Mössbauer Isomer Shifts inSm149Compounds

The isomer shift (IS) of the 22.5-keV $\gamma$ transition of ${\mathrm{Sm}}^{149}$ has been studied for various ionic, semiconducting, and metallic compounds using Mössbauer spectroscopy. We found changes as large as 0.9 mm/sec between trivalent and divalent compounds and 0.3 mm/sec between metallic and ionic trivalent compounds. The divalent compounds show a range of IS due to covalency effects, and the intermetallic compounds, which all have Sm in the trivalent state, show the extra electronic density due to the conduction electrons. The IS obtained for the semiconducting Sm${\mathrm{B}}_6$, -0.4 mm/sec, and for the chalcogenide ${\mathrm{Sm}}_3$ ${\mathrm{S}}_4$, -0.19 mm/sec, are anomalous. We calculated the difference in the mean-square charge radius between the 22.5-keV state and the ground state $\delta 〈{\mathcal{r}}^2〉=1.2\mathrm{}\times {10}^{-3\mathrm{}}$ ${\mathrm{fm}}^2$. We also found the conduction-electron density at the nucleus of Sm metal ${\left|\Psi \right|}^2\left(\mathrm{CE}\right)=0.95\mathrm{}\times {10}^{26}$ ${\mathrm{cm}}^{-3\mathrm{}}$. The solid-state and nuclear results are discussed in the light of current theories.