Transition to intermediate valence state and x-ray photoemission in Sm/sub 1-x/Gd/sub x/S

We report a systematic x-ray photoemission study of the alloys Sm1/sub -//subx/Gd/subx/S for 0 < or = x < or = 15.5% which exhibit a first-order, black-to-gold phase transition, at x/subc/ approx. 16%, analogous to the one reported for SmS under pressure. We show that substitutional Gd produces a shift of the 4f levels of neighboring Sm atoms of about 0.5 eV, which to good approximation is independent of x. This allows us to obtain a reasonable fit to the Sm2+ → Sm3+ photoemission line shape using a model which has some similarities with the Jaccarino-Walker model for magnetic alloys. It assumes the existence of only two different kinds of Sm2+ ions in Sm1/sub -//subx/Gd/subx/S for x approximately-less-than 16%. We discuss possible reasons for the fact that Gd does not show the usual clustering effects known to occur in many substitutional rare-earth alloys. Lattice-constant measurements for various substituents further illustrate the importance of electronic effects in the phase transition