Heteroanionic stabilization of Ni1+ with nonplanar coordination in layered nickelates

We present electronic structure calculations on layered nickelate oxyfluorides derived from the Ruddlesden-Popper aristotype structure in search of unidentified materials that may host nickelate superconductivity. By performing anion exchange of oxygen with fluorine, we create two heteroanionic La₂NiO₃F polymorphs and stabilize Ni¹⁺ in four-coordinate and five-coordinate square planar and square pyramidal geometries, respectively. We further predict chemical reactions with high thermodynamic driving forces to guide their synthesis. These oxyfluorides are weakly correlated antiferromagnetic insulators, and their nonmagnetic phases exhibit quasi-two-dimensional Fermi surfaces dominated by Ni d_x²-y² states, which strikingly resemble undoped cuprate superconductors. We discuss how the oxyfluoride anion chemistry promotes Ni-O covalency and single-band character that is more similar to the cuprates than homoanionic infinite-layer nickelates. We use our understanding to propose doping strategies and layered LaSrNiO₂F₂ and La₃Ni₂O₄F₃ nickelate oxyfluorides with tunable electronic and magnetic structures for experimentation.