A-Site Cation Disorder Engineering in Ruddlesden–Popper Layered Perovskite Oxide La ₂ (Ba,Sr)In ₂ O ₇ for Hybrid Improper Ferroelectricity

The strategic design of ferroelectrics exhibiting reversible spontaneous polarization remains a pivotal challenge in functional materials research. In this work, we demonstrate A-site cation disorder engineering in Ruddlesden–Popper layered perovskite oxides La2Ba1–xSrxIn2O7 to achieve room-temperature hybrid improper ferroelectricity. Systematic substitution of Sr2+ for Ba2+ drives symmetry transitions from a centrosymmetric tetragonal P42/mnm phase (0 ≤ x ≤ 0.3) to orthorhombic nonpolar Amam (0.3 ≤ x ≤ 0.4) and polar A21am (0.5 ≤ x ≤ 0.9) phases. Multimodal characterization, combining synchrotron X-ray and neutron diffraction, nonlinear optical spectroscopy, and electric polarization versus electric field hysteresis loops, reveals switchable polarization in the A21am phase arising from trilinear coupling with nonpolar oxygen octahedral rotations and tilts. Crucially, Sr/La disorder at the A-sites plays a key role in enhancing the octahedral rotations, a prerequisite for polar symmetrical stabilization, while simultaneously suppressing the octahedral elongation (deformation) due to the electrostatic interaction-induced rumpling at the perovskite-rocksalt layer interfaces. First-principles calculations elucidate that Sr/La disorder mitigates the interlayer rumpling, enabling oxygen octahedral distortions necessary for ferroelectricity. This work establishes cation disorder engineering as a versatile strategy to design room-temperature ferroelectric/magnetoelectric materials in layered perovskites, advancing the coupling between structural distortions and functional responses in complex oxides.