2016
ACL
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K.K.Inglis, J.P.Corley, P.Florian, J.Cabana, R.D.Bayliss, F.Blanc, 'Structure and Sodium Ion Dynamics in Sodium Strontium Silicate Investigated by Multinuclear Solid-State NMR', Chem. Mat. (2016) doi:10.1021/acs.chemmater.6b00941
The high oxide ion conductivity of the proposed sodium strontium silicate ion conductors Sr0.55Na0.45SiO2.775 (> 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 crystallisation 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 Sr0.55Na0.45SiO2.775 material. These insights demonstrate that the high conductivity observed in Sr0.55Na0.45SiO2.775 are due to Na conduction in the Na2O.2SiO2 glass and this motivates the quest for the discovery of low temperature fast ion conductors in non-crystalline solids.
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