Fig. 1.

Lattice and electronic structure of ${\mathrm{N}\mathrm{d}\mathrm{N}\mathrm{i}\mathrm{O}}_y$ thin films. (A) Atomic-resolution HAADF-STEM imaging of an undoped ${\mathrm{N}\mathrm{d}\mathrm{N}\mathrm{i}\mathrm{O}}_2$ film shows mostly well-ordered epitaxial structure with some visible crystalline defects. (B) STEM-EELS Nd, Ni, and Ti elemental mapping confirms an abrupt interface with ${\mathrm{S}\mathrm{r}\mathrm{T}\mathrm{i}\mathrm{O}}_3$. (C) Observed variations in O-K near-edge fine structure across different regions in the same reduced nickelate film (blue) necessitate local measurements to avoid contributions from defects and partially unreduced regions. For comparison, reference spectra from bulk oxides are also shown. (D) EELS O-K edge of ${\mathrm{N}\mathrm{d}\mathrm{N}\mathrm{i}\mathrm{O}}_2$ and a ${\mathrm{N}\mathrm{d}\mathrm{N}\mathrm{i}\mathrm{O}}_3$ film before reduction to the infinite-layer phase. The disappearance of the first peak (highlighted in gray), ascribed to metal-oxygen hybridization, indicates filling of the Ni states upon reduction from $d^7$ to $d^9$. (Scale bars, 2 nm.)