Experimental demonstration of hybrid improper ferroelectricity and presence of abundant charged walls in (Ca,Sr)$_{3}$Ti$_{2}$O$_{7}$ crystals

TL;DR

This paper experimentally confirms room-temperature switchable polarization and charged domain walls in (Ca,Sr)$_{3}$Ti$_{2}$O$_{7}$ crystals, demonstrating hybrid improper ferroelectricity in these Ruddlesden-Popper compounds.

Contribution

First experimental demonstration of ferroelectricity and charged domain walls in (Ca,Sr)$_{3}$Ti$_{2}$O$_{7}$, validating theoretical predictions of hybrid improper ferroelectricity.

Findings

Room-temperature switchable polarization confirmed in (Ca,Sr)$_{3}$Ti$_{2}$O$_{7}$.

Presence of abundant charged domain walls with distinct conduction properties.

Observation of complex ferroelectric domain structures from orthorhombic twins.

Abstract

Standing on successful first principles predictions for new functional ferroelectric materials, a number of new ferroelectrics have been experimentally discovered. Utilizing trilinear coupling of two types of octahedron rotations, hybrid improper ferroelectricity has been theoretically predicted in ordered perovskites and the Ruddlesden-Popper compounds (Ca$_{3}$Ti$_{2}$O$_{7}$, Ca$_{3}$Mn$_{2}$O$_{7}$, and (Ca/Sr/Ba)$_{3}$(Sn/Zr/Ge)$_{2}$O$_{7}$). However, the ferroelectricity of these compounds has never been experimentally confirmed and even their polar nature has been under debate. Here we provide the first experimental demonstration of room-temperature switchable polarization in the bulk crystals of Ca$_{3}$Ti$_{2}$O$_{7}$ as well as Sr-doped Ca$_{3}$Ti$_{2}$O$_{7}$. In addition, (Ca,Sr)$_{3}$Ti$_{2}$O$_{7}$ is found to exhibit an intriguing ferroelectric domain structure resulting from orthorhombic twins and (switchable) planar polarization. The planar domain structure accompanies abundant charged domain walls with conducting head-to-head and insulating tail-to-tail configurations, which exhibit two-order-of-magnitude conduction difference. These discoveries provide new research opportunities not only on new stable ferroelectrics of Ruddlesden-Popper compounds, but also on meandering conducting domain walls formed by planar polarization.