The crystal and defect structures of polar KBiNb₂O₇

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, 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 Bi³⁺ cations parallel to the y-axis. HAADF-STEM images reveal that KBiNb₂O₇ 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 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.