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