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Polar Structures of KNdNb2O7 and KNdTa2O7

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posted on 02.09.2020 by Subhadip Mallick, Alexandra S. Gibbs, Weiguo Zhang, P. Shiv Halasyamani, Nicole A. Benedek, Michael A. Hayward
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 AAB2O7 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 AAB2O7 phases is discussed with reference to hybrid improper ferroelectric behavior.

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