Mössbauer Effect Study of Fe₃(CO)₁₂

The Mössbauer effect spectrum of Fe ₃ (CO) ₁₂ has been measured from 4.2 to 295 K, and the results, when analyzed in terms of the two expected quadrupole doublets, reveal an unusual temperature dependence, first in the quadrupole interaction for the unique nonbridged iron site, second in the relative area of the two components of the quadrupole doublet associated with the two chemically equivalent carbonyl-bridged iron sites, and third in the relative area of these two quadrupole doublets. The first of these may be understood in terms of a negative quadrupole interaction for the unique iron site, produced principally by a negative, temperature-dependent, valence contribution to the electric field gradient. The second results from the large vibrational anisotropy associated with the bridged iron sites, which leads to the Goldanskii-Karyagin effect and an asymmetric doublet. An analysis based on the single-crystal thermal ellipsoid anisotropy of the bridged iron sites is successful in explaining the observed asymmetry. The third results from the different temperature dependence of the recoil-free fractions observed for the two chemically quite dissimilar iron sites.