Temperature-Induced Valence Transition in EuNi2(Si0.20Ge080)2 Studied by Hard X-ray Photoemission Spectroscopy

Full text: It is well known that many Eu compounds with the ThCr2Si2 tetragonal structure exhibit temperature dependence of the mean valence between Eu3+ (4f6) and Eu2+(4f7) configurations. EuNi2(Si1-xGex)2 (0.70 ≤ x ≤ 0.82) is one of the materials which exhibit a relatively sharp temperature induced valence transition. The mean valence of Eu ion in EuNi2(Si0.20Ge0.80)2 changes from nearly trivalent to nearly divalent around the critical temperature (Tv) of 80 K with in- creasing temperature, as detected by early Eu L edge x-ray absorption spectra (XAS). In this study to investigate Eu 3d states below and above Tv, hard x-ray photoemission spectroscopy (PES) was performed at 40 K and 120 K. The probing depth of the hard x-ray used (hv = 5.95 keV) being much larger than that of soft x-ray, the Eu 3d hard x-ray PES spectra showed clear features derived from chemically shifted divalent and trivalent states. The intensities of the Eu 3d spectra showed an obvious temperature dependence below and above Tv. The Eu 3d hard x-ray PES spectra of EuNi2(Si0.20Ge0.80)2 indicates that the mean valence at 40 K is about 2.70 and that at 120 K is 2.35 by using the integrated intensity of trivalent and divalent components, consistent with Eu L edge XAS. Eu 3d-4 f resonant PES indicates a dramatic enhancement of bulk Eu 4 f states, which shows a temperature dependence consistent with hard x-ray PES