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: ashis@vbphysics.net.in
Department of Chemistry, University of Mainz,
55099, Mainz, Germany, E-mail: guetlich@uni-mainz.de
Prussian Blue (PB) Analogue
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 = diand 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-induced metal-to-metal electron
transfer and magnetism.
For further details see the follwoing references:
- M. Verdaguer et al., Coord. Chem. Rev. 190-192 (1999) 1023;
- T. Yokoyama, H. Tokoro, S.-i. Ohkoshi, and K. Hashimoto Phys. Rev. B 2002, 66, 184111;
- A. Goujon, F. Varret, V. Escax, A. Bleuzen, M. Verdaguer Polyhedron, 2001, 20 (11-14), 1347-1354;
- V. Ksenofontov, G. Levchenko, S. Reiman, P. Gütlich, A. Bleuzen, V. Escax, M. Verdaguer, Phys. Rev. B
2003, 68, 024415;
- 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.
K0.2MnII.66MnIII1.44[FeII0.2FeIII0.8(CN)6]O0.66(CH3COO)1.32]·7.6H2O
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]·7.6H2O
have indicated a ferrimagnetic phase transition around 8 K along with a ferromagnetic
phase transition around 2 K. The compound exhibits metamagnetic transition around 3
K observed in the magnetic measurements. Furthermore, the compound exhibits a thermal
anomaly around 185 K arising due to a glass transition.
15
3
8
8 kOe
-1
7
6
1.5
5
0
1
7 kOe
2
2.0
2.5
3.0
T/K
M / NAB
10
Cmag / JK mol
-1
-1
Cmag / JK mol
-1
9 kOe
6 kOe
5 kOe
4 kOe
3 kOe
1
2 kOe
1 kOe
0.6 kOe
0.2 kOe
11
21
T/K
Magnetic Transitions
31
0
0
10
20
T/K
Metamagnetic Transition
Mössbauer Spectroscopy
100
II
Fe
96
III
Fe
300 K
Mössbauer spectroscopic studies of this
compound were done at various
temperatures.
92
Rel. Transmission (%)
100
96
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, t2g3 eg1, S = 2) and
MnII (high spin, t2g3 eg2, S = 5/2) mixed
valence states.
11 K
92
100
96
92
10 K
100
96
5K
92
100
96
4.2 K
92
-5.0
-2.5
0.0
v /mm s
2.5
-1
5.0
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.
Metal-to-Metal Electron Transfer
Mössbauer spectroscopy successfully detects
the phenomenon of metal to metal electron
transfer between Mn and Fe ions possibly
through the
12
[FeIII (t2g5, S = ½) –CN- MnII (t2g3 eg2, S =5/2)]
to [FeII (t2g6, S = 0)–CN- MnIII (t2g3 eg1, S = 2)]
process.
III
Fe /Fe
II
9
At temperatures above the magnetic transition
the compound exists as a mixture of
6
[FeIII(S = ½) –CN- MnII(S = 5/2)] and
3
0
100
200
300
T/K
Temperature dependence of the population
ratio of FeIII and FeII low spin species
obtained from Mössbauer spectroscopy
[FeII (S = 0) –CN- MnIII (S = 2)] states,
whereas below the magnetic transition the
former state predominates.
Glass Transition
3.2
12
3.0
II
9
III
Fe /Fe
-2
(Cp / T) / JK mol
-1
3.1
2.9
6
2.8
2.7
120
140
160
180
200
220
T/K
From Calorimetry
240
3
0
100
200
300
T/K
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