Study of thermal spin crossover in [Fe(II)(isoxazole)(6)](BF(4))(2) with Mössbauer spectroscopy.
ABSTRACT (57)Fe Mössbauer spectroscopy of the mononuclear [Fe(II)(isoxazole)(6)](BF(4))(2) compound has been studied 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 has been constructed with the least-square fitted data obtained from the Mössbauer spectra measured at various temperatures in the 240-60 K range during the cooling and heating cycle. The compound exhibits a temperature-dependent two-step spin transition phenomenon with T(SCO) (step 1) = 92 and T(SCO) (step 2) = 191 K. The compound has three high-spin Fe(II) sites at the highest temperature of study; among them, two have slightly different coordination environments. These two Fe(II) sites are found to undergo a spin transition, while the third Fe(II) site retains the high-spin state over the whole temperature range. Possible reasons for the formation of the two steps in the spin transition curve are discussed. The observations made from the present study are in complete agreement with those envisaged from earlier magnetic and structural studies made on [Fe(II)(isoxazole)(6)](BF(4))(2), but highlights the nature of the spin crossover mechanism.
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ABSTRACT: The 57Fe Mössbauer spectroscopy of mononuclear [Fe(II)(isoxazole)6](ClO4)2 has been studied to reveal the thermal spin crossover of Fe(II) between low-spin (S=0) and high-spin (S=2) states. Temperature-dependent spin transition curves have been constructed with the least-square fitted data obtained from the Mössbauer spectra measured at various temperatures between 84 and 270 K during a cooling and heating cycle. This compound exhibits an unusual temperature-dependent spin transition behaviour with TC(↓)=223 and TC(↑)=213 K occurring in the reverse order in comparison to those observed in SQUID observation and many other spin transition compounds. The compound has three high-spin Fe(II) sites at the highest temperature of study of which two undergo spin transitions. The compound seems to undergo a structural phase transition around the spin transition temperature, which plays a significant role in the spin crossover behaviour as well as the magnetic properties of the compound at temperatures below TC. The present study reveals an increase in high-spin fraction upon heating in the temperature range below TC, and an explanation is provided.Journal of Physics and Chemistry of Solids 11/2008; · 1.59 Impact Factor
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ABSTRACT: A one-dimensional iron(II) spin-crossover compound [Fe(3py-im)(2)(NCS)(2)].7H(2)O (1) (3py-im = 2,4,5-tris(4-pyridly)-imidazole) has been solvothermally synthesized and structurally characterized. Compound 1 crystallizes in the monoclinic space group P2/c with a = 11.9078(2), b = 9.9474(1), c = 17.7290(3) and beta = 102.361(2) degrees at 105 K. Studies on the variable-temperature magnetic susceptibilities and Mössbauer spectra suggest that compound undergoes incomplete spin transition behaviour. Pressure effects on the transition behaviour have also been investigated, the thermal-induced spin transition becomes more gradual and the critical temperature shifts towards slightly higher temperature range when external pressure increases. However, the spin transition can not be completed by applying external pressure even as high as 0.79 GPa.Dalton Transactions 03/2010; 39(9):2288-92. · 4.10 Impact Factor
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ABSTRACT: The aim of this paper is to highlight some selected research activities on molecular magnetic materials using Mössbauer spectroscopy as a technique carried out in our laboratory in recent years. The first part of the present article is devoted to the studies of the various magnetic interactions, metal-to-metal electron-transfer phenomenon, glass transition occurring in molecular magnetic materials, whereas the second part deals with the iron(II) high spin (S = 2)–low spin (S = 0) transition phenomenon occurring in some isoxazole ligand based iron(II) compounds as examples with unusually complicated spin transition behaviour. Also, an example of a dinuclear a spin crossover compound of iron(II) is described, where Mössbauer spectroscopy has most convincingly unraveled the mechanism of the spin transition process. Finally, an example from our most recent studies of spin crossover materials exhibiting both thermal spin crossover and liquid crystalline properties in the same temperature interval near room temperature will be presented.06/2009: pages 3-19;