Valence modulations in CeRuSn

TL;DR

CeRuSn exhibits temperature-dependent structural and valence state transitions involving coexistence and modulation of Ce ions, with new spectroscopic evidence showing partial valence change during these transitions.

Contribution

This study provides new x-ray absorption data revealing valence state variations in CeRuSn across structural transitions, highlighting a valence-modulated state.

Findings

Valence state of Ce ions varies with temperature.

Structural modulation transitions involve Ce valence changes.

New spectroscopic evidence supports a valence transition similar to cerium's gamma-alpha transition.

Abstract

CeRuSn exhibits an extraordinary room temperature structure at 300~K with coexistence of two types of Ce ions, namely trivalent Ce$^{3+}$ and intermediate valent Ce$^{(4-\delta)+}$, in a metallic environment. The ordered arrangement of these two Ce types on specific 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 diffraction revealed that at low temperatures the doubling of the CeCoAl type structure is replaced by a different modulated ground state, approximating a near tripling of the basic CeCoAl cell. The transition is accompanied by a significant contraction of the $c$ axis. We present new x-ray absorption near-edge spectroscopy data at the Ce L$_{3}$ 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 significant variations as function of temperature that are consistent with a transition of a fraction of Ce$^{3+}$ ions to the intermediate valence state, analogous to the $\gamma \rightarrow \alpha$ transition in elemental cerium, when cooling through the structural transitions of CeRuSn. Such results in a valence-modulated state.