Hard x-ray photoemission study of the temperature-induced valence transition system EuNi2(Si1-xGex)2

We investigated the bulk-derived electronic structure of the temperature-induced valence transition system EuNi2(Si1-xGex)2 (x=0.70, 0.79, and 0.82) by means of hard x-ray photoemission spectroscopy (HAXPES). The HAXPES spectra clearly show distinct temperature dependencies in the spectral intensities of the Eu2+ and Eu3+3d components. For x=0.70, the changes in the Eu2+ and Eu3+3d spectral components with temperature reflect a continuous valence transition, whereas the sudden changes for x=0.79 and 0.82 reflect first-order valence transitions. The Eu 3d spectral shapes for all x and particularly the drastic changes in the Eu3+3d feature with temperature are validated by a theoretical calculation based on the single-impurity Anderson model (SIAM). SIAM analysis reveals that the valence transition for each x is controlled by the c-f hybridization strength and the charge-transfer energy. Furthermore, the c-f hybridization strength governs the valence transition of this system, which is either first order or continuous, consistent with Kondo volume collapse.