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.