ANALYSIS OF THE X-RAY-ABSORPTION NEAR-EDGE-STRUCTURE SPECTRA OF LA1-XNDXNIO3 AND LANI1-XFEXO3 PEROVSKITES AT THE NICKEL K-EDGE

We present here the experimental and theoretical analysis of the x-ray near-edge absorption spectra of LaNiO3, NdNiO3, Nd1-xLaxNiO3, and NdN i1-xFexO3 solid solutions at the Ni K edge. All the compounds show ver y similar experimental data characterized by the presence of a clear p reedge feature that lies at about 16 eV below the main first peak of t he edge. In order to clarify either the origin of this peak and the ot her part of the spectra, several theoretical calculations have been ma de on the basis of multiple-scattering theory, using a self-consistent charge density and potential in different atomic clusters. Good agree ment with experimental data has been obtained and the analysis indicat es that the anomalous preedge structure can be only explained on the b asis of a large mixing of 3d(7) and 3d(8)L nickel configurations in th e ground state.