Phase variation of ferroelectric Li2Sr1-xCax(Nb1-xTax)(2)O-7 by selective reinforcement in the (Nb,Ta)-O covalent bonds

A phase variation of a pseudo-Roddlesden-Popper-type ferroelectric oxide Li2Sr1-xCax(Nb1-xTax)(2)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 P2(1)cn phases. At low x, we observe large and divergent dielectric anomalies associated with phase transitions from Cmcm to P2(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.