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

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• 作者：Kenneth K. Inglis ; John P. Corley ; Pierre Florian ; Jordi Cabana ; Ryan D. Bayliss ; Frédéric Blanc
• 刊名：Chemistry of Materials
• 出版年：2016
• 出版时间：June 14, 2016
• 年：2016
• 卷：28
• 期：11
• 页码：3850-3861
• 全文大小：637K
• 年卷期：0
• ISSN：1520-5002

文摘

The high oxide ion conductivity of the proposed sodium strontium silicate ion conductors Sr_0.55Na_0.45SiO_2.775 (>10^–2 S·cm^–1 at 525 °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 ¹⁷O, ²³Na, and ²⁹Si solid-state NMR spectroscopy data and a detailed investigation of the Na ion dynamics by high temperature ²³Na NMR line shape analysis and relaxation rates measurements. Both ²³Na and ²⁹Si NMR spectra demonstrate the absence of Na doping in strontium silicate SrSiO₃ and the presence of an amorphous phase identified as Na₂O·2SiO₂ glass as the Na-containing product. Devitrification at 800 °C yields crystallization of the Na₂O·2SiO₂ glass into the known crystalline α-Na₂Si₂O₅ phase which was positively identified by its XRD pattern and the extensive and clear ¹⁷O, ²³Na, and ²⁹Si NMR fingerprints. High temperature ²³Na NMR reveals that the Na ions are mobile in the Na₂O·2SiO₂ amorphous component below its glass transition temperature (∼450 °C). In contrast, ²³Na NMR data obtained on the crystalline α-Na₂Si₂O₅ shows limited Na dynamics below ∼650 °C, and this result explains the large discrepancy in the conductivity observed in the literature which strongly depends on the thermal history of the Sr_0.55Na_0.45SiO_2.775 material. These insights demonstrate that the high conductivity observed in Sr_0.55Na_0.45SiO_2.775 is due to Na conduction in the Na₂O·2SiO₂ glass, and this motivates the quest for the discovery of low temperature fast ion conductors in noncrystalline solids.