The recoilless-absorption spectra of the 21.6-keV $\gamma$ rays of ${\mathrm{Eu}}^{151}$ in ${\mathrm{Eu}}_x{\mathrm{La}}_{1-x}{\mathrm{Rh}}_2$ are composed of a few lines which exhibit strongly temperature-dependent isomer shifts. The results are interpreted in terms of fast fluctuations of electrons between a localized $4f$ level and the conduction band. The energy released by removing an electron from the $4f$ level to the conduction band (0.01-0.1 eV) depends significantly on the number of La nearest neighbors. The levels have a width of ∼ 0.01 eV.

• Received 17 June 1974

DOI:https://doi.org/10.1103/PhysRevLett.33.890