Electronic configuration of samarium sulphide and related compounds: Mössbauer-effect measurements and a model

${}_{}{}^{149}{}_{}{}^{}\mathrm{Sm}$ Mössbauer spectra have been measured at room temperature for SmS under applied pressures $P=0-11\mathrm{}$ kbar and for ${\mathrm{Sm}}_{1-x}{\mathrm{Y}}_x\mathrm{S}$, $0\le x\le 0.34$. There is a sharp increase in isomer shift at $P=6\mathrm{}$ kbar and $x=0.15\mathrm{}$, respectively, accompanying the transformation from the black to the gold phase, but there is no corresponding increase in linewidth. The samarium configuration, assumed to be a mixture of $4f^6$ and $4f^{5}5d^1$, is deduced as a function of $P$ or $x$. The results suggest that the $5d$ electron in the intermediate valence configuration is rather localized, but they are insensitive to whether the lattice is rigid or relaxed. Any fluctuations between the two configurations are more rapid than ${10}^{-9\mathrm{}}$ sec. Caculations have been made with a Falicov Hamiltonian in which a term for $4f-5d$ hybridization is included. The model accounts for the observed variations of the samarium electron configuration. It is demonstrated, both theoretically and experimentally, that yttrium doping is not simply equivalent to pressure.