Cation Distributions and Anion Disorder in Ba₃Nb<i>M</i>O_8.5 (<i>M</i> = Mo, W) Materials: Implications for Oxide Ion Conductivity

Abstract

Competitive oxide ion conductivity was identified recently in members of the Ba3Nb1–y(Mo1–xWx)1+yO8.5+1/2y (0 ≤ x ≤ 1, −0.3 ≤ y ≤ 0.2) series, which adopt a disordered rhombohedral “hybrid” structure combining features of the 9R perovskite and palmierite structures. We report the first growth of Ba3NbMoO8.5 and Ba3NbWO8.5 single crystals from molten phases and their characterizations using single-crystal X-ray diffraction data between 120 and 473 K. Structure refinements reveal a previously unreported splitting of the central Nb/M cation site, rationalized by bonding considerations, which imposes limitations on the material stoichiometry and possible arrangements of cations within the face-sharing polyhedral stacks. The analysis of atomic displacement parameters and bond valence energy landscapes (BVELs) gives new insight into the probable low-energy pathways for oxide ion diffusion in the hybrid structure, indicating that they are three-dimensional and involve all crystallographically distinct oxygen sites. Evidence for considerable static disorder of the oxide ions at temperatures below the onset of significant conductivity is also discussed.