Nature of the Phase Transitions Leading to Hybrid Improper Ferroelectricity in Ca3X2O7*

Detailed structural and optical measurements reveal that the tilt and rotation distortion relative to the high symmetry phase driving ferroelectricity in the system Ca₃X₂O₇ system (X=Mn and Ti) condense at different temperatures. The condensation of the rotation and tilt distortions at distinctly different temperatures is unexpected. Experimental results, combined with DFT simulation of the atomic force constants, suggest that this loss of a polar state is driven by the relative strength of the A-O bonds to the X-O bonds. Raman measurements under isotropic pressure are used to assess the stability of the tilt and rotational distortions.