Probing the Interplay between Ferroelectricity and Metallicity in KNbO3/BaTiO3 Superlattices through Strain Engineering

The discovery of polar metals challenges long‐standing assumptions about the incompatibility between polar order and metallicity. However, despite recent progress, the mechanism allowing the coexistence of these supposedly forbidden properties remains unclear. Herein, carriers are introduced by constructing a symmetric polar/nonpolar interface in the KNbO3/BaTiO3 (KNO/BTO) superlattice (SL), enabling the coexistence of ferroelectric and metallic properties. A first‐principles analysis of the evolution of polar distortion and metallicity in the KNO/BTO SL under biaxial strain in the face to explore the dependence of the two at the microscopic level is performed. It is found that compressive strain enhances polarization distortion while reducing conductivity from three dimensions to two dimensions, indicating a weak coupling mechanism between carriers and polarization. Furthermore, it is shown that the presence of holes strengthens the TiO (NbO) covalent bonds, resulting in an unexpected enhancement of the polarization properties of the KNO/BTO SL. Therefore, it is proposed using the carrier screening effect and covalent bonding synergies as a framework for understanding the interaction between ferroelectricity and free carriers, rather than just a single factor therein. These results provide new theoretical insights for designing robust polar metals in perovskite ferroelectrics.