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Complex Structural Phase Transitions of the Hybrid Improper Polar Dion–Jacobson Oxides RbNdM2O7 and CsNdM2O7 (M = Nb, Ta)

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posted on 2020-05-07, 09:02 authored by Tong Zhu, Alexandra S. Gibbs, Nicole A. Benedek, Michael A. Hayward
Recently, there has been much interest in hybrid improper ferroelectrics materials that adopt polar, ferroelectric structures due to a complex tilting and twisting of the MO6 octahedra which constitute perovskite and related structures. Using a combination of synchrotron X-ray powder diffraction (XRD) 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, aac+/–(aac+) distorted I2cm phase to an antipolar, ab0c/–(ab0)c distorted Cmca phase at T = 790 and 500 K for M = Nb and Ta, respectively. There is a subsequent transition to an a0a0c/a0a0c distorted I4/mcm structure at 865 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, aac+ distorted P21am structure to an antipolar, ab0c distorted C2/m phase at T = 625 and 330 K for M = Nb and Ta, respectively, with a subsequent phase transition to the undistorted P4/mmm aristotype structure at 800 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 corresponding aristotype P4/mmm structures (calculated from first-principles density functional theory (DFT)) yields a strong linear relation, suggesting that Tc is not proportional to the enthalpy change at the ferroelectric phase transition.

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