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 La 2 Ba 1– x Sr x In 2 O 7 to achieve room-temperature hybrid improper ferroelectricity. Systematic substitution of Sr 2+ for Ba 2+ drives symmetry transitions from a centrosymmetric tetragonal P 4 2 / mnm phase (0 ≤ x ≤ 0.3) to orthorhombic nonpolar Amam (0.3 ≤ x ≤ 0.4) and polar A 2 1 am (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 A 2 1 am 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.