A STRUCTURAL INVESTIGATION OF CAO-AL2O2 GLASSES VIA AL-27 MAS-NMR

Al-27 isotropic chemical shifts and quadrupolar coupling constants hav e been obtained for glasses along the CaO-Al2O3 join, prepared by fast (splat) quench techniques and slow cooling in containerless levitatio n experiments. Quench rates with the two techniques were 10(6)-10(7) d egrees/s and 200-300 degrees/s. Fast-quenched glasses with CaO:Al2O3 < 1 contain Al in V- and VI-coordination to oxygen, in addition to the d ominant IV-coordinated species. Glasses along the CaO-CaAl2O4 portion of the join contain only IV-coordinated aluminium. The average isotrop ic chemical shift of these glasses increases with increasing CaO conte nt, which may reflect a decrease in the polymerization state of the al uminate units, although a similar trend can not be detected in the cor responding crystals, ?The chemical shifts for the fast quenched glasse s are displaced systematically to smaller values, which can be associa ted with a rearrangement of Ca and Al in the next nearest neighbour co ordination environment about the central Al atom. The fast quenched gl asses have a less ordered, more distorted structure, as revealed by la rger values of the quadrupolar coupling constant, and Al XANES spectra .