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要旨:
Non-centrosymmetric EuT Ge-3 (T = Co, Ni, Rh, and Ir) possesses magnetic Eu2+ ions, and antiferromagnetic ordering appears at low temperatures. Transition-metal substitution leads to changes in the unit-cell volume and in the magnetic ordering. However, the magnetic ordering temperature does not scale with the volume change, and the Eu valence is expected to remain divalent. Here we study the bulk electronic structure of non-centrosymmetric EuT Ge-3 (T = Co, Ni, Rh, and Ir) by hard x-ray photoelectron spectroscopy. The Eu 3d core-level spectrum confirms the robust Eu2+ valence state against the transition-metal substitution with a small contribution from Eu3+. The estimated Eu mean valence is around 2.1 in these compounds, as confirmed by multiplet calculations. In contrast, the Ge 2p spectrum shifts to higher binding energy upon changing the transition metal from 3d to 4d to 5d elements, hinting at a change in the Ge-T bonding strength. The valence bands of the different compounds are found to be well reproduced by ab initio band structure calculations.
