Nickelates have emerged alongside cuprates as an important class of materials for studying unconventional superconductivity. The synthesis of infinite-layer nickelates, which appear to be most similar to high-Tc cuprates, requires a complex topochemical process that makes it difficult to determine the stoichiometry. For this reason, the valence electron configuration of nickel in the superconducting phase has not been conclusively determined experimentally.

In my talk, I will present our recent findings from x-ray spectroscopy. In superlattices with repeating interfaces of NdNiOx-SrTiO3 heterostructures, we observed modulations in the depth-resolved nickel valence configuration due to a reconstructed ligand field at the interface to SrTiO3 and considerable disorder in the oxygen removal sites [1]. In PrNiOx thin films at various intermediate stages of topotactic reduction with x = 2-3, we find that even the most reduced films do not exhibit a pure Ni1+/3d9 configuration [2]. The quantitative analysis shows that there is an average of 1.35 holes in the nickel 3d states and superconducting samples have even higher values. These results call into question previous findings regarding the doping range within which superconductivity occurs in infinite-layer nickelates and/or suggests a different doping mechanism in compounds without alkaline-earth doping.

[1] R. A. Ortiz et al., Phys. Rev. Materials 9, 054801 (2025), [2] R. Pons et al., arXiv:2601.07710v2 (2026)