Valence transitions of Sm in monosulfide solid solutions

Magnetic-susceptibility and lattice-parameter measurements indicate that Sm in ${\mathrm{Sm}}_{1-x}{\mathrm{Y}}_x\mathrm{S}$ for $x>\mathrm{}0.15\mathrm{}$, in ${\mathrm{Sm}}_{1-x}{\mathrm{La}}_x\mathrm{S}$ for $x>\mathrm{}0.33\mathrm{}$, and in ${\mathrm{Sm}}_{1-x}{\mathrm{Gd}}_x\mathrm{S}$ for $x>\mathrm{}0.17\mathrm{}$ is in an intermediate valence state at $T=295\mathrm{}$ K. A transition from this intermediate valence state to a more divalent but still intermediate valence state is observed on cooling and on heating from room temperature in ${\mathrm{Sm}}_{1-x}{\mathrm{Y}}_x\mathrm{S}$ for $0.15<x<\mathrm{}0.27\mathrm{}$. For $x>\mathrm{}0.47\mathrm{}$, Sm remains intermediate valent on cooling to $T=4.2\mathrm{}$ K and on heating to $T=900\mathrm{}$ K. Its susceptibility is independent of temperature below $T=100\mathrm{}$ K. Related effects are observed in the ${\mathrm{Sm}}_{1-x}{\mathrm{La}}_x\mathrm{S}$ and in the ${\mathrm{Sm}}_{1-x}{\mathrm{Gd}}_x\mathrm{S}$ systems. A model of interconfiguration fluctuations including multiple energy levels in both configurations and crystal-field effect may explain these results.