The Crystal Structure of Ba₃Nb₂O₈ Revisited: A Neutron Diffraction and Solid-State NMR Study

The structure of Ba₃Nb₂O₈ has been investigated using high resolution neutron powder diffraction. Our results show that, while the structure has some features in common with the 9R perovskite and palmierite structures, it is a new and distinct structure. It is shown to follow a (chh)(hhc)(chh) sequence with BaO_3-δ packing layers and is a cation- and anion-deficient 9H perovskite polytype. Nb atoms occupy octahedral sites with vacancies between hexagonal close-packed layers. Isolated, corner-sharing and face-sharing Nb-O octahedra all occur within the unit cell. The identification of purely octahedral Nb is supported by solid-state ⁹³Nb wideline NMR measurements. A two-component line shape was detected: a narrow featureless resonance with an isotropic chemical shift of δ_iso -928 ± 5 ppm consistent with regular Nb octahedra, and a much broader featureless resonance with an approximate isotropic chemical shift in the range δ_iso ∼ -944 to -937 ± 10 ppm consistent with Nb octahedra influenced by O vacancies. These are both characteristic of 6-fold oxo-coordinated Nb environments. The highly distorted octahedral environments in Ba₃Nb₂O₈ make it a potential candidate for dielectric and photocatalytic applications.