27Al Isotropic chemical shifts and quadrupolar coupling constants have been obtained for glasses along the CaO-Al2O3 join, prepared by fast (splat) quench techniques and slow cooling in containerless levitation experiments. Quench rates with the two techniques were 106-107°/s and 200-300°/s. Fast-quenched glasses with CaO:Al2O3 < 1 contain Al in V-and VI-coordination to oxygen, in addition to the dominant IV-coordinated species. Glasses along the CaO-CaAl2O4 portion of the join contain only IV-coordinated aluminium. The average isotropic chemical shift of these glasses increases with increasing CaO content, which may reflect a decrease in the polymerization state of the aluminate units, although a similar trend can not be detected in the corresponding crystals. The chemical shifts for the fast quenched glasses are displaced systematically to smaller values, which can be associated with a rearrangement of Ca and Al in the next nearest neighbour coordination environment about the central Al atom. The fast quenched glasses have a less ordered, more distorted structure, as revealed by larger values of the quadrupolar coupling constant, and Al XANES spectra.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Ceramics and Composites
- Condensed Matter Physics
- Materials Chemistry