Experimental demonstration of hybrid improper ferroelectricity and presence of abundant charged walls in (Ca,Sr)${}_3$Ti${}_2$O${}_7$ crystals

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.

References in corpus (3)

• Conduction at domain walls in insulating Pb(Zr${}_{0.2}$Ti${}_{0.8}$)O${}_3$ thin films
• Coercive fields in ultrathin BaTiO3 capacitors
• Coexistence of Weak Ferromagnetism and Ferroelectricity in the High Pressure LiNbO3-type Phase of FeTiO3

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