cm0c02846_si_002.cif (1.75 kB)

Polar Structures of KNdNb2O7 and KNdTa2O7

Download (1.75 kB)
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.