Isothermal evolution of phase composition, structural parameters, and ionic conductivity in Na1+Al Ge2-(PO4)3 glass-ceramics

Precursor glasses with composition Na1+xAlxGe2-x(PO4)3 (NAGP) (0.6 ≤ x ≤ 1.0) are converted into Na-superionic conductor (NASICON) glass-ceramics by thermal treatments with varied duration and annealing temperature. Detailed X-ray powder diffraction with Rietveld refinement and 31P and 27Al solid-state nuclear magnetic resonance (NMR) spectroscopy show that extended annealing at the crystallization temperature leads to a progressive de-alumination, segregation of T-AlPO4 and other crystalline phases, accompanied by the formation of amorphous material. These results suggest that an earlier formed aluminum super-saturated structure equilibrates by losing aluminum upon extended annealing. However, the ionic conductivity of glass-ceramics is less affected than would be predicted by the Al loss encountered in the NASICON phase, suggesting that ionic conductivity in these samples is not only controlled by the composition of the NASICON phase but is further influenced by the other phases present, either by contributing directly to ion transport or by facilitating interparticle contacts.