Structure and Sodium Ion Dynamics in Sodium Strontium Silicate Investigated by Multinuclear Solid-State NMR

摘要

The high oxide ion conductivity of the proposed sodium strontium silicate ion conductors Sr0.55Na0.45SiO2.775 (>10(-2) S.cm(-1) at 525 degrees C) and its unusual alkali metal substitution strategy have been extensively questioned in the literature. Here, we present a comprehensive understanding of the structure of this material using a combination of XRD and multinuclear O-17, Na-23, and Si-29 solid-state NMR spectroscopy data and a detailed investigation of the Na ion dynamics by high temperature Na-23 NMR line shape analysis and relaxation rates measurements. Both Na-23 and Si-29 NMR spectra demonstrate the absence of Na doping in strontium silicate SrSiO3 and the presence of an amorphous phase identified as Na2O center dot 2SiO(2) glass as the Na-containing product. Devitrification at 800 degrees C yields crystallization of the Na2O center dot 2SiO(2) glass into the known crystalline alpha-Na2Si2O5 phase which was positively identified by its XRD pattern and the extensive and clear O-17, Na-23, and Si-29 NMR fingerprints. High temperature Na-23 NMR reveals that the Na ions are mobile in the Na2O center dot 2SiO(2) amorphous component below its glass transition temperature (similar to 450 degrees C). In contrast, Na-23 NMR data obtained on the crystalline alpha-Na2Si2O5 shows limited Na dynamics below similar to 650 degrees C, and this result explains the large discrepancy in the conductivity observed in the literature which strongly depends on the thermal history of the Sr0.55Na0.45SiO2.775 material. These insights demonstrate that the high conductivity observed in Sr0.55Na0.45SiO2.775 is due to Na conduction in the Na2O center dot 2SiO(2) glass, and this motivates the quest for the discovery of low temperature fast ion conductors in noncrystalline solids.