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The Crystal Structure of Ba<sub>3</sub>Nb<sub>2</sub>O<sub>8</sub> Revisited: A Neutron Diffraction and Solid-State NMR Study
The
structure of Ba<sub>3</sub>Nb<sub>2</sub>O<sub>8</sub> 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 (<i>chh</i>)Â(<i>hhc</i>)Â(<i>chh</i>) sequence with BaO<sub>3−δ</sub> 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 <sup>93</sup>Nb
wideline NMR measurements. A two-component line shape was detected:
a narrow featureless resonance with an isotropic chemical shift of
δ<sub>iso</sub> −928 ± 5 ppm consistent with regular
Nb octahedra, and a much broader featureless resonance with an approximate
isotropic chemical shift in the range δ<sub>iso</sub> ∼
−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 Ba<sub>3</sub>Nb<sub>2</sub>O<sub>8</sub> make it a potential candidate
for dielectric and photocatalytic applications