Glass-to-crystal transition in the NASICON glass-ceramic system Na1+xAlxM2−x(PO4)3 (M=Ge, Ti)

The glass-to-crystal transition of Na1+xAlxGe2−x(PO4)3 (NAGP), and Na1+xAlxTi2−x(PO4)3 (NATP), both crystallizing in variants of the Na-superionic conducting (NASICON) structure, has been investigated by solid-state NMR. The ceramic materials produced by annealing the precursor glasses above the glass transition temperature are candidate materials for solid-state separator membranes in sodium ion batteries. The different local structural environments involving both network former and network modifier species have been characterized by comprehensive 23Na, 27Al and 31P magic angle spinning nuclear magnetic resonance (MAS NMR) experiments. In crystalline Na1+xAlxGe2−x(PO4)3 samples multiple phosphate environments are observed, corresponding to n Al and 4-n Ge species in their second coordination spheres. In contrast, no site resolution is observed in the analogous Na1+xAlxTi2−x(PO4)3 (NATP) system. This can be understood on the basis of X-ray powder diffraction (XRD) data, which reveal a significant lattice expansion in the former, but no lattice expansion in the latter material. 27Al MAS-NMR data reveal that in the glassy state, Al occurs with coordination numbers four, five and six, with the fraction of four-coordinated Al being substantially higher in the NATP glasses than in the NAGP glasses. 23Na MAS-NMR and spin echo decay measurements reveal distinct differences between glassy and crystallized materials with regard to the local environments and the spatial distributions of the sodium ions.