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

• Published in: Chemistry of materials 2020-05, Vol.32 (10), p.4340-4346
• Main Authors: Zhu, Tong, Gibbs, Alexandra S, Benedek, Nicole A, Hayward, Michael A
• Format: Article
• Language: English
• ISSN: 0897-4756; 1520-5002
• DOI: 10.1021/acs.chemmater.0c01304

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, a – a – c +/–(a – a – c +) distorted I2cm phase to an antipolar, a – b 0 c –/–(a – b 0)c – distorted Cmca phase at T = 790 and 500 K for M = Nb and Ta, respectively. There is a subsequent transition to an a 0 a 0 c –/a 0 a 0–c – 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, a – a – c + distorted P21 am structure to an antipolar, a – b 0 c – 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 T c 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 T c is not proportional to the enthalpy change at the ferroelectric phase transition.