Temperature- and Composition-Dependent Valence Mixing of Sm in Cation-and Anion-Substituted SmS Observed by X-ray Photoemission Spectroscopy

We have determined the interconfiguration mixing ratios (Sm⁺³/Sm⁺²) of 1.4 and 4.1 for Sm0.81{Y}0.19$S and Sm${As}0.18S0.82 at 295 K from their x-ray photoemission spectra. These ratios are observed to decrease reversibly by a factor of 2 at 110 K. The agreement of these results with valences deduced from lattice-constant measurements is direct evidence that configuration mixing is responsible for the anomalous temperature- and composition-dependence of the lattice constant.