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Polar Structures of KNdNb2O7 and KNdTa2O7
Chemistry of Materials ( IF 7.2 ) Pub Date : 2020-08-25 , DOI: 10.1021/acs.chemmater.0c02846 Subhadip Mallick 1 , Alexandra S. Gibbs 2 , Weiguo Zhang 3 , P. Shiv Halasyamani 3 , Nicole A. Benedek 4 , Michael A. Hayward 1
Chemistry of Materials ( IF 7.2 ) Pub Date : 2020-08-25 , DOI: 10.1021/acs.chemmater.0c02846 Subhadip Mallick 1 , Alexandra S. Gibbs 2 , Weiguo Zhang 3 , P. Shiv Halasyamani 3 , Nicole A. Benedek 4 , Michael A. Hayward 1
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Na-for-Rb cation exchange followed by K-for-Na cation exchange of RbNdM2O7 (M = Nb, Ta) yields the corresponding, metastable KNdM2O7 phases. Synchrotron X-ray and neutron powder diffraction data, combined with powder SHG data, reveal that the KNdM2O7 phases adopt a polar structure (space group Im2m) consisting of NdM2O7 perovskite double sheets stacked in a (0, 1/2, z) manner with K+ cations ordered within the 6-coordinate prismatic interlayer sites. The perovskite double sheets adopt an (a0b+c0/a0-b+c0) tilting distortion; however, unlike other A′AB2O7 phases, this distortion is not the origin of the noncentrosymmetric structure, which is attributed to a second-order Jahn–Teller distortion of the MO6 units. First-principles density functional theory (DFT) calculations confirm that the polar Im2m phase is more stable than the corresponding centrosymmetric alternative. The role of the A′- and A- cations in directing the stacking patterns and tilting distortions of A′AB2O7 phases is discussed with reference to hybrid improper ferroelectric behavior.
中文翻译：
KNdNb 2 O 7和KNdTa 2 O 7的极性结构
Na-Rb阳离子交换，然后Kb-Na阳离子交换RbNdM 2 O 7（M = Nb，Ta），产生相应的亚稳KNdM 2 O 7相。同步加速器X射线和中子粉末衍射数据结合粉末SHG数据显示，KNdM 2 O 7相采用极性结构（空间群Im 2 m），该结构由NdM 2 O 7钙钛矿双片堆叠成（0， 1/2，z）方式，K +阳离子在6坐标棱柱形夹层位点内有序排列。钙钛矿双张采用一个（一个0b + c 0 / a 0 -b + c 0）倾斜变形；然而，不像其它甲' AB 2 ö 7个阶段，这种失真是不非中心对称的结构，这归因于二阶的MO的杨-特勒畸变的原点6单位。第一原理密度泛函理论（DFT）计算确认该极性林2米相比相应中心对称替代更稳定。所述的作用甲' -和甲-在引导堆叠模式和倾斜的扭曲阳离子甲“ AB 2 ö 7个相参照混合不当铁电性能的讨论。
更新日期：2020-09-22
中文翻译：
KNdNb 2 O 7和KNdTa 2 O 7的极性结构
Na-Rb阳离子交换，然后Kb-Na阳离子交换RbNdM 2 O 7（M = Nb，Ta），产生相应的亚稳KNdM 2 O 7相。同步加速器X射线和中子粉末衍射数据结合粉末SHG数据显示，KNdM 2 O 7相采用极性结构（空间群Im 2 m），该结构由NdM 2 O 7钙钛矿双片堆叠成（0， 1/2，z）方式，K +阳离子在6坐标棱柱形夹层位点内有序排列。钙钛矿双张采用一个（一个0b + c 0 / a 0 -b + c 0）倾斜变形；然而，不像其它甲' AB 2 ö 7个阶段，这种失真是不非中心对称的结构，这归因于二阶的MO的杨-特勒畸变的原点6单位。第一原理密度泛函理论（DFT）计算确认该极性林2米相比相应中心对称替代更稳定。所述的作用甲' -和甲-在引导堆叠模式和倾斜的扭曲阳离子甲“ AB 2 ö 7个相参照混合不当铁电性能的讨论。
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