Improper ferroelectricity has been attracted many attentions because of its great potential application in creating room temperature multiferroicity with strong magnetoelectric coupling. Recently, the hybrid improper ferroelectricity (HIF) has been proposed by the first-principle calculation, and the ferroelectricity is induced by a complex distortion pattern consisting of two oxygen octahedron tilting modes, such as a-a-c0 and a0a0c+ (in Glazer's notation) in perovskite [1]. The HIF should widely exist in the perovskites and layered perovskites since the oxygen octahedron rotations are ubiquitous distortions in the perovksite-related materials. In the present talk, the experimental evidences of HIF in Ruddlesden-Poper Ca3(Ti,Mn)2O7 and (Sr,Ca)3Sn2O7 ceramics have been shown. The room temperature ferroelectric hysteresis loops are observed in these ceramics, and a polar orthorhombic structure with two oxygen tilting modes has been confirmed by the X-ray powder diffraction. A first-order phase transition around 1100 K in Ca3Ti2O7 was evidenced, and the temperatures of phase transitions decrease linearly with increasing of the contents of Mn4+ and Sr2+ ions [2].
References
[1] N.A. Benedek, and C.J. Fennie, Phys. Rev. Lett., 106, 07204 (2011).
[2] X.Q. Liu, J.W. Wu, X.X. Shi, H.J. Zhao, H.Y. Zhou, R.H. Qiu, W.Q. Zhang, and X.M. Chen, Appl. Phys. Lett., 106, 202903 (2015) .