  Mixed-valence phenomena occurring in the "black" (B) and "gold" (G) phases of
Sm1-x Yx 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 \alpha(T), as well as in the Sm mean-square displacements
denote the onset of the B-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 Sm2+ spin-orbit component close to 36 meV. A
strongly overdamped Sm3+ contribution appears only for x >= 0.33 near
room-temperature. The quasielastic signal is strongly suppressed below 70 K,
reflecting the formation of the singlet mixed-valence ground state. Quite
remarkably, the signal around 36 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 SmB6 .
