  The metal-insulator transitions and the intriguing physical properties of
rare-earth perovskite nickelates have attracted considerable attention in
recent years. Nonetheless, a complete understanding of these materials remains
elusive. Here, taking a NdNiO3 thin film as a representative example, we
utilize a combination of x-ray absorption and resonant inelastic x-ray
scattering (RIXS) spectroscopies to resolve important aspects of the complex
electronic structure of the rare-earth nickelates. The unusual coexistence of
bound and continuum excitations observed in the RIXS spectra provides strong
evidence for the abundance of oxygen 2p holes in the ground state of these
materials. Using cluster calculations and Anderson impurity model
interpretation, we show that these distinct spectral signatures arise from a Ni
3d8 configuration along with holes in the oxygen 2p valence band, confirming
suggestions that these materials do not obey a conventional positive
charge-transfer picture, but instead exhibit a negative charge-transfer energy,
in line with recent models interpreting the metal to insulator transition in
terms of bond disproportionation.
