Electronic and Magnetic Properties of Nickel Oxychlorides with Tetragonal Crystal Structures

The discovery of pressure-induced superconductivity in Ruddlesden–Popper nickelates has motivated the search for ambient-pressure tetragonal superconductors. Using first-principles calculations, we investigate the electronic and magnetic properties of three-layered nickel oxychlorides: Sr 2 NiO 3 Cl, Sr 3 Ni 2 O 5 Cl 2, and the hypothetical reduced phase Sr 3 Ni 2 O 4 Cl 2 . In agreement with the previous studies, although ferromagnetic interactions predominate in Sr 2 NiO 3 Cl, the underlying mechanism is governed by the Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction rather than superexchange, which is attributed to the coexistence of localized 3d z 2 moments and the itinerant 3d x 2– y 2 -derived carriers. In Sr 3 Ni 2 O 5 Cl 2, the dominant RKKY interaction within the bilayer perovskite blocks combined with strong antiferromagnetic (AFM) superexchange coupling along the c axis yields A-type AFM ordering. Furthermore, the uniform Ni 3+ oxidation state and pronounced buckling of the NiO 2 basal plane distinguish it from high-pressure superconductor La 3 Ni 2 O 7, which might be one reason potentially contributing to the absence of superconductivity. Removal of the inner-apical oxygen from the precursor Sr 3 Ni 2 O 5 Cl 2 to form Sr 3 Ni 2 O 4 Cl 2 not only converts the Ni oxidation state but also profoundly alters the electronic and magnetic properties, inducing G-type AFM ordering and insulating behavior. Collectively, these results reveal profound tunability of the electronic and magnetic properties in layered nickel oxychlorides and may offer valuable insights for designing ambient-pressure tetragonal new Ni-based potential superconductors.