Exchange interactions, charge delocalization, and spin relaxation in a mixed-valence di-iron complex studied by Mössbauer spectroscopy

Abstract

Exchange interactions and charge transfer in the Fe2+Fe3+ pair of the mixed valence [Fe2S2(dimethylmethanebisbenzimidazolate)2]3− trianion have been studied by analysis of Mössbauer 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 low-temperature spectra reveal a ground state with total spin St=1/2 and hyperfine parameters intermediate between values for a Fe2+Fe3+ localized mixed-valence pair and a fully delocalized system where the two iron atoms are equivalent. A consistent set of hyperfine parameters has been derived by fitting the spectra with a stochastic relaxation model taking into account spin relaxation in the St=1/2 state and electron hopping between the iron ions. An interpretation of the values of the hyperfine parameters has been given by solving a spin Hamiltonian, which includes antiferromagnetic and double exchange in an asymmetric Fe2+Fe3+ pair and which allows partial electron delocalization. Using the value a2=0.8 for the delocalization coefficient and an estimate of Δ=105 cm−1 for the difference between the St=1/2 and the first excited St=3/2 state we have derived limits for the exchange-coupling constant J, the double-exchange parameter B, and the energy difference EA−EB arising from the two possible configurations FeA2+FeB3+ and FeA3+FeB2+, i.e., 70 cm−1≲J≲300 cm−1, 0<‖B‖≲395 cm−1, and 0<‖EA−EB‖≲590 cm−1.