From perovskite to infinite-layer nickelates: Hole concentration from x-ray absorption

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 PrNiO$_x$ 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 exhibits a pure d$^9$ configuration. Our quantitative analysis using the charge sum rule shows that even when films are maximally reduced, the spatially averaged number of nickel 3d holes is 1.38. 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, structural defects, and oxygen nonstoichiometry.