  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.
