Phonon coupled cooperative low-spin 1A1high-spin 5T2 transition in [Fe(phen)2(NCS)2] and [Fe(phen)2(NCSe)2] crystals |
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Authors: | M. Sorai S. Seki |
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Affiliation: | Department of Chemistry, Faculty of Science, Osaka University, Toyonaka, Osaka 560, Japan |
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Abstract: | Heat capacities of [Fe(phen)2(NCS)2] and [Fe(phen)2(NCSe)2] were measured between 135 and 375 K. A heat capacity anomaly due to the spin-transition from low-spin 1A1 to high-spin π2 electronic ground state was found at 176·29 K for the SCN-compound and at 231·26 K for the SeCN-compound, respectively. Enthalpy and entropy of transition were determined to be ΔH = 8·60 ± 0·14 kJ mol?1 and ΔS = 48·78 ± 0·71 J K?1 mol?1 for the SCN-compound and ΔH = 11·60 ± 0·44 kJ mol?1 and ΔS = 51·22 ± 2·33 J K?1 mol?1 for the SeCN-compound. To account for much larger value of ΔS compared with the magnetic contribution, we suggest that there is significant coupling between electronic state and phonon system. We also present a phenomenological theory based on heterophase fluctuation. Gross aspects of magnetic, spectroscopic, and thermal behaviors were satisfactorily accounted for by this model. To examine closely the transition process, infrared spectra were recorded as a function of temperature in the range 4000 ? 30 cm?1. The spectra revealed clearly the coexistence of the 1A1, and the 5T2 ground states around Tc. |
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