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 combined high-pressure powder synchrotron x-ray diffraction experiment and $abinitio$ study on the bilayered Ruddlesden-Popper phases ${\mathrm{Ca}}_3{\mathrm{Mn}}_2{\mathrm{O}}_7$ and ${\mathrm{Ca}}_3{\mathrm{Ti}}_2{\mathrm{O}}_7$. In both compounds we observe a first-order phase transition, that in combination with our density functional theory calculations, we can confidently assign as being between polar $A{2}_{1}am$ and nonpolar $Acaa$ structures. Interestingly, we show that while the application of pressure ultimately favors a nonpolar 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.

• Received 9 December 2021
• Revised 1 February 2024
• Accepted 29 February 2024

DOI:https://doi.org/10.1103/PhysRevB.109.094107

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Published by the American Physical Society