Nature of the Structural Symmetries Associated with Hybrid Improper Ferroelectricity in Ca3X2O7

TL;DR

This study investigates the structural symmetries and phase transitions in hybrid improper ferroelectric Ca3X2O7 (X=Mn, Ti), revealing distinct temperature-dependent behaviors of lattice distortions and their potential for strain engineering.

Contribution

It provides detailed measurements and modeling of the symmetry changes and lattice mode behaviors in Ca3X2O7, highlighting differences between Mn and Ti variants and their response to pressure.

Findings

Tilt and rotation distortions occur at different temperatures.

Tilt angle vanishes abruptly at ~400 K for Mn, continuously for Ti.

Polyhedral tilts are very soft and can be suppressed by low pressure.

Abstract

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 T$_T$ ~ 400 K for Ca3Mn2O7 (and continuously for X=Ti) and the rotation mode amplitude is suppressed at much higher temperatures T$_R$ ~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 and2.3 GPa) indicating their softness. These results indicate that the Ca3Mn2O7 system provides a new platform for strain engineering of ferroelectric properties in film based systems with substrate induced strain.