Thermal-induced changes in molecular magnets based on Prussian blue analogues

Article abstract of DOI:10.1021/jp0555551

The thermal-induced changes in molecular magnets based on Prussian blue analogues, M₃[Fe(CN)₆]₂·xH₂O (M = Mn, Co, Ni, Cu, Zn, and Cd), were studied from infrared, X-ray diffraction, thermo-gravimetric, Moessbauer, and magnetic data. Upon being heated, these materials loose the crystalline water that enhances the interaction between the metal centers, as has been detected from Moessbauer spectroscopy data. At higher temperatures, a progressive decomposition process takes place, liberating CN^- groups, which reduces the iron atom from Fe(III) to Fe(II) to form hexacyanoferrates(II). The exception corresponds to the cobalt compound that undergoes an inner charge transfer to form Co(III) hexacyanoferrate(II). In the case of zinc ferricyanide, the thermal decomposition is preceded by a structural transformation, from cubic to hexagonal. For M = Co, Ni, Cu, and Zn the intermediate reaction product corresponds to a solid solution of M(II) ferricyanide and ferrocyanide. For M = Mn and Cd the formation of a solid solution on heating was not detected. The crystal frameworks of the initial M(II) ferricyanide and of the formed M(II) ferrocyanide are quite different. In annealed Mn(II) ferricyanide samples, an increasing anti-ferromagnetic contribution on heating, which dominates on the initial ferrimagnetic order, was observed. Such a contribution was attributed to neighboring Mn(II) ions linked by aquo bridges. In the anhydrous annealed sample such interaction disappears. This effect was also studied in pure Mn(II) ferrocyanide. The occurrence of linkage isomerism and also the formation of Ni(III), Cu(III), and Zn(III) hexacyanoferrates(II) were discarded from the obtained experimental evidence.

Full text of DOI:10.1021/jp0555551

7296
J. Phys. Chem. B 2006, 110, 7296-7303
Thermal-Induced Changes in Molecular Magnets Based on Prussian Blue Analogues
R. Mart´ınez-Garcia,† M. Knobel,‡ and E. Reguera*,†,§
Institute of Science and Technology of Materials, San Lazaro and L, 10400 HaVana, Cuba, Institute of Physics
“Gleb Wataghin”, UNICAMP, 13083-970 Campinas SP, Brazil, and Center of Applied Science and AdVanced
Technologies of IPN, Mexico D.F., Mexico
ReceiVed: September 29, 2005; In Final Form: February 17, 2006
The thermal-induced changes in molecular magnets based on Prussian blue analogues, M3[Fe(CN)6]2‚xH2O
(M ) Mn, Co, Ni, Cu, Zn, and Cd), were studied from infrared, X-ray diffraction, thermo-gravimetric,
Mo¨ssbauer, and magnetic data. Upon being heated, these materials loose the crystalline water that enhances
the interaction between the metal centers, as has been detected from Mo¨ssbauer spectroscopy data. At higher
temperatures, a progressive decomposition process takes place, liberating CN- groups, which reduces the
iron atom from Fe(III) to Fe(II) to form hexacyanoferrates(II). The exception corresponds to the cobalt
compound that undergoes an inner charge transfer to form Co(III) hexacyanoferrate(II). In the case of zinc
ferricyanide, the thermal decomposition is preceded by a structural transformation, from cubic to hexagonal.
For M ) Co, Ni, Cu, and Zn the intermediate reaction product corresponds to a solid solution of M(II)
ferricyanide and ferrocyanide. For M ) Mn and Cd the formation of a solid solution on heating was not
detected. The crystal frameworks of the initial M(II) ferricyanide and of the formed M(II) ferrocyanide are
quite different. In annealed Mn(II) ferricyanide samples, an increasing anti-ferromagnetic contribution on
heating, which dominates on the initial ferrimagnetic order, was observed. Such a contribution was attributed
to neighboring Mn(II) ions linked by aquo bridges. In the anhydrous annealed sample such interaction
disappears. This effect was also studied in pure Mn(II) ferrocyanide. The occurrence of linkage isomerism
and also the formation of Ni(III), Cu(III), and Zn(III) hexacyanoferrates(II) were discarded from the obtained
experimental evidence.
I. Introduction
tronic configurations for cobalt and copper atoms are t2g6eg1 and
3
t2g6eg , respectively. The formation of these species also
Prussian blue (PB) analogues, probably the oldest studied
family of coordination compounds, have received renewed
attention as prototypes of molecular magnets in the past decade.1
Several PB analogues are included in those molecular materials
with the highest temperature of magnetic ordering (Tc).2 In some
compositions, unusual effects have been observed. For instance,
Co(III) ferrocyanide, which is a nonmagnetic material, becomes
magnetic when it is illuminated at low temperatures.3 This effect
in cobalt iron cyanide has been intensively studied as a prototype
of a photoinduced molecular magnet3,4 in recent years, and it
has encouraged the study of other transition metal ferricyanides,
particularly the heat-treatment effect probably oriented to find
a similar charge-transfer process in other Prussian blue
analogues.5-9 For cobalt(II), nickel(II), copper(II), and zinc(II)
ferricyanides, the formation of cobalt(III), nickel(III), copper-
(III), and zinc (III) ferrocyanides has been reported.5-9 However,
nickel(III), copper (III), and zinc(III) are unusual species not
detected even in ozonized ferrocyanides.10 For cobalt(II) and
copper(II) ferricyanides, the formation on heating of CoII-CN-
FeIII and CuII-CN-FeIII species has also been reported.5,7 This
is an unexpected result since all of the known stable hexacyanide
complexes are formed with transition metals where the inner
metal has unoccupied the eg orbitals to favor a strong cation-
ligand interaction with the formation of low-spin complexes.
In CoII-CN-FeIII and CuII-CN-FeIII hexacyanides, the elec-
supposes the occurrence of a linkage isomerization process up
to now only observed in ferrous chromicyanide and mangani-
cyanide.11
The behavior of Prussian blue analogues on heating has been
studied;12 ferricyanides reduce to ferrocyanides releasing CN-
anions. In this process the liberated CN- groups play the role
of the reducing agent. In the exhaust reaction gases, C2N2 has
been detected through the use of mass spectrometry and IR
spectroscopy. A similar mechanism is present during the
reduction of ferricyanides to ferrocyanides by milling or
sonication.13 In the milling, the colliding points of the micro-
crystals behave as “hot points” where CN- groups are liberated.
From all this evidence, we believe that the reported thermal-
induced changes in some molecular magnets based on PB
analogues5-9 are related to a simple reduction of ferricyanides
to ferrocyanides without the presence of other heat-induced
effects, with the exception of the cobalt analogue. In this paper,
we present experimental results that support this hypothesis. In
addition, some interesting new effects were observed and are
discussed.
This work concerns mainly those heat-induced changes in
divalent transition metal ferricyanides for heating below 200
°C. Above this temperature ferrocyanides are always formed,
and they remain stable approximately up to 350 °C. Their final
decomposition leads to the reduction of the transition metals
involved, in some cases forming alloys of small particle size.
Such metallic end products have been appropriately char-
acterized.5-8
* Author to whom correspondence should be addressed. E-mail: ereguera@
yahoo.com.
† Institute of Science and Technology of Materials.
‡ UNICAMP.
§ Center of Applied Science and Advanced Technologies of IPN.
10.1021/jp0555551 CCC: $33.50 © 2006 American Chemical Society
Published on Web 03/18/2006

Thermal-Induced Changes in Molecular Magnets
J. Phys. Chem. B, Vol. 110, No. 14, 2006 7297
II. Experimental Methods
Mn(II), Co(II), Ni(II), Cu(II), cadmium, and zinc ferricyanides
were obtained from aqueous solutions of these divalent cations
and of ferricyanic acid, prepared in situ,14 to obtain a product
free of the accompanying alkaline cation. The obtained pre-
cipitates were filtered, washed several times with distilled water,
and then dried in air until constant weight. According to
chemical analysis, the M:Fe atomic ratio obtained for all the
studied compositions was 3:2. Such an atomic ratio corresponds
to compounds with the following formula unit, M3[Fe(CN)6]2‚
xH2O. From now on these compounds will be labeled as M3-
Fe2. The same ferricyanides were also obtained from potassium
ferricyanides to discard any probable effect of the accompanying
alkaline metal when it remains in the obtained material. For
excess of the transition metal, a complex salt practically free
of potassium was obtained. As reference samples, the corre-
sponding divalent transition metal ferrocyanides were prepared
using the same procedure. The nature of all the studied
compounds was established from infrared (IR), X-ray diffraction
(XRD), thermo-gravimetry (TG), Mo¨ssbauer, and magnetic data.
The heat treatments were carried out under an argon
atmosphere (99.99%) during 3 h at 60, 80, 100, 120, 140, 160,
180, and 200 °C, allowing sample cooling until room temper-
ature within the furnace used. The obtained annealed samples
were then characterized using the mentioned techniques.
The IR spectra were recorded in Nujol mulls between KBr
windows. On milling and pressing processes with KBr, ferri-
cyanides reduce to ferrocyanides.15 Mo¨ssbauer spectra were run
at room temperature in a constant acceleration spectrometer
operated in the transmission mode with a 57Co/Rh source. To
obtain spectra of anhydrous samples a special vacuum glass
cell with windows transparent to γ-rays was used. The obtained
spectra were fitted using a least-squares minimization algorithm
and pseudo-Lorentzian line shape to obtain the values for isomer
shift (δ), quadrupole splitting (∆), line width (Γ), magnetic
hyperfine field (Bh), and relative area (A). The values of δ are
reported relative to sodium nitroprusside. The magnetic mea-
surements were done at low temperature using a superconduct-
ing quantum interference device (SQUID) magnetometer (Quan-
tum Design MPMS XL7). The TG curves were recorded in the
high-resolution mode using a TA Instrument (TG-2950 model)
thermo-balance.
Figure 1. Powder XRD patterns of Cu(II) ferricyanide and of the heat-
induced decomposition product. Inset A: Dependence of the cell edge
size on the annealing temperature. Inset B: Cell contraction in Cu(II)
ferricyanide on the water removal (XRD patterns recorded under
vacuum).
The cell edge corresponds to the Fe-CtN-M-NtC-Fe
chain length. In these structures, both metal centers have
octahedral coordination. The iron atom is linked to the C ends
of six CN ligands. The outer metal, M, has a mixed coordination
sphere formed by four N ends from the CN groups plus two
coordinated water molecules, M(NC)4(OH2)2. According to their
formula unit, M3[Fe(CN)6]2‚xH2O, 33% of the building unit (Fe-
(CN)6) sites remain vacant in the material framework, creating
a network of pores of about 8.5 Å that communicate by windows
of c.a. 4.5 Å. These pores are usually filled with zeolitic waters
hydrogen bonded to the coordinated ones. These weakly bonded
water molecules can be removed by heating at relatively low
temperatures (discussed below). Zinc ferricyanide appears
dimorphic, cubic (Fm3hm), and hexagonal (R3hc). In the hexago-
nal phase the zinc atom is found tetrahedrally coordinated to N
ends of CN ligands. This structure is free of vacancies but
preserves a porous feature, with pores of about 5 × 8 × 13 Å3
communicating by windows of c.a. 5 Å.24 These pores are free
of water molecules because they have a practically nonpolar
surface. This hexagonal phase is formed when the synthesis is
carried out from hot solutions and also when the cubic phase is
dehydrated by heating (discussed below).
Powder XRD patterns were recorded in Bragg-Brentano
geometry in a D5000 diffractometer (Siemens) with monochro-
matic Cu KR radiation in the (10-110)/2θ angular range, at a
step of 0.025°/2θ with a counting time of 10 s. Some powder
XRD patterns were recorded at the XPD beamline of the LNLS
synchrotron radiation facilities (in Campinas, Brazil). The
DicVol program16 was used to assign Miller indexes to the
powder XRD patterns. The cell parameters were calculated
through a profile fitting using the FullProf’1998 code.17
The reference compounds, divalent transition metal ferro-
cyanides, crystallize with a higher diversity of structures,
depending on the M metal involved. For Mn and Cd, simple
and mixed salts can be obtained. The simple salts, M2[Fe(CN)6]‚
8H2O, crystallize with a monoclinic cell (P21/n),25 while the
mixed ones, MA2[Fe(CN)6]‚xH2O (A, alkaline cation), adopt a
cubic or pseudo-cubic structure depending on the alkaline metal
involved, which is occupying interstitial sites in the material
framework.23 For Co, Ni, Cu, and Zn, only mixed compositions
are known, with a cubic or pseudo-cubic structure22,23 for the
first three and with the R3hc hexagonal structure for Zn (space
group already mentioned for zinc ferricyanide24). In mixed zinc
ferrocyanide, the alkaline metal is located within the porous
framework. In transition metal ferrocyanides, the alkali metal
always behaves as a charge balance cation, and it is an extra
framework and exchangeable species.
III. Results and Discussion
A. Crystal and Electronic Structure of the Studied
Materials. Except for Cu(2+), the studied ferricyanides crystal-
lize with the cubic cell (Fm3hm), which is typical of Prussian
blue analogues. Cu(2+) ferricyanide appears reported as
Fm3hm;7,18 however, the appearance of a series of weak peaks
in its powder XRD pattern (Figure 1) that do not correspond to
that space group discards such an assignment. This compound
was found to be also cubic but within the Pm3hm space group.
This space group has also been reported for ferric ferrocyanide.19
The crystal structure of Prussian blue analogues (Fm3hm) is
known, and their cell edge values have been reported.18,20-23
In hexacyanoferrates, the iron atom coordinated to the C ends
of the CN groups is always found in a low-spin configuration,
and their Mo¨ssbauer spectra are single lines for ferrocyanides
and a quadrupole splitting doublet for ferricyanides (Figure 2).
Both the single lines and the quadrupole splitting doublets have
relatively low isomer shift values (Table 1) due to the existence

7298 J. Phys. Chem. B, Vol. 110, No. 14, 2006
Mart´ınez-Garcia et al.
porous framework of these materials, which facilitates the
removed water molecules release from the solid. It seems that
the dehydration process is accompanied by the decomposition
of a small sample fraction (particularly the sample fraction of
smaller particle size). The TG curve of the anhydrous sample
shows a slight but continued weight loss up to a certain
temperature where a pronounced evolution of a gaseous species
is observed. For copper, this temperature is about 150 °C, while
for Mn, Ni, Cd, and Zn it is above 225 °C. Such rapid weight
loss for the anhydrous sample is not observed for the cobalt
complex. This compound remains stable up to 350 °C. That
information on the stability of the anhydrous phases, derived
from the TG curves, should be taken cautiously. Even for the
most stable compounds, a pronounced decomposition degree
with formation of ferrocyanide is observed on prolonged heating,
even at 140 °C. Infrared spectra of the annealed samples provide
conclusive evidence in that sense. The formed ferrocyanides
remain stable up to 350 °C.
C. Heat-Induced Changes as Detected from IR Data. For
all the studied compounds the ν(CN) band of the ferrous species,
always present in the as-synthesized samples as an impurity,
gains in intensity on heating at the expense of a weakening of
the ferricyanide band (Figures 3 and 4). However, each of the
studied compositions shows certain peculiarities on heating.
When the copper compound, the less stable of the studied
ferricyanides (Figure 5), is heated at 160 °C, the ferricyanide
band at 2174 cm-1 disappears, and a broad band at 2106 cm-1
due to copper ferrocyanide formation appears (Figure 3A). For
the nickel compound, one of the most stable ferricyanides, an
increase in the intensity of the ferrocyanide band on heating
was observed but accompanied by a band of lower frequency
at 2040 cm-1 (Figure 3C). That low-frequency band was
attributed to a ferrocyanide with CN groups unlinked at the N
end, as a decomposition byproduct. For instance, cesium
ferrocyanide has its ν(CN) vibration at that frequency of 2040
cm-1. When cobalt ferricyanide is heated, an inner charge
transfer takes place, forming a stable ferrocyanide without
decomposition, according to
Figure 2. Mo¨ssbauer spectra, at room temperature, of Mn3Fe2: (a)
hydrated sample; (b) anhydrous sample; (c) anhydrous thermal decom-
position product from an annealing of 200 °C; (d) hydrated thermal
decomposition product from an annealing of 380 °C. Inset: Mn2[Fe-
(CN)6]‚xH2O (Mn2Fe) in its hydrated and anhydrous forms.
of a strong π back-bonding effect from the iron atom toward
the CN ligands. This effect reduces the number of 3d electrons
on the metal and their shielding of the s electron density at the
iron nucleus. That π back-bonding effect is more pronounced
in ferrocyanides than that in ferricyanides due to the higher 3d
electrons availability in the former ones. From this fact, the
isomer shift values in ferricyanides and ferrocyanides have only
a small difference of about 0.05 mm/s. However, that amount
is sufficient to differentiate them. The quadrupole splitting in
ferricyanides is mainly related to the existence of an unpaired
electron in their t2g orbitals, which creates an electric field
gradient around the iron nucleus. This unpaired electron is also
responsible for the magnetic order at low temperature in
ferricyanides.
Co3[Fe(CN)6]2 f
(Co2+)3-x(Co3+)x[(FeIII)2-x(FeII)x(CN)12] f
(Co2+)(Co3+)2[FeII(CN)6]2
The IR spectrum is probably the simplest and most efficient
sensor to differentiate ferro- from ferricyanides. Ferricyanides
have their ν(CN) absorption band 80 cm-1 above the frequency
of this vibration in ferrocyanides (Figures 3 and 4). Table S1
of the Supporting Information contains the observed frequency
for this vibration in all of the studied ferricyanide samples,
before and after the heat-induced effects. The ν(CN) band is
also sensitive to the oxidation state and electronic configuration
of the metal bonded to the N end. An increment in the valence
This mixed valence state system in cobalt iron cyanide is
characterized by a broad ν(CN) absorption at 2114 cm-1, 32
cm-1 above the expected frequency value for Co(II) ferrocyanide
(Figure 3B). In the formed mixed valence system, the Co(III)
atom adopts a low-spin configuration where the eg orbitals are
free of electrons. This favors a large charge subtraction from
the CN group, through its weakly antibonding 5σ orbital. That
broad band at 2114 cm-1 probably results from contributions
of Co(III)-NC-Fe(II) and Co(II)-NC-Fe(II) chains within
the same framework. Such a charge-transfer process was not
observed for the remaining ferricyanides.
For manganese and cadmium, the heating process leads to
the formation of anhydrous M2[Fe(CN)6] (M ) Mn, Cd) as the
main reaction product, with a ν(CN) absorption band of a
relatively low frequency, around 2030 cm-1 (Figures 4A and
4C). In contact with humid air these anhydrous phases incor-
porate water to form octahydrates, M2[Fe(CN)6]‚8H2O. That
process is accompanied by a ν(CN) frequency shift up to
approximately 2060 cm-1 and by the appearance of new
absorption bands in the 900-700 cm-1 region. These new bands
are assigned to rocking, F(H2O), and wagging, w(H2O), motions
of this metal increases the ν(CN) frequency by 10-25 cm-1
.
For instance, in manganese hexacyanoferrates, when the Mn
ion goes from a valence of 4+ to 3+, a frequency shift of -24
cm-1 is observed.26 The frequency shift is -14 cm-1 when the
Mn atom valence goes from 3+ to 2+.26 With regards to
intensity, ferrocyanide bands are very intense and broad, whereas
ferricyanide bands are narrow and of medium intensity.
B. Dehydration and Thermal Stability. In ferricyanides both
coordinated and zeolitic waters are weakly bonded, and under
the material heating the crystal water evolves at a relatively
low temperature, usually below 100 °C (Figure 5). The mass
loss corresponds to up to 30% of the sample weight, which
means up to 14 water molecules per formula unit; 6 of them
are coordinated. This easy dehydration is also related to the

Thermal-Induced Changes in Molecular Magnets
J. Phys. Chem. B, Vol. 110, No. 14, 2006 7299
TABLE 1: Mo1ssbauer Parameters at Room Temperature of the Studied Ferricyanide Samples in the Hydrated and Anhydrous
States and of the Resulting Ferrocyanides by Thermal-Induced Decomposition
δa
(mm/s)
∆
Γ
A
(%)
sample
(mm/s)
(mm/)]
assignmentb
Mn3[Fe(CN)6]2‚xH2O
0.11
0.10
0.07
0.18
0.27
0.97
0.21
0.29
0.46
0.26
0.26
100
Fe(III) LS
Fe(III) LS
Fe(II) LS
Fe(II) LS
anhydrous
decomposition product (anhydrous)
decomposition product (hydrated)
100
100
Cd3[Fe(CN)6]2‚xH2O
0.11
0.10
0.08
0.17
0.26
0.88
0.20
0.31
0.48
0.25
0.27
100
100
Fe(III) LS
Fe(III) LS
Fe(II) LS
Fe(II) LS
anhydrous
decomposition product (anhydrous)
decomposition product (hydrated)
100
Co3[Fe(CN)6]2‚xH2O
anhydrous
decomposition product (anhydrous)
0.12
0.10
0.17
0.10
0.17
0.11
0.43
0.97
0.31
0.56
0.48
0.39
0.47
0.41
100
100
79
21
78
Fe(III) LS
Fe(III) LS
Fe(II) LS
Fe(III) LS
Fe(II) LS
Fe(III) LS
0.91
0.44
decomposition product (hydrated)
22
Ni3[Fe(CN)6]2‚xH2O
anhydrous
decomposition product
0.11
0.10
0.17
0.49
0.99
0.34
0.55
0.42
100
100
100
Fe(III) LS
Fe(III) LS
Fe(II) LS
Cu3[Fe(CN)6]2‚xH2O
anhydrous
decomposition product
0.10
0.09
0.16
0.54
0.91
0.31
0.47
0.32
100
100
100
Fe(III) LS
Fe(III) LS
Fe(II) LS
Zn3[Fe(CN)6]2‚xH2O (cubic)
anhydrous (cubic)
anhydrous (hexagonal)
decomposition product
0.12
0.11
0.12
0.18
0.35
0.49
0.18
0.38
0.42
0.38
0.26
100
100
100
100
Fe(III) LS
Fe(III) LS
Fe(III) LS
Fe(II) LS
a Isomer shift values are reported relative to sodium nitroprusside. The fitting error in the values of δ, ∆, and Γ remains below 0.01 mm/s. b LS
) low spin.
Figure 3. IR spectra (CN stretching region) of (A) Cu3Fe2 and of its
thermal decomposition products, indicated is the annealing temperature;
(B) Co3Fe2 and of annealed samples of this compound where the
formation of low-spin Co(III) ferrocyanide is detected; (C) Ni3Fe2 before
and after its annealing at 160 °C where the formation of two species
of ferrocyanide are observed.
Figure 4. IR spectra (CN stretching region) of (A) Mn3Fe2 and of its
thermal decomposition products and of the Mn2[Fe(CN)6]‚8H2O (Mn2-
FeII) reference sample in its hydrated and partially dehydrated states;
(B) Zn3Fe2 and of annealed samples of this compound where the
decomposition process appears preceded by a structural transformation
from cubic and hexagonal; (C) Cd3Fe2 before and after its annealing at
160 °C where the formed Cd(II) ferrocyanide appears in its anhydrous
state.
of the water molecules that are forming bridges between two
neighboring M(II) atoms. In Mn(II) and Cd(II) ferrocyanides
the metals are found in the following sequence -Fe-M-M-
Fe-, where neighboring M metals remain linked by water
bridges.25,27 When such aquo bridges are removed, the ν(CN)
frequency decreases about 30 cm-1. Zinc ferricyanide also shows
an interesting behavior. Its dehydration on heating induces a
structural transformation from cubic to hexagonal, observed in
the corresponding IR spectra as a ν(CN) frequency shift of 26
cm-1 (Figure 4B). In the tetrahedral coordination, the zinc atom
takes part in a stronger interaction with the CN ligand, probably
subtracting a higher electron density from the CN group, mainly
from its 5σ orbital. A reduction in the coordination number leads
to an increase in the positive charge on the metal, which, in
turn, increases the σ donation increasing the ν(CN) frequency.28
The anhydrous hexagonal phase has a relatively high stability
on heating, because in this structure the two metal centers have
saturated their coordination sphere through a strong bond to
CN ligands, but it also decomposes on heating, forming a Zn-
(II) ferrocyanide where the Zn(II) atom, according to the

7300 J. Phys. Chem. B, Vol. 110, No. 14, 2006
Mart´ınez-Garcia et al.
have different bonding properties. From this fact, the coordina-
tion environment of this metal is slightly distorted and since
the metal centers remain strongly linked through CN ligands,
such local distortion is partially transmitted to the iron atom
environment. This explains the observed quadrupole splitting
value in ferricyanides. When these two coordinated water
molecules are removed on dehydration, all of the interaction of
the outer metal is with the CN groups, increasing the charge
subtraction from the ligands through their 5σ orbitals. This
induces a higher π back-donation from the iron atom toward
the CN groups, creating certain charge asymmetry around the
iron atom, in addition to the mentioned local deformation. This
is observed as a pronounced increase in the value of ∆ on
dehydration. When the anhydrous sample is heated at a higher
temperature, an increasing ferrocyanide fraction appears that
becomes dominant from a certain temperature where the sample
decomposes as a whole (Figures 1-5). The formed ferrocyanide
is usually accompanied by a new secondary high-spin Fe(II)
phase, detected as a weak broad quadrupole doublet (Figure
2c). A similar doublet has been observed during the pyrolysis
of Prussian blues, and it has been tentatively assigned to a
decomposition byproduct or to the formation of ferrous ferro-
cyanide.29 When the sample is heated at a higher temperature,
a partial decomposition of the previously formed ferrocyanide
is favored, and metallic species appear5-8 that oxidize when
they are exposed to air, forming iron oxides (Figure 2d). In the
pyrolyzed samples, other decomposition byproducts could also
be detected. For the particular cases of Mn(II) and Cd(II)
ferricyanides, the Mo¨ssbauer spectra of the formed ferrocyanides
appear as practically an unresolved doublet with an atypically
low value of the isomer shift (Figure 2c, Table 1). When the
obtained ferrocyanide sample rehydrates, that unresolved doublet
is replaced by a single line with the isomer shift value expected
for a ferrocyanide (Figure 2d). The rehydration process leads
to the formation of M2[Fe(CN)6]‚8H2O (monoclinic, space group
P21/n). Such an assignment agrees with the observed behavior
for pure M2[Fe(CN)6]‚8H2O samples on dehydration (Figure
2, inset). For the zinc complex, the heat-induced structural
change, from cubic to hexagonal, is also detected in the obtained
Mo¨ssbauer spectra. In contrast to the cubic phase, a very low
value of quadruple splitting (∆) indicates the formation of the
hexagonal modification. In the R3hc structure the Zn atom is
coordinated to only four N atoms in a tetrahedral configuration,
which favors a relatively high symmetry for the charge
distribution around the iron nucleus.
Figure 5. Thermo-gravimetric curves for the studied transition metal
ferricyanide samples. Cu3Fe2 appears as the less stable ferricyanide. In
the cobalt complex a thermal-induced inner charge transfer with the
formation of a highly stable low-spin Co(III) ferrocyanide takes place.
observed frequency (2099 cm-1) for the ν(CN) vibration,
preserves the tetrahedral coordination. For an octahedral coor-
dination that vibration falls at 2091 cm-1, for instance, in mixed
zinc-cesium ferrocyanide (Fm3hm).
According to the IR results discussed above, with the
exception of the cobalt complex, the remaining ferricyanides
decompose on heating, forming M(II) ferrocyanides as the main
intermediate products. No formation of Ni(III), Cu(III), and Zn-
(III) ferrocyanides and of CoII-CN-FeIII and CuII-CN-FeIII
species was detected. The existence of these last isomers is
highly improbable since it requires a CN ligand flipping. Such
a linkage isomerization process is only favored when, in the
new configuration, the metal that is bonded to the C end has
filled or partially filled the t2g orbitals and emptied the eg ones.
This leads to a higher stability of the new solid network through
a stronger metal-ligand bond. For instance, ferrous chromi-
cyanide and manganicyanide undergo CN ligand flipping to
form Cr(II) and Mn(II) ferrocyanides11 because in the former
compounds the CN ligands are interacting with only three and
four t2g electrons, respectively, while in the resulting ferrocya-
nides such an interaction takes place with six t2g electrons. This
favors a stronger M-CN bond through a higher π back-donation
from the iron atom toward the CN group. Such a CN flipping
process is unfavorable for Co(II) and Cu(II) because they have
(t2g)5(eg)2 and (t2g)6(eg)3 as electronic configurations, respec-
tively, without the possibility of forming stable octahedral
complexes with CN groups at the C end. In the case of cobalt,
the option to achieve a higher stability is in the observed charge
transfer to form Co(III) ferrocyanide where the cobalt atom
remains bonded to the N end but changing its configuration to
low spin. This contributes to a higher stability of the complex
through a stronger bond of both metal centers to the CN ligands.
Once ferricyanides have decomposed and ferrocyanides are
formed, the last ones remain stable up to 350 °C where evidence
of their final decomposition appears. The liberated CN- groups
reduce the transition metals involved to their metallic state. Such
end solid byproducts have been well-identified and characterized
from XRD, Mo¨ssbauer, and magnetic data.5-8
E. Head-Induced Structural Changes. Figure 1 shows a
sequence of powder XRD patterns for a Cu(II) ferricyanide
sample submitted to heat treatment. On the reduction reaction
with the formation of ferrocyanide, a progressive peak shift to
higher values of 2θ is observed, indicating the occurrence of a
cell contraction (Figure 1, inset A) but preserving the material
framework. On being heated, within the material framework
CN- groups are liberated, which reduce neighboring iron atoms
to form Cu(II)-NtC-Fe(II) chains. Within the sample bulk,
this process takes place randomly, and the net effect is
appreciated as a cell contraction since in the formed ferrocyanide
the C-Fe interatomic distance is smaller than that in the starting
ferricyanide. Such behavior on heating was observed for Ni-
(II), Cu(II), and Zn(II) ferricyanides. For the Co(II) complex
the cell contraction and the solid solution formation are related
to the already mentioned inner charge transfer. For these four
metals the formed ferrocyanide preserves the crystal framework
of the parent ferricyanide, and the partially reduced material
appears as a solid solution of M-NtC-Fe(III) and M-Nt
D. Mo1ssbauer Spectra. The Mo¨ssbauer data obtained provide
additional insight on the heat-induced changes in the studied
molecular magnets. On the ferricyanide sample dehydration, an
increase in the quadrupole splitting (∆) value was observed
(Figure 2b, Table 1). In the crystal structure of the M3Fe2
complexes studied, the outer metal (M) has a mixed coordination
sphere formed by four N atoms from CN ligands plus two water
molecules. The CN group (at the N end) and the water molecule

Thermal-Induced Changes in Molecular Magnets
J. Phys. Chem. B, Vol. 110, No. 14, 2006 7301
C-Fe(II) chains within the same network. Except for Co, in
the formed ferrocyanides a fraction of M(II) atoms probably
remains occupying interstitial sites as charge balance cations.
For Mn(II) and Cd(II) ferricyanides, from a certain degree of
decomposition the formation of M2[Fe(CN)6] is detected, which
incorporates water to form M2[Fe(CN)6]‚8H2O (monoclinic P21/
n).25,27
It seems that the accompanied secondary byproducts during
the heat-induced reduction process of ferricyanides to ferro-
cyanides usually have low crystalline order. No reflections
attributed to such decomposition byproducts were observed in
the corresponding powder XRD patterns.
When the heating was carried out in a vacuum and the powder
XRD pattern was recorded under this condition, a significant
cell contraction on the crystal water removal was observed
(Figure 1, inset B). This is equivalent to a shortening of the
M-NtC-Fe-CtN-M chain length and to a stronger interac-
tion among the metals through the ligands. This corroborates
the evidence discussed above from Mo¨ssbauer spectra of
anhydrous ferricyanides, observed as a large increase in the
value of ∆ on the water removal process.
F. Heat-Induced Changes in the Magnetic Properties. In
ferricyanides, the magnetic order is established by an indirect
exchange interaction between the metallic centers through the
CN bridges. According to the results from IR, Mo¨ssbauer, and
XRD data discussed above, once the studied ferricyanides have
decomposed, ferrocyanides are obtained where poor magnetic
order is expected. In ferrocyanides, all of the magnetic interac-
tion proceeds from the M metal centers because the iron(II)
remains in a low-spin state (S ) 0). However, as will be
discussed below, for Mn an interesting magnetic interaction was
found.
For the studied ferricyanides, three different magnetic be-
haviors are expected. Zinc and cadmium ferricyanides may show
a weak anti-ferromagnetic order. For Ni and Cu ferricyanides,
a ferromagnetic order may be present, while, for Mn and Co,
mixed ferro- and anti-ferromagnetic interactions are expected
for a net ferrimagnetic order. The estimated values for the Curie
(Tc) and Curie-Weiss (θCW) temperatures, Curie constant (C),
Ne´el temperature (TNe´el), residual magnetization (MR), magne-
tization at 5173 kA/m (6.5 T) (M6.5T), and coercive field (HC)
for the studied transition metal ferricyanide samples are reported
in Table S2 of the Supporting Information.
Figure 6. Field-cooled magnetization curves as a temperature function,
(MFC(T)), at an applied field of 0.4 kA/m (5 Oe), for Cu3Fe2 and Ni3-
Fe2 before and after their annealing. Inset: MFC vs T curves for the
Ni(II) complex samples. The samples are (a) the as-synthesized
ferricyanides, (b) annealed at 80 °C, (c) annealed at 100 °C, (d) annealed
at 120 °C, (e) annealed at 140 °C, and (f) annealed at 160 °C.
presence of a net anti-ferromagnetic interaction, and its relatively
low absolute value indicates that it is weak. The weaker
interaction corresponds to the hexagonal phase, although the
1
metallic centers FeIII (t2g1, S ) /2) are located at a shorter
distance from one another. It seems other factors are contributing
to a reduction in the effective interaction between the unpaired
electrons on the iron atoms, probably the occurrence of certain
deviation of the Fe-CtN-Zn-NtC-Fe chain from the
linearity, related to the tetrahedral coordination of the Zn atom
to the NC ligands. For a tetrahedral coordination the N-Zn-N
angle is >90 °. This reduces the overlapping density from
neighboring FeIII ions. Such deviation is absent in the cubic
phase where the N-Zn-N angle is 90°. The cubic structure is
formed by a three-dimensional network of Fe-CtN-Zn-Nt
C-Fe linear chains. This favors an optimal overlapping of t2g
orbitals from neighboring FeIII atoms and a higher θCW value.
The second studied magnetic behavior corresponds to Ni and
Cu ferricyanides. These compounds have ferromagnetic order
because the magnetic interaction results from orthogonal orbitals.
Low-spin Fe (III) has an unpaired electron in π orbitals, t2g5 (S
Zinc ferricyanide is an example of the first type of behavior.
In this complex the superexchange is established through the
)
1/2), while, nickel(II) and copper(II) have only unpaired
5
3
electrons in σ orbitals, eg2 and eg , respectively. Figure 6 shows
the field-cooled magnetization (MFC(T)) curves as a function
of temperature, at an applied field of H ) 0.4 kA/m (5 Oe), for
Cu(II) and Ni(II) ferricyanides before and after their annealing
at 80, 100, 120, 140, and 160 °C. The reduction of ferricyanide
to ferrocyanide on heating is accompanied by a decrease in the
measured value for MFC. Below 20 K for copper and 25 K for
nickel, the magnetization saturates in the characteristic way for
a ferromagnetic material. From the maximum in the dMFC/dT
curve, the critical (Curie) temperature (Tc) for the onset of long-
range ferromagnetic order was estimated, which resulted in 17.9
and 18.9 K for Cu(II) and Ni(II) ferricyanide, respectively.
As has been discussed above, for nickel and copper and for
cobalt and zinc as well, the reduction of ferricyanide to
ferrocyanide on heating takes place through the formation of a
solid solution of ferric and ferrous species. From this fact, as
the amount of Ni(II) or Cu(II) ferrocyanide in the annealed
sample increases, the material shows a ferrimagnetic behavior
as a net effect from combined ferro- and anti-ferromagnetic
overlapping of identical t2g orbitals from iron atoms, and it
has anti-ferromagnetic nature as a result of the Pauli exclusion
principle.30 As has been already discussed, the heat-induced
dehydration of zinc ferricyanide induces a structural transforma-
tion to form the hexagonal R3hc phase. In both crystalline phases,
cubic and hexagonal, the overlapping density is small because
the superexchange interaction is established among the second
nearest neighbors, an unfavorable situation for a magnetic
interaction. In the cubic structure, the FeIII ions appear at a
distance of 10.2 Å, and at 7.4 Å for the hexagonal one. From
this fact the stronger magnetic interaction could be expected
for the R3hc phase.
From a linear fitting, according to the Curie-Weiss law ø-1
) (T - θCW)/C, of the reciprocal mass susceptibility (1/øp) vs
temperature curve, the values for the Curie constant C and for
the Curie-Weiss temperature (θCW) in the zinc ferricyanide
phases were estimated. The obtained θCW values are -5.1(1)
and -3.7(1) K for cubic and hexagonal zinc ferricyanide,
respectively. The negative sign of θCW corresponds to the

7302 J. Phys. Chem. B, Vol. 110, No. 14, 2006
Mart´ınez-Garcia et al.
Figure 7. Field-cooled magnetization curves as a temperature function,
(MFC(T)), at an applied field of 0.4kA/m (5 Oe), for Mn3Fe2 before
and after its annealing. The samples are (a) as-synthesized Mn(II)
ferricyanide, (b) annealed at 100 °C, (c) annealed at 120 °C, and (d)
annealed at 140 °C (d). Inset: The corresponding temperature
dependence of the reciprocal mass susceptibility (1/øp(T)).
Figure 8. Reciprocal mass susceptibility (1/øp) versus temperature (T)
curves for a pure Mn(II) ferrocyanide, Mn2[Fe(CN)6]‚8H2O, sample.
Inset: Coordination environment for the Fe and Mn atoms in this
compound. Two neighboring Mn atoms appear linked through two aquo
bridges favoring their anti-ferromagnetic interaction.
interaction between Mn(II) atoms with an estimated θCW of -3.5
K was observed (Figure 8). Such coupling disappears when the
sample is dehydrated, and it is then restored when it is
rehydrated. To the best of our knowledge, an analogue magnetic
interaction between two manganese atoms through aquo bridges
within a basically nonmagnetic matrix has not previously been
reported.
interactions, the last ones among M(II) ions in the formed
ferrocyanide. For the nickel compound, the anti-ferromagnetic
contribution is detected from an annealing of 120 °C, with a
Ne´el temperature of 22.9 K, which increases up to 24 K for the
samples annealed at 140 and 160 °C. For the copper complex,
which is a less stable compound, the anti-ferromagnetic
contribution is detected from an annealing temperature of 100
°C. For the sample at 140 °C, the resulting magnetic order is
very weak with a θCW value of only 2.3 K.
Conclusions
Cobalt ferricyanide shows a behavior on heating very similar
to that observed for the nickel analogue but with a lower stability
and with an anti-ferromagnetic contribution that is present from
the as-prepared sample. The heat-induced charge transfer favors
a fast dilution effect of the Co(II)-NtC-Fe(III) chains and a
progressive decrease in the resulting magnetic order. From an
initial Tc value of 13.5 K, with a θCW of 11.8 K (for the
nonannealed sample), these parameters have fallen to 4 and 0.4
K, respectively, for the sample heat-treated at 120 °C.
Manganese ferricyanide showed a unique behavior on heating.
The ferrimagnetic order observed for the as-prepared compound
appears combined to a relatively strong anti-ferromagnetic
contribution for samples annealed above 100 °C. Such a
dominant anti-ferromagnetic contribution is observed as a
relatively large increase in the zero field-cooling magnetization
(MFC) (Figure 7), which reaches a maximum value and then
falls on sample cooling. With a higher annealing temperature,
the more pronounced is that effect. That increasing anti-
ferromagnetic contribution was assigned to neighboring Mn-
(II) atoms in the formed Mn2[Fe(CN)6]‚8H2O ferrocyanide.
Unlike the remaining ferrocyanides, in this compound two
neighboring M atoms remain bridged by two aquo bridges,21,25,27
which allows a strong anti-ferromagnetic interaction between
them through the oxygen atoms of the water molecules. These
Mn(II) atoms remain at a relatively short distance (3.67 Å).27
This favors a significant electron cloud overlapping, although
the Mn-O-Mn angle, close to 100°, is an unfavorable factor
for a strong magnetic interaction.
On being heated divalent transition metal ferricyanides reduce
to ferrocyanides. No occurrence of linkage isomerism was
observed. The formation of Ni(III), Cu(III), and Zn(III) hexa-
cyanoferrates(II) was also discarded. Only for cobalt, an inner
charge transfer with formation of low-spin Co(III) ferrocyanide
was detected. Except for the case of Mn(II) and Cd(II)
ferricyanides, the thermal decomposition and the accompanied
reduction process led to the formation of a solid solution of
ferric and ferrous species within the same framework. For Mn-
(II) and Cd(II), the reduction process on heating leads to the
formation of an anhydrous M2[Fe(CN)6] phase, which in contact
with humid air incorporates water to form octahydrates, M2-
[Fe(CN)6]‚8H2O. The hydration process is relatively fast for
the manganese complex and slow for the cadmium one. The
formed Mn2[Fe(CN)6]‚8H2O shows a strong anti-ferromagnetic
interaction attributed to neighboring Mn(II) atoms linked by
aquo bridges. Upon being heated, cubic zinc ferricyanide
dehydrates and undergoes a structural transformation to a
hexagonal phase where the Zn(II) atom is found tetrahedrally
coordinated to N ends of CN ligands. In this hexagonal phase
the FeIII atoms are involved in a weak anti-ferromagnetic
interaction due to the tetrahedral coordination for the Zn atom.
The crystal water removal in the studied family of molecular
magnets has a pronounced effect on the metal center interaction
that is detected through the Mo¨ssbauer data.
Acknowledgment. R.M. thanks the support provided by the
CLAF-ICTP program for his Ph.D. studies. The access to the
LNLS synchrotron radiation facilities is gratefully recognized.
The support from FAPESP and CNPq (Brazilian agencies) is
acknowledged. The authors thank C. Vazquez from IIM-UNAM
for the TG data acquisition.
Such an assignment for the dominant anti-ferromagnetic
contribution in annealed Mn(II) ferricyanide samples was
corroborated from a parallel study in a pure Mn2[Fe(CN)6]‚
8H2O sample. In this compound a strong anti-ferromagnetic

Thermal-Induced Changes in Molecular Magnets
J. Phys. Chem. B, Vol. 110, No. 14, 2006 7303
(7) Ng, C. W.; Ding, J.; Shi, Y.; Gan, L. M. J. Phys. Chem. Solids
Supporting Information Available: Frequency values for
the CN stretching vibration, ν(CN), in the studied transition
metal ferricyanides and in their heat-induced decomposition
products, the values obtained for the Curie (Tc) and Curie-
Weiss (θCW) temperatures, Curie constant (C), Ne´el temperature
(TNe´el), residual magnetization (MR), magnetization at 5173
kA/m (6.5 T) (M6.5T), and coercive field (HC) in the studied
transition metal ferricyanides samples before and after their
annealing, and the pseudo-Lorentzian line shape function used
for the Mo¨ssbauer spectra fitting. This material is available free
of charge via the Internet at http://pubs.acs.org.
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