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

Abstract

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 demonstrate that an A-site-deficient perovskite Y${}_{1/3}$TaO${}_3$ exhibits room-temperature ferroelectricity in a $Pb{2}_{1}m$ phase, enabled by ordered vacancies coupled with TaO${}_6$ octahedral rotations. Defect-ordered perovskites are frequently trapped in centrosymmetric incommensurate states due to competing structural instabilities; we circumvent this by favoring rotational over polar instability through compositional selection. Unlike canonical improper ferroelectrics that are \textit{ferrielectric}, the vanishing dipoles on vacancy layers in Y${}_{1/3}$TaO${}_3$ allow for a net ferroelectric alignment of local dipoles, resulting in enhanced polarization. Upon heating, Y${}_{1/3}$TaO${}_3$ transforms to a paraelectric incommensurate phase at $\simeq$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.

11 pages, 10 figures

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