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Complex structural phase transitions of the hybrid improper polar Dion-Jacobson oxides RbNdM2O7 and CsNdM2O7 (M = Nb, Ta)

DOI: 10.1021/acs.chemmater.0c01304 DOI Help

Authors: Tong Zhu (University of Oxford) , Alexandra S. Gibbs (ISIS Facility) , 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: April 2020
Diamond Proposal Number(s): 13284

Abstract: There has been much interest recently in hybrid improper ferroelectrics – materials which adopt polar, ferroelectric structures due to a complex tilting and twisting of the MO6 octahedra which constitute perovskite and related struc-tures. Using a combination of synchrotron X-ray powder diffraction and high-resolution neutron powder diffraction data the temperature-dependent phase transitions of a series of n = 2 Dion-Jacobson oxides have been investigated. RbNdM2O7 undergoes a transition from a polar, aˉaˉc+/-(aˉaˉc+) distorted I2cm phase to an antipolar, aˉb0cˉ/-(aˉb0)cˉ distorted Cmca phase at T = 790 K and 500 K for M = Nb and Ta respectively. There is a subsequent transition to an a0a0cˉ/a0a0-cˉ distorted I4/mcm structure at 865 K and 950 K for M = Nb and Ta respectively, before a transition to the undistorted P4/mmm aristotype structure. In contrast CsNdM2O7 undergoes a transition from a polar, aˉaˉc+ distorted P21am structure to an antipolar, aˉb0cˉ distorted C2/m phase at T = 625 K and 330 K for M = Nb and Ta respectively, with a subsequent phase transition to the undistorted P4/mmm aristotype structure at 800 K and 820 K for M = Nb and Ta respectively. A plot of Tc against the relative stability of the 4 polar Dion-Jacobson phases compared to the corre-sponding aristotype P4/mmm structures (calculated from first-principles DFT), yields a strong linear relation suggest-ing Tc is not proportional to the enthalpy change at the ferroelectric phase transition.

Journal Keywords: Group theory; Chemical structure; Polarization; Phase transitions; Materials

Diamond Keywords: Ferroelectricity

Subject Areas: Materials, Chemistry

Instruments: I11-High Resolution Powder Diffraction

Added On: 29/04/2020 10:01

Discipline Tags:

Inorganic Chemistry Materials Science Quantum Materials Chemistry

Technical Tags:

Diffraction X-ray Powder Diffraction