Mixed-valence phenomena occurring in the “black” $\left(B\right)$ and “gold” $\left(G\right)$ phases of ${\mathrm{Sm}}_{1-x}{\mathrm{Y}}_x\mathrm{S}$ have been studied by x-ray diffraction, x-ray absorption spectroscopy, and inelastic neutron scattering. Lattice-constant and phonon-dispersion results confirm that the valence instability occurs already inside the $B$ phase. On the other hand, pronounced temperature anomalies in the thermal expansion $\mathrm{<ruby><rb>\alpha </rb><rt>あるふぁ</rt></ruby>}\left(T\right)$, as well as in the Sm mean-square displacements, denote the onset of the $B\text{−}G$ transition for the compositions $x=0.33$ and 0.45. It is argued that these anomalies primarily denote an effect of electron-phonon coupling. The magnetic spectral response, measured on both powder and single crystals, is dominated by the ${\mathrm{Sm}}^{2+}$ spin-orbit component close to $36\mathrm{meV}$. A strongly overdamped ${\mathrm{Sm}}^{3+}$ contribution appears only for $x\ge 0.33$ near room temperature. The quasielastic signal is strongly suppressed below $70\mathrm{K}$, reflecting the formation of the singlet mixed-valence ground state. Quite remarkably, the signal around $36\mathrm{meV}$ is found, from the single-crystal spectra, to arise from two distinct, dispersive, interacting branches. The lower peak, confirmed to exist from $x=0.17$ to $x=0.33$ at least, is tentatively ascribed to an excitation specific to the mixed-valence regime, reminiscent of the “exciton” peak reported previously for ${\mathrm{SmB}}_6$.

• Received 17 March 2006

DOI:https://doi.org/10.1103/PhysRevB.74.035114