The crystal and defect structures of polar KBiNb 2 O 7
The crystal and defect structures of polar KBiNb 2 O 7
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极性KBiNb 2 O 7 的晶体和缺陷结构
DOI：
10.1039/d1dt04064b
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发表时间：
2022
影响因子：
4
通讯作者：
Hayward, Michael A.
中科院分区：
文献类型：
--
作者：
Mallick, Subhadip;Zhang, Weiguo;Batuk, Maria;Gibbs, Alexandra S.;Hadermann, Joke;Halasyamani, P. Shiv;Hayward, Michael A.
以RbBiNb2O7为原料，经过一系列阳离子交换反应，首先将RbBiNb2O7转化为LiBiNb2O7，然后再进行k - to - li阳离子交换生成KBiNb2O7。中子、同步加速器x射线和电子衍射数据的结合表明，KBiNb2O7采用极性层状钙钛矿结构（空间群A11m），其中BiNb2O7层以（0,1 / 2,z）排列堆叠，K+阳离子位于可用的10坐标层间阳离子位的一半。平行于y轴的Bi3+阳离子的大位移破坏了相的反转对称性。HAADF-STEM图像显示，KBiNb2O7表现出频繁的堆叠错误，将（0，½，z）层堆叠转换为（½，0,z）层堆叠，反之亦然，基本上切换了材料的x轴和y轴。通过拟合从KBiNb2O7收集的SXRD数据的复杂衍射峰形状，估计每层有大约9%的机会是缺陷的，这是由于材料中缺乏NbO6的协同倾斜，这限制了与每个缺陷相关的晶格应变。
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.