Fig. 4. Outline of the multistage reduction process of nickelate films and high-resolution STEM images of a partially reduced film showing 3a0 ordering.
a, Schematic of the reduction pathway from the perovskite NdNiO3 to the intermediate oxygen-deficient phases of Nd3Ni3O7 or Nd3Ni3O8 and the infinite-layer NdNiO2; the corresponding samples measured in this study that lie on this pathway are also shown. b–d, STEM images of a mixed-phase sample (sample J) containing Nd3Ni3O7,8 intermediate phases. Fourier transform image (b) and corresponding high-angle annular dark-field image (c) of a defective region are shown. The positions of the third-order peaks in the Fourier transform are circled in green. An annular bright-field image showing two filled rows of apical oxygens (d), followed by one row of missing apical oxygen positions, corresponding to the schematic for Nd3Ni3O8 in a. In this image, the apical oxygen chains run into the page.