Heteroanionic Stabilization of Ni¹⁺ with Nonplanar Coordination in Layered Nickelates

We present electronic structure calculations on layered nickelate oxyfluorides derived from the Ruddlesden-Popper arisotype structure in search of unidentified materials that may host nickelate superconductivity. By performing anion exchange of oxygen with fluorine, we design two heteroanionic polymorphs with Ni¹⁺ in 4-coordinate and 5-coordinate square planar and square pyramidal geometries, respectively. We suggest chemical reactions to guide their synthesis. These oxyfluorides are weakly correlated antiferromagnetic insulators and their nonmagnetic phases exhibit quasi-2D Fermi surfaces dominated by Ni d_x2−y2 states, which strikingly resemble undoped cuprate superconductors. We use our understanding to propose doping strategies and layered nickelate oxyfluorides with tunable electronic and magnetic structures for experimentation.