Hybrid improper ferroelectricity in A-cation ordered perovskite BaSrBi₂O₆: a first-principles study

We investigate structural, electronic, and ferroelectric properties of A-cation ordered perovskite BaSrBi₂O₆ using first-principles density functional theory. We systematically search for the metastable structures by constructing the initial structures with A-site orderings and octahedral rotation patterns commonly observed in perovskite oxides. In all of the metastable structures, we find that the breathing distortions observed in the bulk BaBiO₃ and SrBiO₃ are maintained, which stabilizes the charge-ordered insulating phase. More importantly, we identify a polar ground state with layered A-cation ordering and a⁻a⁻c⁺ octahedral rotation pattern with a net polarization of 5.7 μC/cm². Our distortion mode analysis shows that the polarization is induced from the hybrid improper ferroelectricity in which the polar A-site displacements from Ba/Sr layered ordering emerge from the coupling to the octahedral rotations. We believe that our method to identify the metastable structures can be used in a wide range of perovskite oxides and the proposed ferroelectric BaSrBi₂O₆ can be fabricated by layer-by-layer growth techniques.