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Hybrid improper ferroelectricity, which is induced by the coupling of oxygen octahedral rotation and tilt in oxide materials, has great potential to create single-phase multiferroic materials with intrinsic electric field-controlled magnetism. Based on previous experiments with bi-layered Ruddlesden-Popper Sr-based materials, hybrid improper ferroelectricity in Sr3Hf2O7 was investigated by first-principle calculations in the present work. The results show that the ferroelectric A21am phase has the lowest energy among all possible phases, indicating that it is the ground state. The spontaneous polarization calculated by the Berry phase approach and Born effective charge model is approximately 4.6 μC/cm2, in agreement with those reported for other Sr-based compounds. Compared to Ca3Ti2O7, the smaller polarization of Sr3Hf2O7 is attributed to the negative contribution of the HfO2 layer to global polarization. The possible ferroelectric switching path was determined using the nudged elastic band method. The energy barriers of the two four-step paths across the non-polar Pbnm and Pnab phases are almost the same, and they are the lowest energies among all possible paths. Competition between these paths can help us understand complex phase transitions in analogous compounds. In summary, hybrid improper ferroelectricity was predicted in Sr3Hf2O7 for the first time, and the possible ferroelectric switching paths were determined. [ABSTRACT FROM AUTHOR]

Hybrid improper ferroelectricity is induced by the coupling of oxygen octahedral rotation and tilt, and it may provide a promising way to develop new room-temperature single-phase multiferroic materials with a strong magnetoelectric effect. Though ferroelectric polarizations are confirmed by the electric hysteresis loops at room temperature, the ferroelectric-paraelectric phase transition has not been well understood. In the present work, the crystal structures, dielectric properties, and phase transition behaviors of Ruddlesden-Popper Sr3–xCaxSn2O7 ceramics are thoroughly determined. Ferroelectricity is confirmed through the polarization-electric field hysteresis loops at room temperature, and the Curie temperature is identified by the variable-temperature differential scanning calorimetric analysis and dielectric characterization. The polarizations and Curie temperatures of the present ceramics increase with increasing the calcium content. An improper ferroelectric transition is experimentally evidenced by a small dielectric constant and a step variation of dielectric constant around the Curie temperature, and the thermal hysteresis of dielectric constant confirms the first order transition nature. Moreover, the fitting result of Landau phase transition theory also suggests the first-order improper ferroelectric transition nature. The variable-temperature Raman result implies a tetragonal paraelectric phase above TC. The present work confidently confirms the first-order hybrid improper ferroelectric transitions in the present ceramics, and ferroelectric properties can be readily tuned by substituting the cations at the A-site. [ABSTRACT FROM AUTHOR]

Hybrid improper ferroelectricity (HIF) has been proposed as a candidate approach to create room- temperature multiferroic materials with an intrinsic electric-field controlled magnetism effect, and HIFs in bi-layered Ruddlesden-Popper Ca3-xSrxTi2O7 ceramics have been confirmed in the present work. The ferroelectric hysteresis loops are observed in these ceramics at room temperature, and the polar orthorhombic structures with oxygen octahedral rotation and tilt modes have been confirmed by room-temperature X-ray powder diffraction. The amplitudes of oxygen octahedral rotation and tilt modes decrease with the increasing x value, which in turn leads to the decrease of remnant polarization in these ceramics. A first-order phase transition has been evidenced by the differential scanning calorimetry measurement, and the temperature of phase transition decreases linearly with the increasing content of strontium cations. It is found that the amplitude of oxygen octahedral distortion is the dominating factor that affects ferroelectric polarization in partial-ordered bi-layered Ca3Ti2O7-based ceramics. [ABSTRACT FROM AUTHOR]

Hybrid improper ferroelectricity is induced by the coupling of oxygen octahedral rotation and tilting, and a promising way to create multiferroic materials with a strong magnetoelectric effect can be proposed. Although some hybrid improper ferroelectric materials with Ruddlesden-Popper structures have been discovered experimentally, the small polarizations and high coercive fields will severely limit their practical applications. To deal with this issue, an alternative way is proposed by the first-principles calculations in the present work. By calculating the energies of phases with oxygen octahedral rotation and tilting in Ca3TiMO7 (M = Si, Ge, Mn, Sn, Hf, and Zr) compounds with five B-site ordering configurations, it is found that the tolerance factor has a considerable impact on their ferroelectric properties. The energy barriers and ferroelectric polarizations increase with decreasing tolerance factor. Moreover, the individual-layer contributions to the total polarizations depend on the space group, which is also decided by the tolerance factor of the compound. Therefore, to enhance the ferroelectric polarization in B-site substituted Ca3Ti2O7, the tolerance factor should be decreased according to the calculation results of this work. [ABSTRACT FROM AUTHOR]

Dielectric characteristics of Ca(Fe1/2Nb1/2)O3 ceramics were investigated in wide temperature and frequency ranges. A very high dielectric constant peak with strong frequency dispersion following the Arrhenius law was observed in the temperature range of 473–673 K, and this high temperature dielectric relaxation reflected the defect ordering induced relaxor ferroelectricity. A dielectric constant step associated with a low temperature dielectric relaxation was observed in the temperature range of 200–500 K, but the dielectric constant step was very low (∼50) and could only be detected in the O2-annealed samples due to the overshadowing amplitude of the high temperature dielectric relaxation. The low temperature dielectric relaxation, following the Arrhenius law, indicated the electronic ferroelectricity due to the ordering among heterovalent iron ions. [ABSTRACT FROM AUTHOR]

Dielectric relaxations of Ba(Fe1/2Ta1/2)O3 ceramics were investigated and discussed over a broad temperature and frequency range. Two dielectric relaxations following Arrhenius law were observed at 153–382 and 440–623 K, where there was a giant dielectric constant step between them. The frequency dependent rapid drop of dielectric constant at 153–382 K was nearly a Debye relaxation with the intrinsic nature, while the high temperature dielectric relaxation with an extremely high dielectric constant peak and very strong frequency dispersion was attributed to the defect ordering but not a typical relaxor ferroelectric behavior. The O2 annealing almost completely suppressed the dielectric peak and subsequently extended the giant dielectric step, while the low temperature dielectric relaxation and the magnitude of such step were not obviously affected. [ABSTRACT FROM AUTHOR]