Relationship between the structural distortion and the Mn electronic state in La_1-xCa_xMnO₃: a Mn K-edge XANES study

A theoretical study of the X-ray absorption near-edge structure (XANES) spectra at the Mn K-edge in the La_1-xCa_xMnO₃ series is reported. The relationship between the edge shift, the Ca-La substitution and the distortion of the MnO₆ octahedra in these systems has been studied. It is shown that, by correctly considering these effects simultaneously, the experimental XANES data are consistent with the presence of two different Mn local environments in the intermediate La_1-xCa_xMnO₃ compounds. By taking into account the energy shift associated with the modification of the MnO₆ distortion as Ca substitutes for La, it is possible to reproduce the XANES spectra of the intermediate-doped compounds starting from the experimental spectra of the end-members LaMnO₃ and CaMnO₃. These results point out the need to re-examine the conclusions derived in the past from the simple analysis of the Mn K-edge XANES edge-shift in these materials. In particular, it is shown that the modification of the Mn K-edge absorption through the La_1-xCa_xMnO₃ series is well reproduced by considering the simultaneous presence of both distorted and undistorted octahedra and, consequently, that the existence of charge-ordering phenomena cannot be ruled out from the XANES data.