Switchable Polarization in an A-Site Deficient Perovskite through Vacancy and Cation Engineering

While defects are unavoidable in crystals and often detrimental to material performance, they can be a key ingredient for inducing functionalities when tailored. Here, we show that an A-site-deficient perovskite Y_1/3TaO₃ exhibits switching-like polarization response at room temperature in the <i>Pb</i>2₁<i>m</i> phase, enabled by ordered vacancies coupled with TaO₆ octahedral rotations. Defect-ordered perovskites are frequently trapped in centrosymmetric incommensurate states due to competing structural instabilities; we circumvent this by favoring rotational over off-centering instability through compositional selection. Unlike canonical improper ferroelectrics that are <i>ferrielectric</i>, the vanishing dipoles on vacancy layers in Y_1/3TaO₃ allow for a net ferroelectric alignment of local dipoles, resulting in enhanced polarization. Upon heating, Y_1/3TaO₃ transforms to a paraelectric incommensurate phase at ≃750 K, whose atomic arrangement mirrors the domain topology observed in hybrid improper ferroelectrics. Superspace analysis of the modulated phase reveals a route to improve room-temperature polarization, achieved through epitaxial strain, as confirmed by our lattice-dynamics calculations. This defect-ordering strategy should be generalizable to other improper ferroelectrics, including magnetoelectric multiferroics, providing a pathway to amplify otherwise limited macroscopic polarization.