EXCHANGE INTERACTIONS, CHARGE DELOCALIZATION, AND SPIN RELAXATION IN A MIXED-VALENCE DIIRON COMPLEX STUDIED BY MOSSBAUER-SPECTROSCOPY

Exchange interactions and charge transfer in the Fe2+Fe3+ of the mixed valence [Fe2S2 (dimethylmethanebisbenzimidazolate)2]3- trianion have been studied by analysis of Mossbauer spectra in the temperature range of 1.5-180 K and in applied fields of 10 mT, 0.35 T, and 6.2 T. The l ow-temperature spectra reveal a ground state with total spin S(t) = 1/ 2 and hyperfine parameters intermediate between values for a Fe2+Fe3localized mixed-valence pair and a fully delocalized system where the two iron atoms are equivalent. A consistent set of hyperfine parameter s has been derived by fitting the spectra with a stochastic relaxation model taking into account spin relaxation in the S(t) = 1/2 state and electron hopping between the iron ions. An interpretation of the valu es of the hyperfine parameters has been given by solving a spin Hamilt onian, which includes antiferromagnetic and double exchange in an asym metric Fe2+Fe3+ pair and which allows partial electron delocalization. Using the value a2 = 0.8 for the delocalization coefficient and an es timate of DELTA = 105 cm-1 for the difference between the S(t) = 1/2 a nd the first excited S(t) = 3/2 state we have derived limits for the e xchange-coupling constant J, the double-exchange parameter B, and the energy difference E(A) - E(B) arising from the two possible configurat ions Fe(A)2+Fe(B)3+ and Fe(A)3+Fe(B)2+, i.e., 70 cm-1 less than or sim ilar to J less than or similar to 300 cm-1, 0 < Absolute value of B le ss than or similar to 395 cm-1, and 0 < \E(A) - E(B)\ less than or sim ilar to 590 cm-1.