Polar Structures of KNdNb2O7and KNdTa2O7

Na-for-Rb cation exchange followed by K-for-Na cation exchange of RbNdM 2 O 7 (M = Nb, Ta) yields the corresponding, metastable KNdM 2 O 7 phases. Synchrotron X-ray and neutron powder diffraction data, combined with powder SHG data, reveal that the KNdM 2 O 7 phases adopt a polar structure (space group Im 2 m ) consisting of NdM 2 O 7 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 MO 6 units. First-principles density functional theory (DFT) calculations confirm that the polar Im 2 m 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.