Mössbauer Effect Studies of Interconfiguration Fluctuations in Metallic Rare Earth Compounds

Recoilless absorption spectra of the 21.6 keV gamma ray of IslEu in EuCu~Si2 and in EuRh2 reveal a single absorption line which has a strongly temperature dependent isomer shift. The recoilless absorption spectra in E ~ ~ L a l ~ R h 2 (0.075 < x < 0.5), Eu.125Pr.87~Rh2 and E u R h ~ ~ p t ~ (0.125 < x < 1) are composed of a few lines, many of which also exhibit strongly temperature dependent isomer shifts. Each line in these spectra is attributed to a different Eu site characterized by the number of La, Pr or Pt nearest neighbors. The isomer shifts of many of the lines correspond to neither Eu3f nor E u ~ + valencies, but fall in between. All the results are interpreted in terms of fast fluctuations (z < 4 x 10-11 s) between the 4f6 and 4f7 configurations. The energy necessary to make an interconfiguration excitation from 4f6 to 4f7, by removing an electron from the conduction band at the Fermi level and placing it in the localized 4f level, is 800 K in EuCu2Si~ and 1 200 K in EuRhz. In the ternary compounds, this energy depends significantly on the number of La, Pr or Pt nearest neighbors of the Eu ions, and is somewhat temperature dependent. The localized 4f levels have an energy width of about 100 K. The recoilless absorption spectra in Yb metal of the 84 keV gamma ray of 17oYb emitted by a 170TmxHo~-xFe2 source are composed of two subspectra, one of which is a 5 line split spectrum and the other is a broadened line at zero velocity. These spectra are also interpreted in terms of interconfiguration fluctuations (4f14 tt4fl').