\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.