Metal-to-Metal Electron Transfer and Magnetic Interactions in a Mixed-Valence Prussian Blue Analogue. A. Bhattacharjee, P. Gütlich et al. Department of Physics, Visva-Bharati University, Santiniketan 731235, India, E-mail: [email protected] Department of Chemistry, University of Mainz,
A. Bhattacharjee, P. Gütlich et al.
Department of Physics, Visva-Bharati University, Santiniketan 731235, India, E-mail: [email protected]
Department of Chemistry, University of Mainz,
55099, Mainz, Germany, E-mail: [email protected]
The hexacyanometalate [B(CN)6]x- ions are well known building blocks used for fabrication of the hetero-metal assemblies exhibiting bulk magnetization, where reaction of the [B(CN)6]x- ions with metal ions gives rise to the so-called ‘Prussian Blue’ (PB) analogues - MA[B(CN)6].(solvent)(M = monovalent alkali metal ion, and A, B = di- and trivalent transition metal ions). These materials exhibit various magnetic properties depending on their transition metal combinations, e.g., high TC magnet, magnetic pole reversal, spin glass behavior and photo-induced magnetic transition.The alkali-doped analogues are among the most extensively studied recent materials of the Prussian Blue family in regard to photo-induced and pressure-inducedmetal-to-metal electron transfer and magnetism.
For further details see the follwoing references:
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- T. Yokoyama, H. Tokoro, S.-i. Ohkoshi, and K. Hashimoto Phys. Rev. B 2002, 66, 184111;
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- A. Bhattacharjee, S. Saha, S. Koner, V. Ksenofontov, S. Reiman, P. Gütlich, J. Magn. Magn. Mater. 2006, 302, 173-180;
- A. Bhattacharjee, S. Saha, S. Koner, Y. Miyazaki J. Magn. Magn. Mater. 2007, 312, 435-442.
Calorimetric study under magnetic field and field dependent magnetization studies of a new PB analogue -K0.2MnII.66MnIII1.44[FeII0.2FeIII0.8(CN)6]O0.66(CH3COO)1.32]·7.6H2Ohave indicated aferrimagnetic phase transition around 8 Kalong with aferromagnetic phase transition around 2 K. The compound exhibitsmetamagnetic transitionaround 3 K observed in the magnetic measurements. Furthermore, the compound exhibits a thermal anomaly around 185 K arising due to aglass transition.
Mössbauer spectroscopic studies of this compound were done at various temperatures.
The Mössbauer spectra obtained at all the measuring temperatures exhibited the existence of both FeIII and FeII in low spin states. Thus, the compound exists in FeIII (low spin, t2g5, S = ½), FeII (low spin, t2g6, S = 0), MnIII (high spin, t2g3eg1, S = 2) and MnII (high spin, t2g3eg2, S = 5/2) mixed valence states.
The onset of magnetic ordering of the FeIII low spin species around 5 K is clearly seen by the broadening of the blue signal, which develops to a reasonably well resolved magnetic sextet at 4.2 K.
Mössbauer spectroscopy successfully detects the phenomenon of metal to metal electron transfer between Mn and Fe ions possibly through the
[FeIII (t2g5, S = ½) –CN- MnII (t2g3eg2, S =5/2)] to [FeII (t2g6, S = 0)–CN- MnIII (t2g3eg1, S = 2)] process.
At temperatures above the magnetic transition the compound exists as a mixture of
[FeIII(S = ½) –CN- MnII(S = 5/2)] and
[FeII (S = 0) –CN- MnIII (S = 2)] states, whereas below the magnetic transition the former state predominates.
Temperature dependence of the population ratio of FeIII and FeII low spin species obtained from Mössbauer spectroscopy
From Mössbauer spectroscopy
A glass transition at 194 K has been observed in the heat capacity study due to freezing of the orientational motion of the H2O molecules present. This phenomenon is reflected in the temperature dependence of the estimated FeIII and FeII concentrations in the present material obtained through Mössbauer spectroscopy. Mössbauer spectroscopy being extremely sensitive to lattice dynamics is able to detect the effect of the glass transition due to the freezing of the orientational motion of the H2O molecules inducing non-rigid / dynamic character in the lattice on and around the glass transition temperature.
Bhattacharjee,et al., J. Magn. Magn. Mater. 302 (2006) 173; J. Magn. Magn. Mater. 312 (2007) 435