Displacement-type ferroelectricity with off-center magnetic ions in perovskite Sr_1-xBa_xMnO₃

We report a ferroelectric transition driven by the off-centering of magnetic Mn4⁺ ions in antiferromagnetic Mott insulators Sr _1-xBa_xMnO₃ with a perovskite structure. As x increases, the perovskite lattice shows the typical soft-mode dynamics, as revealed by the momentum-resolved inelastic x-ray scattering and far-infrared spectroscopy, and the ferroelectricity shows up for x0.45. The observed polarization is comparable to that for a prototypical ferroelectric BaTiO ₃. We further demonstrate that the magnetic order suppresses the ferroelectric lattice dilation by ∼70% and increases the soft-phonon energy by ∼50%, indicating the largest magnetoelectric effects yet attained.