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Fe(II) High Spin ↔ Low Spin Transition in [Fe II (isoxazole) 6 ](X) 2 , X = BF 4 , ClO 4

Fe(II) High Spin ↔ Low Spin Transition in [Fe II (isoxazole) 6 ](X) 2 , X = BF 4 , ClO 4. A. Bhattacharjee, P. Gütlich et al. Department of Physics, Visva-Bharati University, Santiniketan 731235, India, E-mail: ashis@vbphysics.net.in Department of Chemistry, University of Mainz,

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Fe(II) High Spin ↔ Low Spin Transition in [Fe II (isoxazole) 6 ](X) 2 , X = BF 4 , ClO 4

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  1. Fe(II) High Spin ↔ Low Spin Transition in [FeII(isoxazole)6](X)2, X = BF4, ClO4 A. Bhattacharjee, P. Gütlich et al. Department of Physics, Visva-Bharati University, Santiniketan 731235, India, E-mail: ashis@vbphysics.net.in Department of Chemistry, University of Mainz, 55099, Mainz, Germany, E-mail: guetlich@uni-mainz.de

  2. ....eg ....t2g Low Spin (1A1) S = 0, Diamagnetic High Spin ( 5T2) S = 2, Paramagnetic E EHL  kBT rHL rFe-N Spin Crossover Research activities in the area of Fe(II) high spin low spin transition materials have been stimulated in recent years by the objective of developing molecular switches or sensors for molecular electronics. In this context hexa-coordinated Fe(II) spin crossover materials have been widely studied, which has been motivated by their favourable response towards a change in temperature or pressure, as well as upon light irradiation as they undergo interconversion from a low spin (S = 0) diamagnetic state to a high spin (S = 2) paramagnetic state under the influence of these external perturbations. For details see: Gütlich, P. and Goodwin, H. A. (Editors) Topics in Current Chemistry (Berlin: Springer-Verlag 2004) Vol. 233-235.

  3. [Fe(II)(isoxazole)6](X)2, X = BF4, ClO4 These mononuclear compounds have been studied by 57Fe Mössbauer spectroscopy to reveal the thermal spin crossover of Fe(II) between low spin (S = 0) and high spin (S = 2) states upon changing temperature as well as the effect of different solvent molecules on the nature of the spin transition. [Fe(II)(isoxazole)6](BF4)2 Hibbs, W., et al., Inorg. Chem. 42 (2003) 5645. [Fe(II)(isoxazole)6](ClO4)2

  4. Mössbauer spectra of [Fe(II)(isoxazole)6](BF4)2 A temperature-dependent spin transition curve has been constructed with the least-squares fitted data obtained from the Mössbauer spectra measured at various temperatures in the 240 – 60 K range during cooling and heating cycle. The compound exhibits a temperature-dependent two-step spin transition phenomenon with TSCO(step 1) = 92 and TSCO(step 2) = 191 K. Bhattacharjee, A., et al., J. Phys.: Condens. Matter. 19 (2007) 406202.

  5. Nature of Spin Transition in [Fe(II)(isoxazole)6](BF4)2 The compound has three high spin Fe(II) sites at the highest temperature of study, two of them have slightly different coordination environment. These two Fe(II) sites are found to undergo spin transition, while the third Fe(II) site retains the high spin state over the whole temperature range. The possibility of existence of similar structures in the high and low temperature phases along with an intermediate structure in the intermediate temperature range may be another viable origin of the two-step spin transition. The Mössbauer spectroscopic study sheds new light on the mechanism of thermal spin crossover in a system with complex interplay of temperature dependent spin transition and structural rearrangements. Bhattacharjee, A., et al., J. Phys.: Condens. Matter. 19 (2007) 406202

  6. Mössbauer spectra of [Fe(II)(isoxazole)6](ClO4)2 57Fe Mössbauer spectroscopy has been performed in the 270 – 84 K range during cooling and heating cycle to reveal the thermal spin crossover of Fe(II) between low spin (S = 0) and high spin (S = 2) states. A temperature-dependent spin transition curve constructed with the least-squares fitted data exhibits an unusual temperature-dependent spin transition behaviour with TSCO(↓) = 223 and TSCO(↑) = 213 K.

  7. Nature of Spin Transition in [Fe(II)(isoxazole)6](ClO4)2 The [Fe(II)(isoxazole)6](ClO4)2compound has three high spin Fe(II) sites at the highest temperature of study. Two of these sites are found to undergo spin transition. The compound seems to undergo structural phase transition around the spin transition temperature which plays a significant role in the magnetic behaviour of the compound at temperatures below TSCO. Comparing the spin transition curves obtained through the Mössbauer spectroscopy with that obtained from the magnetic measurements, it is found that Mössbauer spectroscopy observes the spin transition “earlier” than the magnetic measurement. This is attributable to the existence of a definite time window for the spectroscopic method while bulk magnetic measurements at thermodynamic equilibrium correspond to no time window. Bhattacharjee, A., et al., unpublished data (2007)

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