4.7 Article

The crystal and defect structures of polar KBiNb2O7

Journal

DALTON TRANSACTIONS
Volume 51, Issue 5, Pages 1866-1873

Publisher

ROYAL SOC CHEMISTRY
DOI: 10.1039/d1dt04064b

Keywords

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Funding

  1. National Science Foundation [DMR-2002319]
  2. Welch Foundation [E-1457]
  3. Engineering and Physical Sciences Research Council [EP/T011130/1] Funding Source: researchfish

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KBiNb2O7 is prepared by cation exchange reactions and has a polar, layered, perovskite structure with a high level of defects.
KBiNb2O7 was prepared from RbBiNb2O7 by a sequence of cation exchange reactions which first convert RbBiNb2O7 to LiBiNb2O7, before KBiNb2O7 is formed by a further K-for-Li cation exchange. A combination of neutron, synchrotron X-ray and electron diffraction data reveal that KBiNb2O7 adopts a polar, layered, perovskite structure (space group A11m) in which the BiNb2O7 layers are stacked in a (0, 1/2, z) arrangement, with the K+ cations located in half of the available 10-coordinate interlayer cation sites. The inversion symmetry of the phase is broken by a large displacement of the Bi3+ cations parallel to the y-axis. HAADF-STEM images reveal that KBiNb2O7 exhibits frequent stacking faults which convert the (0, 1/2, z) layer stacking to (1/2, 0, z) stacking and vice versa, essentially switching the x- and y-axes of the material. By fitting the complex diffraction peak shape of the SXRD data collected from KBiNb2O7 it is estimated that each layer has approximately a 9% chance of being defective - a high level which is attributed to the lack of cooperative NbO6 tilting in the material, which limits the lattice strain associated with each fault.

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