Solid‐state NMR investigation of fast sodium ion‐conducting glass–ceramics: The system Na3+3x−yRE1−xPySi3−yO9 (RE = Sc, Y)

Abstract

AbstractGlasses and glass–ceramics of composition Na3+3x−yRE1−xPySi3−yO9 were synthesized using RE = Sc and Y, x = 0.4, and y = 0.0 and 0.3 to obtain multiple‐phase glass–ceramics containing the highly conducting Na5RESi4O12 (N5) phase. In addition, the two model compounds Na5ScSi4O12 and Na5InSi4O12 were synthesized. Samples were characterized at two distinct annealing stages using X‐ray powder diffraction, electrical conductivity measurements, and multinuclear solid‐state magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. The N5 phase is dominantly formed for Sc‐containing glass–ceramics (both with y = 0.0 and 0.3) at crystallization temperatures above 900°C. For the other glass–ceramics, the crystallized phases were dominantly Na3RESi2O7 (N3), RE = Sc and Y, and Na9YSi6O18 (N9) phases. 29Si MAS‐NMR peak assignments were done with the aid of 29Si{45Sc} rotational echo adiabatic passage double resonance (REAPDOR) experiments. 29Si and 23Na MAS‐NMR spectra reveal complex phase compositions and local environment distributions, which could be largely assigned based on known semiempirical chemical shift correlations with average Si–O and Na–O bond distances. 31P MAS and 31P{45Sc} REAPDOR NMR results suggest the presence of orthophosphate groups, arguing against the literature model of isostructural substitution of silicon by phosphorus in the N5 phase.