Valence modulations in CeRuSn

CeRuSn exhibits an extraordinary room temperature structure at 300 K with the coexistence of two types of Ce ions, namely trivalent ${\mathrm{Ce}}^{3+}$ and intermediate-valent ${\mathrm{Ce}}^{(4-\delta )+}$, 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 ${\mathrm{L}}_3$ absorption edge, measured on a freshly cleaved surface of a CeRuSn single crystal. In contrast to our previous report, the new data exhibit small but significant variations as a function of temperature that are consistent with a transition of a fraction of ${\mathrm{Ce}}^{3+}$ ions to the intermediate valence state, analogous to the $\gamma \to \alpha$ transition in elemental cerium, when cooling through the structural transitions of CeRuSn. Such results in a valence-modulated state.

Figure 1

Experimental XANES spectra at the Ce ${\mathrm{L}}_3$ edge in CeRuSn at 100 K and 300 K. The ratio of intensities of the spectral features indicated by $4f^1$ and $4f^0$ allows an estimate of the ${\mathrm{Ce}}^{3+}$/${\mathrm{Ce}}^{4+}$ ratio and thus of the average Ce valence. The difference between the 100 K and 300 K data is highlighted by the difference curve, where a second data set taken at 200 K is also shown. The $\approx 3\%$ reduction of the $4f^1$ at the expense of the $4f^0$ feature observed at 100 K and 200 K is associated with a change of the average valence in the $\approx$3$c$ modulated ground state with respect to the $2c$ room temperature phase.