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Polar Structures of KNdNb2O7 and KNdTa2O7
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DOI：10.1021/acs.chemmater.0c02846.png)
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Abstract
En 中文
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 (a(0)b(+)c(0)/a(0)-b(+)c(0)) tilting distortion; however, unlike other A'AB(2)O(7) 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'AB(2)O(7) phases is discussed with reference to hybrid improper ferroelectric behavior.
Keywords:
BILBAO CRYSTALLOGRAPHIC SERVER
INITIO MOLECULAR-DYNAMICS
TOTAL-ENERGY CALCULATIONS
EXHIBITING ION-EXCHANGE
LAYERED PEROVSKITES
LONE PAIRS
FERROELECTRICITY
ORIGIN
TRANSITION
DISTORTION
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