Polar structures of KNdNb2O7 and KNdTa2O7

Journal article

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.

Authors: Mallick, S; Gibbs, AS; Zhang, W; Halasyamani, PS; Benedek, NA

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: American Chemical Society
• Journal: Chemistry of Materials
• Volume: 32
• Issue: 18
• Pages: 7965-7972
• Publication date: 2020-08-25
• Acceptance date: 2020-08-24
• DOI: 10.1021/acs.chemmater.0c02846
• EISSN: 1520-5002
• ISSN: 0897-4756
• Language: English
• Keywords: perovskites; chemical structure; FFR; group theory; cations; polarization