Nature of the structural symmetries associated with hybrid improper ferroelectricity in C a3X2 O7 (X=Mn and Ti)

In hybrid improper ferroelectric systems, polarization arises from the onset of successive nonpolar lattice modes. In this work, measurements and modeling were performed to determine the spatial symmetries of the phases involved in the transitions to these modes. Structural and optical measurements reveal that the tilt and rotation distortions of the MnO₆ or TiO₆ polyhedra relative to the high symmetry phases driving ferroelectricity in the hybrid improper Ca₃X₂O₇ system (X=Mn and Ti) condense at different temperatures. The tilt angle vanishes abruptly at TT∼400 K for Ca₃Mn₂O₇ (and continuously for X=Ti) and the rotation mode amplitude is suppressed at much higher temperatures TR∼1060 K. Moreover, Raman measurements in Ca₃Mn₂O₇ under isotropic pressure reveal that the polyhedral tilts can be suppressed by very low pressures (between 1.4 and 2.3 GPa) indicating their softness. These results indicate that the Ca3Mn2O7 system provides a platform for strain engineering of ferroelectric properties in film-based systems with substrate-induced strain.