\23Na,27Al and31P NMR and X-ray powder diffraction study of Na/Ca/Al phosphate glasses and ceramics

Abstract

The techniques of X-ray powder diffraction and solid-state magic angle spinning (MAS) 23 Na, 27 Al, and 31 P nuclear magnetic resonance (NMR) spectroscopy have been combined to investigate the speciation in a series of glasses and glass ceramics of general formula (P 2 O 5 ) 0.45 (CaO) 0.24 (Na 2 O) 0.31-x (Al 2 O 3 ) x ,x=0.0–0.05. The principal phosphate species are shown to be various P 2 O 7 4- containing phases, and cylic trimetaphosphates bridged by Ca, i.e. Na 4 Ca(PO 3 ) 6 (instead of open-chain metaphosphates). Higher concentrations of Al 2 O 3 result in glass ceramics which are phosphate-depolymerised (Q 2 →Q 1 ) with respect to the parent glasses. At a lower level of Al 2 O 3 (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 Na 5 Ca 2 Al(PO 4 ) 4 , and the 31 P 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.