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Mixed-valence phenomena occurring in the "black" ͑B͒ and "gold" ͑G͒ phases of Sm 1−x Y x 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 ␣͑T͒, as well as in the Sm meansquare 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 Sm 2+ spin-orbit component close to 36 meV. A strongly overdamped Sm 3+ contribution appears only for x Ն 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 SmB 6 .

Samarium chalcogenides undergo isostructural phase transitions under pressure and samarium ions turn into an intermediate valency state (IVS).The thermo EMF S and electroresistance p of SmS, SmSe and SmTe under pressure to 12 GPa and in the temperature range 30-400 "C have been measured to investigate properties of the IVS and its stability limits. Hydrostatic pressure to 8 GPa and quasi-hydrostatic pressure to 12 GPa on monocrystals have been realised in 'toroid' cells. It is found that the IVS is stabilised after phase transitions at pressures of 0.65 GPa in SmS, 3.4 GPa in SmSe and 5.2 GPa in SmTe. Transition to IVS is accompanied by sharp anomalies in p and S. It is shown that IVS exists in a wide pressure range (at least to 12 GPa). The zone structure model for these compounds is suggested.

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