An unresolved question in solid state physics concerns the nature of charge transfer mechanisms in the late transition metal compounds. This paper shows that a structure similar to the doubly peaked feature detected in the Ni 2p X-ray photoelectron spectra of NiO and attributed to non-local screening is observed also in the Ni 3s spectrum. However, in this case the spectrum is fully accounted for on the basis of charge transfer, near degeneracy, and exchange mechanisms and neglecting non-local screening. In addition, it is demonstrated that non-local screening mechanisms are not observed in La2NiO4 where such an effect, on the basis of the Ni-O octahedra network, is expected. Finally, the significant differences among the Ni 2p XPS spectra in NiO, Ni0.1Mg0.9O and La2NiO4 suggest that core line spectra in nickel oxide based compounds cannot be evaluated simply on the basis of the Ni-O cluster network but it is necessary to consider the long range crystal structure effects.
