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Title: The crystal and defect structures of polar KBiNb 2 O 7
KBiNb 2 O 7 was prepared from RbBiNb 2 O 7 by a sequence of cation exchange reactions which first convert RbBiNb 2 O 7 to LiBiNb 2 O 7 , before KBiNb 2 O 7 is formed by a further K-for-Li cation exchange. A combination of neutron, synchrotron X-ray and electron diffraction data reveal that KBiNb 2 O 7 adopts a polar, layered, perovskite structure (space group A 11 m ) in which the BiNb 2 O 7 layers are stacked in a (0, ½, 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 Bi 3+ cations parallel to the y -axis. HAADF-STEM images reveal that KBiNb 2 O 7 exhibits frequent stacking faults which convert the (0, ½, z ) layer stacking to (½, 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 KBiNb 2 O 7 it is estimated that each layer has approximately a 9% chance of being defective – more » a high level which is attributed to the lack of cooperative NbO 6 tilting in the material, which limits the lattice strain associated with each fault. « less
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Publication Date:
Journal Name:
Dalton Transactions
Page Range or eLocation-ID:
1866 to 1873
Sponsoring Org:
National Science Foundation
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