Ferroelectricity of Dion-Jacobson layered perovskites CsNdNb$_2$O$_7$ and RbNdNb$_2$O$_7$

TL;DR

This study confirms ferroelectricity in Dion-Jacobson layered perovskites CsNdNb$_2$O$_7$ and RbNdNb$_2$O$_7$ at room temperature, demonstrating larger remanent polarization than previous reports and elucidating the underlying hybrid improper ferroelectric mechanism.

Contribution

It provides experimental evidence of room-temperature ferroelectricity in these specific layered perovskites and confirms the theoretical hybrid improper ferroelectric mechanism involved.

Findings

Remanent polarization of 2-3 μC/cm² observed.

CsNdNb₂O₇ shows a dielectric anomaly at 625 K.

RbNdNb₂O₇ exhibits no dielectric anomaly up to 773 K.

Abstract

Crystallography and dielectric properties in Dion-Jacobson layered perovskites, CsNdNb$_2$O$_7$ and RbNdNb$_2$O$_7$, have been examined in dense polycrystalline samples, and polarization hysteresis loops that substantiate ferroelectricity have been observed at room temperature. The theoretical mechanism for the spontaneous polarization, "hybrid improper ferroelectric mechanism,'' induced by a combination of two types of non-polar octahedral rotations, is confirmed in these two phases. Our samples show remanent polarizations of 2-3 $\mu$C/cm$^2$, which are much larger than those obtained in polycrystalline samples with the hybrid improper ferroelectricity reported so far. A dielectric constant in CsNdNb$_2$O$_7$ exhibits an anomaly at 625 K, corresponding to the ferroelectric transition, as previously revealed by x-ray and neutron diffractometry. No dielectric anomaly is observed for RbNdNb$_2$O$_7$ throughout the temperature range studied here ($\leq$ 773 K), which is consistent with the previous diffractometry showing the persistence of polar $I2cm$ symmetry up to 790 K.