A large piezoelectric response in ferroelectric ceramics is typically associated with extrinsic contributions from ferroelectric domain structures. However, such domain structures cannot be expected in systems with pseudo-cubic symmetry. In this study, we determine the mechanism of significant piezoelectricity and ferroelectricity in 0.3BaTiO3–0.1Bi(Mg1/2Ti1/2)O3–0.6BiFeO3 ceramic with a perovskite-type pseudo-cubic symmetry. Synchrotron radiation X-ray diffraction reveals that the Bi ions in this ceramic essentially prefer to be off-centered at six sites by approximately 0.4 Å, in the cubic <100> directions. A phase transition occurs at TC ~725 K. However, the crystal seems to present a cubic symmetry even at room temperature. The large piezoelectric response is caused by the combinational partial ordering of the off-centered Bi ions, adapted to any direction of the applied electric field to the ceramic grains. The proposed mechanism for the emergence of a high polarization in the above system will enable designing novel Pb-free ceramics by controlling the fluctuated and off-centered ions under an applied electric field.

Contributions to the piezoelectric response in pseudocubic 0.3BaTiO3-0.1Bi(Mg1/2Ti1/2)O3-0.6BiFeO3 ceramics were investigated by synchrotron X-ray diffraction under electric fields. All of the lattice strain determined from the 110, 111, and 200 pseudocubic diffraction peaks showed similar lattice strain hysteresis that was comparable to the bulk butterfly-like strain curve. It was suggested that the hysteresis of the lattice strain and the lack of anisotropy were related to the complex domain structure and the phase boundary composition.

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Crystallography and dielectric properties in Dion–Jacobson layered perovskites, CsNdNb2O7 and RbNdNb2O7, have been examined in dense polycrystalline samples, and polarization hysteresis loops that substantiate ferroelectricity have been observed at room temperature. The theoretical mechanism for the spontaneous polarization, “hybrid improper ferroelectric mechanism,” induced by a combination of two types of non-polar octahedral rotations, is confirmed in these two phases. Our samples show remanent polarizations of 2–3 μC cm−2, which are much larger than those obtained in polycrystalline samples with the hybrid improper ferroelectricity reported so far. A dielectric constant in CsNdNb2O7 exhibits an anomaly at 625 K, corresponding to the ferroelectric transition, as previously revealed by X-ray and neutron diffractometry. No dielectric anomaly is observed for RbNdNb2O7 throughout the temperature range studied here (≤773 K), which is consistent with the previous diffractometry showing the persistence of polar I2cm symmetry up to 790 K.

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