Change in Eu 3d hard X-ray photoelectron spectra through the temperature-induced valence transition of EuPd2Si2

Full text : EuPd2Si2 undergoes an abrupt valence transition at around 160 K: The Eu mean valence changes from 2.8 below 130 K to 2.3 above 180 K. In order to understand the mechanism of valence transition, it had been investigated the electronic structure of EuPd2Si2 by means of temperature-dependent hard X-ray photoelectron spectroscopy (HX-PES). The HX-PES experiments were performed at the undulator beamline BL15XU of SPring-8. The temperature-dependent HX-PES spectra clearly show the valence transition, namely, the intensities of the divalent and trivalent Eu 3d components are abruptly changed, as shown in this article. The Eu 3d spectral shape, especially the drastic change in trivalent Eu 3d feature with temperature, cannot be reproduced by the atomic calculation that had been utilized for the spectral analysis of Eu compounds so far. Therefore, there were done some calculations based on the Anderson model in which the hybridization of the trivalent Eu 4f electrons with the conduction electrons and the f-f hybridization between the divalent and trivalent Eu with anisotropic symmetry are taken into account, and it became possible to reproduce the temperature dependence of Eu 3d spectral features reasonably well. According to the model calculation, the change in the trivalent Eu 3d spectral shape through the valence transition is closely related to the degree of admixture of the excitation state and the ground states of trivalent Eu