Mössbauer effect study of filled antimonide skutterudites

The iron-57 M\"ossbauer spectra of a series of Ce-filled antimonide skutterudites, CeₓFe_4-yCoySb₁₂, where x varies from 0.22 to 0.98 and y varies from 0 to 3.5, have been measured at 295 K. In addition, the spectra of Ce0.60Fe₂Co₂Sb₁₂ and Ce0.82Fe₃CoSb₁₂ have been measured from 85 to 295 K and the spectra of Ce0.35FeCo₃Sb₁₂ and Ce0.98Fe₄Sb₁₂ have been measured from 4.2 to 295 K. The spectra, all of which consist of a simple quadrupole doublet, show no evidence for any long-range magnetic ordering of any Fe magnetic moments. The 295-K quadrupole splitting increases linearly from 0.104 mm/s at $x=0.22$ to 0.415 mm/s at $x=0.98,$ an increase which results from the increasing hole concentration in the valence band of these compounds. The quadrupole splitting of approximately 0.16 mm/s observed for Ce0.35FeCo₃Sb₁₂ is virtually independent of temperature between 4.2 and 295 K and results predominantly from a lattice contribution to the electric-field gradient. In contrast, in Ce0.98Fe₄Sb₁₂, the quadrupole splitting is constant at approximately 0.49 mm/s below approximately 70 K, and decreases with T3/2 above 70 K. The 295-K isomer shift increases linearly from 0.345 mm/s at $x=0.22$ to 0.386 mm/s at $x=0.98,$ an increase which results from a linear expansion of the unit-cell volume with increasing x. The temperature dependencies of the isomer shift and of the logarithm of the spectral absorption area in Ce0.98Fe₄Sb₁₂ and Ce0.35FeCo₃Sb₁₂ yield effective M\"ossbauer temperatures of approximately 500 and 400 K, respectively, values which are substantially higher than the Debye temperatures of approximately 300 K observed for these compounds. These differences indicate that the high-frequency vibrations of the iron sublattice are weakly coupled to the lower frequency vibrations of both the partially filled cerium sublattice and the antimony sublattice.