Coexisting Mechanisms for the Ferroelectric Phase Transition in Li<sub>2</sub>SrNb<sub>2</sub>O<sub>7</sub>
Yasuhide Mochizuki, Takayuki Nagai, Hirokazu Shirakuni, Akitoshi Nakano, Fumiyasu Oba, Ichiro Terasaki, Hiroki Taniguchi
Abstract
Pseudo-Ruddlesden–Popper-type Li2SrNb2O7 (LSNO), which belongs to a layered perovskite-type family, exhibits a first-order ferroelectric phase transition at 217 K. Our first-principles calculations and dielectric measurements show that Ca-substitution for Sr in LSNO enhances the rotation of NbO6 octahedra and Nb5+ displacements to induce a robustly stabilized ferroelectric P21cn phase. The present study has demonstrated the two coexisting mechanisms in the ferroelectric phase transition of LSNO: the second-order Jahn–Teller effect and the rotational-distortion driven hybrid-improper ferroelectricity. The origin of unique ferroelectric phase transition of LSNO has been clarified to be softening of the zone-boundary Y2– mode. The present study provides a new insight into designing ferroelectricity in layered perovskite-type oxides with the second-order Jahn–Teller effect and the rotations of octahedra.