Electronic structure of SmS doped with divalent and trivalent ions

We have studied SmS doped with Ca and trivalent ions via the Mössbauer spectroscopy of divalent ${}_{}{}^{153}{}_{}{}^{}\mathrm{Eu}$ impurity probe ions. The isomer shift of the Mössbauer resonance provides information on the electron density changes at the Eu nucleus, and these can be analyzed to show the importance of the different mechanisms involved as the electronic structure of the SmS changes with doping. After removing the effects of lattice dilatation, we find that in the semiconducting phase, the "extra" electron is mostly localized on the trivalent ion, and the electron density at the Eu decreases with doping. In the metallic phase, the electron density increases with doping, which we explain by a small $6s$ component in the conduction band. At the metallic transition, the only electron density change is that arising from the volume change, showing that the conduction band has little $s$ character. Other recent experiments on SmS are discussed in the light of the model developed here. Small amounts of ${\mathrm{Eu}}^{3+}$ are observed as after effects of the radioactive decay. Magnetic ordering is observed at 6 K in samples heavily doped with Gd. Mössbauer resonance lines of some of the samples are broadened due to incompletely relaxed paramagnetic hfs. The observed dependence of the linewidth on doping is qualitatively explained by considering the different spin-relaxation channels opened by the different dopants.