Effect of cation ordering and pressure on n=2 Ruddlesden-Popper oxides from first principles

Octahedral tilts and rotations are ubiquitous in perovskite oxides and couple strongly to the electronic and magnetic properties. Furthermore, the interplay of octahedral rotations and layering can enable novel functionalities. For example, it has recently been shown that in n=2 Ruddlesden-Popper A₃B₂O₇ layered perovskites, two octahedral rotations of different symmetries can induce a polarization via a trilinear coupling mechanism, known as hybrid improper ferroelectricity. While there has been extensive work on engineering octahedral rotation amplitudes and patterns in ABO₃ perovskites, this has been much less studied in the Ruddlesden-Popper phases. By performing first-principles density functional theory calculations for a range of A₃B₂O₇ materials, we explore two different approaches for engineering octahedral rotations in these systems. First, we consider the impact of A-site cation ordering (e.g. A₂A’B₂O₇) on the energetics of a range of structural phases. Second, we elucidate the effect of hydrostatic pressure on octahedral rotation amplitudes and in turn on the energetics of the phases. We hence provide possible mechanisms to control the stability between polar and non-polar phases as well as tune structural distortions necessary for hybrid improper ferroelectricity.