Phase variation of ferroelectric Li2Sr1−xCax(Nb1−x

A phase variation of a pseudo-Roddlesden-Popper-type ferroelectric oxide ${\mathrm{Li}}_{2}{\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{({\mathrm{Nb}}_{1\ensuremath{-}x}{\mathrm{Ta}}_{x})}_{2}{\mathrm{O}}_{7}$ is systematically investigated using dielectric measurements, second harmonic generation, and x-ray diffraction experiments. We find an exotic $x$-T phase diagram, including paraelectric Cmcm, antiferroelectric Pmcn, and in-plane antiferroelectric and out-of-plane ferroelectric $P{2}_{1}cn$ phases. At low $x$, we observe large and divergent dielectric anomalies associated with phase transitions from Cmcm to $P{2}_{1}cn$, whereas these become small kinks at high $x$. Structural analyses reveal an internal distortion, and the rotation of octahedra strongly depends on $x$, which is consistent with the lattice dynamics obtained by a density functional theory calculation. These results demonstrate the great tunability of dielectric properties in layered perovskite-type oxides by tuning the chemical bonding state in the octahedron.