## Abstract

Exchange interactions and charge transfer in the Fe^{2+}Fe ^{3+} pair of the mixed valence [Fe_{2}S _{2}(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 S _{t}=1/2 and hyperfine parameters intermediate between values for a Fe^{2+}Fe^{3+} 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 S_{t}=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 Fe ^{2+}Fe^{3+} pair and which allows partial electron delocalization. Using the value a^{2}=0.8 for the delocalization coefficient and an estimate of Δ=105 cm^{-1} for the difference between the S_{t}=1/2 and the first excited S_{t}=3/2 state we have derived limits for the exchange-coupling constant J, the double-exchange parameter B, and the energy difference E_{A}-E_{B} arising from the two possible configurations Fe_{A}^{2+}Fe_{B} ^{3+} and Fe_{A}^{3+}Fe_{B}^{2+}, i.e., 70 cm^{-1}≲J≲300 cm^{-1}, 0< |B| ≲395 cm ^{-1}, and 0< |E_{A}-E_{B}| ≲590 cm ^{-1}.

Original language | English |
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Journal | The Journal of Chemical Physics |

Volume | 99 |

Issue number | 9 |

Pages (from-to) | 6421-6428 |

Number of pages | 8 |

ISSN | 0021-9606 |

DOIs | |

Publication status | Published - 01.01.1993 |