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

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^–b⁰c^–/–(a^–b⁰)c^– distorted Cmca phase at T = 790 and 500 K for M = Nb and Ta, respectively. There is a subsequent transition to an a⁰a⁰c^–/a⁰a⁰–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 P2₁am structure to an antipolar, a^–b⁰c^– 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.