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The polar structures of KNdNb2O7 and KNdTa2O7

DOI: 10.1021/acs.chemmater.0c02846 DOI Help

Authors: Subhadip Mallick (University of Oxford) , Alexandra S. Gibbs (ISIS Facility) , Weiguo Zhang (University of Houston) , P. Shiv Halasyamani (University of Houston) , Nicole A. Benedek (Cornell University) , Michael Hayward (University of Oxford)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Chemistry Of Materials

State: Published (Approved)
Published: August 2020
Diamond Proposal Number(s): 13284

Abstract: Na-for-Rb cation exchange followed by K-for-Na cation exchange of RbNdM2O7 (M = Nb, Ta) yields the corresponding, metasta-ble 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 double perovskite sheets stacked in a (0, ½, z) manner with K+ cations ordered within the 6-coordinate prismatic inter-layer 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 non-centrosymmetric structure, which is attributed to a second-order Jahn-Teller distortion of the MO6 units. First-principles DFT calculations confirm the polar Im2m phase is more stable than the corresponding centrosymmetric alternative. The role of the A’- and A- cations in di-recting the stacking patterns and tilting distortions of A’AB2O7 phases is discussed with reference to hybrid improper ferroelectric behavior.

Subject Areas: Chemistry, Materials

Instruments: I11-High Resolution Powder Diffraction

Other Facilities: ISIS