiumsilicate gels and glasses with compositions near to that of mullite (approximate to 72 wt.% Al2O3, 28 wt.% SiO2) is re-aximated. Analyses of Al-27 MAS NMR spectroscopy and pair correlation functions 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 and of AlO6-octahedra in the gels and glasses. The (Al/Si)O-4 to AlO6 ratios of non-crystalline aluminium silicates are similar to that of mullite and comparable tricluster units of tetrahedra may occur in both, mullite and its noncrystalline counterparts. There is less evidence that 5-fold coordinated Al exists in aluminium-silicate gels and glasses as was reported frequently in the literature.