Polar metals in strain-engineered KNbO₃/CaNbO₃ superlattices: a first-principles study

Polar metals have generated significant interest since the ferroelectric-like structural transition in metallic LiOsO₃ was discovered. Herein, we report on a strain-modulated polar metal in the ferroelectric/metal superlattice of 1 : 1 KNbO₃/CaNbO₃. Using first-principles calculations, we have investigated the structural distortions, including polar distortions and octahedral rotations, and layer-by-layer electronic structures in the KNbO₃/CaNbO₃ superlattice under different epitaxial strains. Along the stacking direction, the superlattice has almost parallel polar displacements under compressive strain, whereas both in-plane and out-of-plane antiferroelectric-like polar displacements are robust under intermediate strain, which is connected to the octahedral tilting pattern and interlayer electron transfer. In addition, the in-plane polar distortions are enhanced by tensile strains and have a sudden increase at 4% tensile strain. The metallicity is mainly contributed by d electrons from Nb atoms. And orbital-resolved electron distributions in each layer show that d-orbital splitting is related not only to the epitaxial strain but also to the direction of polar displacements. Our results suggest an efficient way to tune polar distortions as well as local metallicity via epitaxial strains in the superlattice.