| Résumé | We present Mn K-edge x-ray absorption near-edge structure (XANES) data for a number of manganite systems as a function of temperature. The 1s absorption edge for the Ca-substituted samples is very sharp, almost featureless, and shifts uniformly upwards with increasing Ca content. The interpretation of this result is controversial because the lack of structure appears difficult to reconcile with a mixture of Mn⁺³ and Mn⁺⁴ ions or with several different Mn-O bond lengths at high T. We propose a possible solution in terms of covalency and considerable overlap of the Mn p states (mostly Mn 4p). The manganite preedge structure is quite similar to that for a large number of other Mn compounds, with two or three small peaks that are ascribed to 1s-3d weakly allowed dipole transitions plus possibly a small quadrupole component. The weak dipole transitions are explained as arising from a hybridization of the Mn 4p state of the excited atom with an odd symmetry combination of Mn 3d states on adjacent Mn atoms. The first preedge peak A₁ has a small shift to higher energy with increasing valence while the next peak A₂ is nearly independent of dopant concentration at 300 K. However, for the colossal magnetoresistance (CMR) samples the A₂ preedge peak shifts to a lower energy below the ferromagnetic transition temperature Tc, resulting in a decrease in the A₂-A₁ splitting by ~0.4 eV. This indicates a change in the higher-energy 3d bands, most likely the minority spin eg, plus some change in covalency. In addition, the amplitudes are temperature dependent for the CMR materials, with the change in A₁, A₂ correlated with the change in sample magnetization. For the charge ordered (CO) sample, the analysis suggests that the change in the preedge is produced by a distortion that increases below TCO. We discuss these results in terms of some of the theoretical models that have been proposed and other recent XANES studies. |
|---|
