A New Approach on the Coordination of Al in Non-Crystalline Gels and Glasses of the System Al_2O_3-SiO_2

摘要

In the present study the idea of five-fold coordinated aluminium in Al_2O_3-rich aluminiumsilicate gels and glasses with compositions near to that of mullite (≈72 wt.% Al_2O_3, 28 wt.% SiO_2) is re-aximated. Analyses of ~(27)Al 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 AlO_6-octahedra in the gels and glasses. The (Al/Si)O_4 to AlO_6 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 non-crystalline counterparts. There is less evidence that 5-fold coordinated Al exists in aluminium-silicate gels and glasses as was reported frequently in the literature.