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Variable-Temperature Multinuclear Solid-State NMR Study of Oxide Ion Dynamics in Fluorite-Type Bismuth Vanadate and Phosphate Solid Electrolytes

preprint
submitted on 12.12.2018 and posted on 13.12.2018 by Matthew Dunstan, David M. Halat, Matthew Tate, Ivana Radosavljevic Evans, Clare P. Grey

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. 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.

Funding

Junior Research Fellowship, Clare College Cambridge

Cambridge Commonwealth Trusts

STFC Early Career Award

NECCES, Award No. DE-SC0012583

History

Email Address of Submitting Author

mtd33@cam.ac.uk

Institution

Department of Chemistry, University of Cambridge

Country

United Kingdom

ORCID For Submitting Author

0000-0002-6319-4231

Declaration of Conflict of Interest

No conflict of interest

Licence

Exports