We have determined the interconfiguration mixing ratios (${\mathrm{Sm}}^{+3\mathrm{}}$/${\mathrm{Sm}}^{+2\mathrm{}}$) of 1.4 and 4.1 for ${\mathrm{Sm}}_{0.81}$${\mathrm{Y}}_{0.19}$S and Sm${\mathrm{As}}_{0.18}$${\mathrm{S}}_{0.82}$ 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.

• Received 17 June 1974

DOI:https://doi.org/10.1103/PhysRevLett.33.820