Topochemical Oxidation of Ruddlesden–Popper Nickelates Reveals Distinct Structural Family: Oxygen-Intercalated Layered Perovskites

Layered perovskites─including the Dion-Jacobson, Ruddlesden-Popper, and Aurivillius families─exhibit a wide range of correlated electron phenomena, from high-temperature superconductivity to multiferroicity. Here, we report a new family of layered perovskites realized through topochemical oxidation of La_<i>n</i>+1Ni_<i>n</i>O_{3<i>n</i>+1+δ} (<i>n</i> = 1-4) Ruddlesden-Popper nickelate thin films. Postgrowth ozone annealing induces a substantial <i>c</i>-axis expansion─17.8% for La₂NiO_4+δ (<i>n</i> = 1)─that monotonically decreases with increasing <i>n</i>. Surface synchrotron X-ray diffraction and coherent Bragg rod analysis (COBRA) reveal that this structural expansion arises from the intercalation of approximately δ ≈ 0.7-1.0 oxygen atoms into interstitial sites within the rock salt spacer layers, far exceeding the previous record of δ ≈ 0.3 for any Ruddlesden-Popper oxide. These oxygen-intercalated phases form a new class of layered perovskites with a spacer layer composition intermediate between the Ruddlesden-Popper and Aurivillius phases. Furthermore, oxygen intercalation induces metallicity, enhances nickel-oxygen hybridization, and suppresses oxygen octahedral rotations, a feature associated with high-temperature superconductivity in Ruddlesden-Popper nickelates. Our work establishes topochemical oxidation as a powerful approach to accessing highly oxidized, metastable phases across a broad range of layered oxide systems, offering new platforms to engineer electronic properties via intercalation chemistry.