Glasses and crystalline A3Al2(PO4)3 (A = Na, Li): an impedance and 31P, 27Al, 23Na and 7Li MAS-NMR study

Abstract

A new vitreous phase with Na₃Al₂(PO₄)₃ composition has been prepared. Its crystallisation has been optimised by annealing at low temperature, giving crystalline c-Na₃Al₂(PO₄)₃ with Sc₂(WO₄)₃-type structure. The direct synthesis of vitreous Li₃Al₂(PO₄)₃ has been attempted but it was unsuccessful. Ion exchange reaction has been carried out in Na₃Al₂(PO₄)₃ glass to obtain the lithiated vitreous solid, v-Li₃Al₂(PO₄)₃. On heating, v-Li₃Al₂(PO₄)₃ decomposes to yield crystalline AlPO₄ and Li₃PO₄ phases. The three solids, with NASICON-like stoichiometries, have been thermal and electrically characterised. DTA and thermodiffractometric studies of the two glass solids have allowed the characterisation of their thermal behaviours including crystallisation. The grain boundary contributions to the total impedance are almost absent in the two vitreous phases. The electrical conductivity values are ∼ 10⁻⁶ S cm⁻¹ at 150 °C. The ³¹P, ²³Na, ²⁷Al and ⁷Li MAS-NMR spectra for these materials provide some insight into the disordered constitution of the vitreous phases and the low mobility of the alkaline cations.