Evolution of crystalline aluminates from hybrid gel-derived precursors studied by XRD and multinuclear solid state MAS NMR: IV: Calcium dialuminate, CaAl4O7 and calcium hexaluminate, CaAl12O19

X-ray amorphous calcium aluminate gels of dialuminate and hexaluminate composition were prepared, the former by two methods, and their thermal crystallisation was monitored by thermal analysis, X-ray powder diffraction and 27Al MAS NMR. Both gel compositions lose a large amount of solvent and organic by-products below 600°C, forming amorphous precursors with 27Al NMR spectra containing resonances corresponding to tetrahedral and octahedral sites, and another site with a peak at 36–39 ppm. The dialuminate gel crystallises abruptly at 900°C to the essentially phase-pure compound containing Al predominantly in two distorted tetrahedral environments. The hexaluminate gel crystallises exothermically at 900°C to γ-alumina spinel, containing only tetrahedral and octahedral Al. The magnetoplumbite structure of the hexaluminate phase appears in an exothermic reaction at 1160°C, this phase-pure reaction product containing Al atoms in one tetrahedral and two distinguishable octahedral sites. The first 43Ca MAS NMR spectra of the crystalline dialuminate and hexaluminate are also reported.