Crystal structures, dielectric properties, and phase transition in hybrid improper ferroelectric Sr3Sn2O7-based ceramics.

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 Sr_3–xCa_xSn₂O₇ 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 T_C. 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]