Feyerherm, R.; Dudzik, E.; Prokes, K.; Mydosh, J. A.; Huang, Y. -K.; Poettgen, R.: Valence modulations in CeRuSn. Physical Review B 90 (2014), p. 041104(R)/1-4
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CeRuSn exhibits an extraordinary room temperature structure at 300 K with coexistence of two types of Ce ions, namely trivalent Ce3+ and intermediate valent Ce(4-x)+, in a metallic environment. The ordered arrangement of these two Ce types on speci c crystallographic sites results in a doubling of the unit cell along the c-axis with respect to the basic monoclinic CeCoAl-type structure. Below room temperature, structural modulation transitions with very broad hysteresis have been reported from measurements of various bulk properties. X-ray di raction revealed that at low temperatures the doubling of the CeCoAl type structure is replaced by a di erent modulated ground state, approximating a near tripling of the basic CeCoAl cell. The transition is accompanied by a signi cant contraction of the c axis. We present new x-ray absorption near-edge spectroscopy data at the Ce L3 absorption edge, measured on a freshly cleaved surface of a CeRuSn single crystal. In contrast to a previous report, the new data exhibit small but signi cant variations as function of temperature that are consistent with a transition of a fraction of Ce3+ ions to the intermediate valence state, analogous to the alpha-gamma transition in elemental cerium, when cooling through the structural transitions of CeRuSn. Such results in a valence-modulated state.