Competing Octahedral Modes in the Hybrid Improper Ruddlesden–Popper Ferroelectric Ln₂SrSc₂O₇

The ferroelectric polarization in hybrid improper ferroelectrics (HIFs) is induced by the combination of oxygen octahedral in-plane rotation (OOR) and out-of-plane tilting (OOT) in perovskite-like structures. Specifically, the widely reported A 2 1 am phase is established by the one-component order parameters (OPs) of the OOR and the OOT in the double-layered Ruddlesden–Popper compounds. In the present work, an unexpected polar P 2 1 am phase is identified in Ln 2 SrSc 2 O 7 ( Ln = Pr, Nd, Sm) ceramics, showing a one-component OP of the OOT and a two-component OP of the OOR and thus leading to the coexistence of the polar and antipolar distortions within the phase. The room-temperature ferroelectricity is approved by the well-defined ferroelectric polarization–electric field hysteresis loops. In situ diffraction analysis reveals a phase transition to the nonpolar Amam phase at high temperatures. The magnitude of ferroelectric polarization is significantly influenced by the degree of A-site cation disorder and the tolerance factor, while the Curie temperature exhibits a linear increase with a decreasing tolerance factor, independent of the degree of A-site cation disorder. This work emphasizes the critical role of complicated oxygen octahedral distortions and offers a strategic pathway for the rational design of advanced HIF materials.