Pressure-Dependent Phase Transitions in Hybrid Improper Ferroelectric Ruddlesden-Popper Oxides

TL;DR

This study investigates how hydrostatic pressure influences phase transitions in hybrid improper ferroelectric Ruddlesden-Popper oxides, revealing pressure can both suppress and enhance polar modes depending on structural responses.

Contribution

It provides new insights into the pressure dependence of phase transitions in Ruddlesden-Popper oxides using combined experimental and computational methods.

Findings

Pressure favors non-polar phases in these materials.

Pressure can increase polar mode amplitudes under certain conditions.

Octahedral tilt and rotation responses explain the pressure effects.

Abstract

The temperature-dependent phase transitions in Ruddlesden-Popper oxides with perovskite bilayers have been under increased scrutiny in recent years due to the so-called hybrid improper ferroelectricity that some chemical compositions exhibit. However, little is currently understood about the hydrostatic pressure dependence of these phase transitions. Herein we present the results of a high-pressure powder synchrotron X-ray diffraction experiment and $ab~initio$ calculations on the bilayered Ruddlesden-Popper phases Ca$_{3}$Mn$_{2}$O$_{7}$ and Ca$_{3}$Ti$_{2}$O$_{7}$. In both compounds we observe a first-order phase transition between polar $A2_{1}am$ and non-polar $Acaa$ structures. Interestingly, we show that while the application of pressure ultimately favours a non-polar phase -- as is commonly observed for proper ferroelectrics -- regions of response exist where pressure actually acts to increase the polar mode amplitudes. The reason for this can be untangled by considering the varied response of octahedral tilts and rotations to hydrostatic pressure and their trilinear coupling with the polar instability.