Heat capacity and phase transitions of the mixed-valence compound [Fe3O(O2CCH3)6(py)3](py)

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Sorai, Michio; Kaji, Kazutoshi; Hendrickson, David N.; Oh, Seung M.
Issue Date
American Chemical Society
J. Am. Chem. Soc. 108, 1986, 702
The heat capacity under constant pressure, C,, of oxo-centered mixed-valence [ Fe30(02CCH3)6(py)3(p] y) has been
measured with an adiabatic calorimeter between 12 and 300 K. Four phase transitions were found to occur at 11 1.4, 112.0,
185.8, and 191.5 K. These transitions can be classified basically into two groups: one is a lower-temperature phase transition
with two C, peaks in the 11 1-1 12 K region and the other is a higher-temperature phase transition evolving from - 11 5 K
to culminate in two C, peaks in the 185-191 K region. The total enthalpy and entropy of these phase transitions were determined
by estimating plausible normal heat capacities to give 4940 J mol-' and (30.58 f 0.83) J K-' mol-', respectively. By comparing
the present results with the 57Fe Mossbauer spectroscopy and the X-ray structural work reported for this complex, it is concluded
that these phase transitions are associated with the intramolecular electron transfer in the mixed-valence Fe30 complexes and
the orientational disordering of the pyridine solvate molecules about the crystallographic C3 axis. DTA data for the solid solutions,
[Fe"',Fe'11,Co*',o(02ccH3)6(py)3](py), indicate that the phase transition having a C, peak at 185.8 K is due to the orientational
order-disorder phenomenon associated with the pyridine solvate molecules. The observed transition entropy is much larger
than the value (RI n 3) expected to result from electronic delocalization in the Fe1112Fe1t1r iads and the orientational disorder
(R In 3) about the C3 axis of the pyridine solvate molecules. Two possibilities to account for the remaining entropy are discussed:
one is associated with changes in the orbital degeneracy of the Fe(I1) ion and the other is the rotation of the pyridine solvate
molecules about their pseudo-C6 axes.
0002-7863 (print)1520-5126 (online)
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College of Engineering/Engineering Practice School (공과대학/대학원)Dept. of Chemical and Biological Engineering (화학생물공학부)Journal Papers (저널논문_화학생물공학부)
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