Electron ptychography reveals oxygen defects and their impact on superconductivity in bilayer nickelates

The discovery of high-temperature superconductivity in bilayer nickelate La₃Ni₂O_7-δ has opened a new frontier in correlated electron systems, inspiring intense efforts to unravel the origin and tunability of its superconducting state. Precise control of oxygen stoichiometry has proven essential: moderate oxidation strengthens superconductivity, whereas high-pressure oxygen annealing completely suppresses it. However, directly visualizing oxygen defects at the atomic scale has remained a longstanding challenge.

Using advanced electron microscopy—specifically, multislice electron ptychography (MEP)—we resolve the three-dimensional distribution of oxygen vacancies and interstitial oxygens in bilayer nickelates. In as-grown La₃Ni₂O_7-δ, oxygen vacancies predominantly occupy inner apical sites and exhibit pronounced nanoscale inhomogeneity, locally disrupting superconductivity. Pr substitution effectively suppresses these vacancies, yielding a nearly full-volume superconducting phase in La₂PrNi₂O₇. In contrast, high-pressure oxygen annealing introduces interstitial oxygens that self-organize into quasi-1D periodic stripes, inducing excess hole doping and an upward shift of the Ni d_z2 band from the Fermi level—thereby destroying superconductivity. These results directly reveal the crucial role of oxygen stoichiometry in nickelate superconductors and provide atomic-scale insights into optimizing their high-temperature superconducting phase.