The difficulty of determining cation concentrations and oxygen stoichiometry in infinite-layer nickelate thin films has so far prevented clear experimental identification of the nickel electron configuration in the superconducting phase. We used soft x-ray absorption spectroscopy to study the successive changes in PrNiOx thin films at various intermediate stages of topotactic reduction with x=2−3. By comparing the Ni-L edge spectra to single and double cluster ligand-field calculations, we find that none of our samples exhibit a pure d9 configuration. Our quantitative analysis using the charge sum rule shows that even when films are maximally reduced, the averaged number of nickel 3d holes is 1.35. Superconducting samples have even higher values, calling into question the previously assumed limit of hole doping. Concomitant changes in the oxygen K-edge absorption spectra upon reduction indicate the presence of oxygen 2p holes, even in the most reduced films. Overall, our results suggest a complex interplay of hole doping mechanisms resulting from self-doping effects and oxygen non-stoichiometry.
@article{arxiv.2601.07710,
title = {From perovskite to infinite-layer nickelates: hole concentration from x-ray absorption},
author = {R. Pons and M. Flavenot and K. Fürsich and E. Schierle and E. Weschke and M. R. Cantarino and E. Goering and P. Nagel and S. Schuppler and G. Kim and G. Logvenov and B. Keimer and R. J. Green and D. Preziosi and E. Benckiser},
journal= {arXiv preprint arXiv:2601.07710},
year = {2026}
}