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# Complex Structural Phase Transitions of the Hybrid
Improper Polar Dion–Jacobson Oxides RbNdM_{2}O_{7} and CsNdM_{2}O_{7} (M = Nb, Ta)

dataset

posted on 2020-05-07, 09:02 authored by Tong Zhu, Alexandra S. Gibbs, Nicole A. Benedek, Michael A. HaywardRecently, there has
been much interest in hybrid improper ferroelectrics
materials that adopt polar, ferroelectric structures due to a complex
tilting and twisting of the MO

_{6}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. RbNdM_{2}O_{7}undergoes a transition from a polar,*a*^{–}*a*^{–}*c*^{+}/–(*a*^{–}*a*^{–}*c*^{+}) distorted*I*2*cm*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*I*4/*mcm*structure at 865 and 950 K for M = Nb and Ta, respectively, before a transition to the undistorted*P*4/*mmm*aristotype structure. In contrast, CsNdM_{2}O_{7}undergoes a transition from a polar,*a*^{–}*a*^{–}*c*^{+}distorted*P*2_{1}*am*structure to an antipolar,*a*^{–}*b*^{0}*c*^{–}distorted*C*2/*m*phase at*T*= 625 and 330 K for M = Nb and Ta, respectively, with a subsequent phase transition to the undistorted*P*4/*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*P*4/*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.