Solid‐state NMR investigation of fast sodium ion‐conducting glass–ceramics: The system Na 3+3 x − y RE 1− x P y Si 3− y O 9 (RE = Sc, Y)

Glasses and glass–ceramics of composition Na 3+3 x − y RE 1− x P y Si 3− y O 9 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 Na 5 RESi 4 O 12 (N5) phase. In addition, the two model compounds Na 5 ScSi 4 O 12 and Na 5 InSi 4 O 12 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 Na 3 RESi 2 O 7 (N3), RE = Sc and Y, and Na 9 YSi 6 O 18 (N9) phases. 29 Si MAS‐NMR peak assignments were done with the aid of 29 Si{ 45 Sc} rotational echo adiabatic passage double resonance (REAPDOR) experiments. 29 Si and 23 Na 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. 31 P MAS and 31 P{ 45 Sc} 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.