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Standing on successful first principles predictions for new functionalferroelectric materials, a number of new ferroelectrics have beenexperimentally discovered. Utilizing trilinear coupling of two types ofoctahedron rotations, hybrid improper ferroelectricity has been theoreticallypredicted 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 ofthese compounds has never been experimentally confirmed and even their polarnature has been under debate. Here we provide the first experimentaldemonstration of room-temperature switchable polarization in the bulk crystalsof Ca$_{3}$Ti$_{2}$O$_{7}$ as well as Sr-doped Ca$_{3}$Ti$_{2}$O$_{7}$. Inaddition, (Ca,Sr)$_{3}$Ti$_{2}$O$_{7}$ is found to exhibit an intriguingferroelectric domain structure resulting from orthorhombic twins and(switchable) planar polarization. The planar domain structure accompaniesabundant charged domain walls with conducting head-to-head and insulatingtail-to-tail configurations, which exhibit two-order-of-magnitude conductiondifference. These discoveries provide new research opportunities not only onnew stable ferroelectrics of Ruddlesden-Popper compounds, but also onmeandering conducting domain walls formed by planar polarization.

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