Coexisting Mechanisms for the Ferroelectric Phase Transition in Li2SrNb2O7

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 P21 cn 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.