Na-23, Al-27 and P-31 NMR and X-ray powder diffraction study of Na/Ca/Al phosphate glasses and ceramics

The techniques of X-ray powder diffraction and solid-state magic angle spinning (MAS) Na-23, Al-27, and P-31 nuclear magnetic resonance (NMR) spectroscopy have been combined to investigate the speciation in a series of glasses and glass ceramics of general formula (P2O5)(0.45)(CaO)(0.24)(Na2O)(0.31-x)(Al2O3)(x), x = 0.0-0.05. The principal phosphate species are shown to be various P2O74- containing phases, and cylic trimetaphosphates bridged by Ca, i.e. Na4Ca(PO3)(6) (instead of open-chain metaphosphates). Higher concentrations of Al2O3 result in glass ceramics which are phosphate-depolymerised (Q(2)-->Q(1)) with respect to the parent glasses. At a lower level of Al2O3 (2%) the aluminium is present in octahedral coordination, while the higher level (5%) results in the formation of tetrahedrally coordinated aluminium. X-Ray powder diffraction of the ceramic with the higher aluminium content indicated the presence of Na5Ca2Al(PO4)(4), and the P-31 NMR Spectrum provides evidence for Q(1)(2) species similar to phosphorus in aluminium metaphosphate. The more detailed structural information available from the aluminium-free glass ceramic, and the similarity in the Q(1)/Q(2) ratio between the glass and its derived ceramic leads to the thesis that the ceramic structure may, in favourable cases, be used to model phosphate speciation in the glass.