Variable-temperature multinuclear solid-state NMR study of oxide ion dynamics in fluorite-type bismuth vanadate and phosphate solid electrolytes

Dunstan, Matthew T.; Halat, David M.; Tate, Matthew L.; Evans, Ivana Radosavljevic; Grey, Clare P.

Abstract

Ionic-conducting materials are crucial for the function of many advanced devices used in a variety of applications, such as fuel cells and gas separation membranes. Many different chemical controls, such as aliovalent doping, have been attempted to stabilise δ-Bi2O3, a material with exceptionally high oxide ion conductivity which is unfortunately only stable over a narrow temperature range. In this study, we employ a multinuclear, variable-temperature NMR spectroscopy approach to characterise and measure oxide ionic motion in the V- and P-substituted bismuth oxide materials Bi0.913V0.087O1.587, Bi0.852V0.148O1.648 and Bi0.852P0.148O1.648, previously shown to have excellent ionic conduction properties (Kuang et al., Chem. Mater. 2012, 24, 2162; Kuang et al., Angew. Chem. Int. Ed. 2012, 51, 690). Two main 17O NMR resonances are distinguished for each material, corresponding to O in the Bi–O and V–O/P–O sublattices. Using variable-temperature (VT) measurements ranging from room temperature to 923 K, the ionic motion experienced by these different sites has then been characterised, with coalescence of the two environments in the V-substituted materials clearly indicating a conduction mechanism facilitated by exchange between the two sublattices. The lack of this coalescence in the P-substituted material indicates a different mechanism, confirmed by 17O T1 (spin-lattice relaxation) NMR experiments to be driven purely by vacancy motion in the Bi–O sublattice. 51V and 31P VT-NMR experiments show high rates of tetrahedral rotation even at room temperature, increasing with heating. An additional VO4 environment appears in 17O and 51V NMR spectra of the more highly V-substituted Bi0.852V0.148O1.648, which we ascribe to differently distorted VO4 tetrahedral units that disrupt the overall ionic motion, consistent both with linewidth analysis of the 17O VT-NMR spectra and experimental results of Kuang et al. showing a lower oxide ionic conductivity in this material compared to Bi0.913V0.087O1.587 (Chem. Mater. 2012, 24, 2162). This study shows solid-state NMR is particularly well suited to understanding connections between local structural features and ionic mobility, and can quantify the evolution of oxide-ion dynamics with increasing temperature.

Citation

Dunstan, M. T., Halat, D. M., Tate, M. L., Evans, I. R., & Grey, C. P. (2019). Variable-temperature multinuclear solid-state NMR study of oxide ion dynamics in fluorite-type bismuth vanadate and phosphate solid electrolytes. Chemistry of Materials, 31(5), 1704-1714. https://doi.org/10.1021/acs.chemmater.8b05143

• Journal Article Type: Article
• Acceptance Date: Feb 8, 2019
• Online Publication Date: Jan 29, 2019
• Publication Date: Mar 12, 2019
• Deposit Date: Jan 31, 2019
• Publicly Available Date: Jan 29, 2020
• Journal: Chemistry of Materials
• Print ISSN: 0897-4756
• Electronic ISSN: 1520-5002
• Publisher: American Chemical Society
• Peer Reviewed: Peer Reviewed
• Volume: 31
• Issue: 5
• Pages: 1704-1714
• DOI: https://doi.org/10.1021/acs.chemmater.8b05143
• Public URL: https://durham-repository.worktribe.com/output/1338564