Thermal decomposition of Prussian blue analogues of the type Fe[Fe(CN)5NO]

Article abstract of DOI:10.1515/znb-2000-0803

The structure and properties of the thermal decomposition products of Fe[Fe(CN)₅NO] · x H₂O (x = 5～6) have been studied by Moessbauer and FT-IR spectroscopy, X-ray diffraction and conductivity measurements. The valence state and coordination environment of the iron ions change dramatically when the nitrosyl ligand is eliminated by heat-treatment under vacuum at 200 °C. The dark blue product obtained by heat-treatment under vacuum at 250 °C is characteristic of the mixed-valence state in Prussian blue analogues. The electrical conductivity of the dark blue product is higher by a factor of 10³ than that of the starting material because of the mixed-valence state between Fe(III)[Fe(II)(CN)₅] and Fe(II)[Fe(III)(CN)₅]. Heat-treatment under vacuum at 350°C yields a new product Fe(II)[Fe(II)(CN)₄] the crystal structure of which is different from that of the starting material. The electrical conductivity of the decomposition product obtained at 350 °C is about 10⁵ times higher than that of the starting material.

Full text of DOI:10.1515/znb-2000-0803

Thermal Decomposition of Prussian Blue Analogues
of the Type Fe[Fe(CN)5NO]
Hidenari Inoue3, Shuichi Narino3, Naoki Yoshioka3, and Ekkehard Fluckb
3 Department of Applied Chemistry, Keio University,
3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan
b Max-Planck-Haus, Berliner Strasse 10, D-69120 Heidelberg, Germany
Reprint requests to Prof. Dr. Dr. h. c. E. Fluck. Fax: +49 (0)6221 486-385
Z. Naturforsch. 55 b, 685-690 (2000); received April 5, 2000
Prussian Blue Analogues, Mixed-Valence State, Thermal Decomposition
The structure and properties of the thermal decomposition products of Fe[Fe(CN)sNO] • x
H2O (x = 5~6) have been studied by Mössbauer and FT-IR spectroscopy, X-ray diffraction, and
conductivity measurements. The valence state and coordination environment of the iron ions
change dramatically when the nitrosyl ligand is eliminated by heat-treatment under vacuum
at 200 °C. The dark blue product obtained by heat-treatment under vacuum at 250 °C is
characteristic of the mixed-valence state in Prussian blue analogues. The electrical conductivity
of the dark blue product is higher by a factor of 103 than that of the starting material because of
the mixed-valence state between Fe(III)[Fe(II)(CN)5] and Fe(II)[Fe(III)(CN)5]. Heat-treatment
under vacuum at 350°C yields a new product Fe(II)[Fe(II)(CN)4] the crystal structure of which
is different from that of the starting material. The electrical conductivity of the decomposition
product obtained at 350 °C is about 105 times higher than that of the starting material.
Introduction
Fe[Fe(CN)5N0]-jtH20 under vacuum. It has been
demonstrated that the physical properties such as
electrical conductivity are dramatically improved
by this treatment.
Prussian blue and its analogues have attracted a
great deal of attention because of their importance in
functional pigments, electrochromic materials and
semiconductors. Their interesting properties are as- Experimental
sociated with the mixed-valence state. Of the syn-
thetic methods of these functional materials, heat-
Materials
Sodium pentacyanonitrosylferrate(II) dihydrate was
treatment under vacuum, i. e. vacuum pyrolysis, has
been a useful means to produce the mixed-valence
states [1]. Although the thermal decomposition of
Prussian blue and its analogues has been reported
in previous papers [2-3], little attention has been
focused on analogues of the type Fe[Fe(CN)5NO].
Surprisingly, Prussian blue analogues of the type
M[Fe(CN)5NO] xH20 (M = Mn, Fe, Co, Ni, Cu
and Zn) have been investigated by a variety of spec-
troscopic methods, e.g. electronic absorption [4], in-
frared absorption [5, 6], Mössbauer [7, 8] and X-ray
photoelectron spectroscopy [9]. However, the ther-
mal decomposition of M[Fe(CN)5N 0] jcH20 has
received less attention, although a few Prussian blue
analogues have been extensively studied [10]. In
the present work we have attempted to produce a
new type of mixed-valence state or electronic state
in macromolecular complexes by heat-treatment of
obtained from Wako Pure Chemical Ind., iron(II) sul-
fate heptahydrate was purchased from Kanto Chem-
icals Co. and used without further purification. All
solvents used were of reagent grade and used as re-
ceived. Fe[Fe(CN)5N0] ^H20 (x = 5~6) was prepared
as follows. A 0.02 M aqueous solution (100 cm3) of
FeS0 4 -7 H2 0 was added to a 0.02 M aqueous solution
(100 cm3) of Na2 [Fe(CN)5N0]-2H20 with stirring. Af-
ter adding 200 cm3 of acetone the mixed solution was al-
lowed to stand overnight. The precipitates obtained were
collected by filtration, washed with distilled water till sul-
fate ions could not be detected in the filtrate, and finally
washed with acetone and dried in air for a week.
Heat-treatment under vacuum
The sample was pre-dried in vacuo at 120 °C for 8 h to
remove water. The sample was then heated under vacuum
in a quartz tube at different temperatures (200 ~ 350 °C)
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H. Inoue et al. ■Thermal Decomposition of Prussian Blue Analogues
for 24 h and subsequently quenched to liquid nitrogen
temperature in a Dewar vessel.
Instruments
The infrared spectra were obtained with a BIO-RAD
FTS-65 infrared spectrometer on KBr pellets and Nu-
jol mulls. The X-ray diffraction powder patterns were
taken at room temperature with a Rigaku-Denki Geiger-
flex RAD-C X-ray diffractometer using graphite-mono-
chromated Cu-KQ radiation (A = 0.1542 nm). Thermo-
gravimetric analyses (TG) and differential thermal anal-
yses (DTA) were carried out simultaneously by using a
Seiko TG/DTA 320 thermal analyzer. About 40 mg of
powdered samples were heated at the constant rate of
5 °Cmin-1 in an atmosphere of argon. The DTA curves
were also recorded at the heating rate of 5 °Cmin_1 with
Q-alumina as reference material in an atmosphere of ar-
gon. The Mössbauer spectra were measured by a Wissel
constant-acceleration transducer coupled with an ORTEC
5500 multichannel analyzer. A 370 MBq 57Co source in
rhodium was purchased from The Radiochemical Centre,
Amersham, England. The spectra obtained at 293 K were
fitted to Lorentzian curves using an iterative least-squares
computer program. The isomer shifts were referred to the
symmetry center of the sextet of natural iron foil kept
at 293 K. The electrical conductivity of the heat-trea-
ted samples was measured with a Takeda-Riken TR-8651
electrometer by the usual current-voltage method. The
details of the conductivity measurements were already
described in [11].
Temperature / °C
Fig. 1. TG and DTA curves of Fe[Fe(CN)5N0] xH20.
Results and Discussion
153 and 262 °C with the evolution of nitrogen oxide
and/or cyanogen gas. This decomposition process
is slightly exothermic in the DTA curve, but not ex-
plicitly observed in the TG curve. Presumably the
loss rate of cyanogen and nitrogen monoxide is very
slow [11], Further heating causes a sharp exother-
mic decomposition at 273 °C and leads to the for-
mation ofFe[Fe(CN)4], followed by an exothermic
decomposition at 320 °C ascribed to the structural
rearrangement of the cyano ligands. The gaseous
products released during the pyrolysis at 250 °C
were analyzed earlier by conventional spectropho-
tometric methods [12].
Therm al analysis
The water in Fe[Fe(CN)5N0] ;cH20 can be re-
moved upon heating under vacuum without any
change in the three-dimensional framework of
Fe[Fe(CN)5NO]. The process is reversible at tem-
peratures lower than 120 °C. As Fig. 1 shows,
the thermogravimetric (TG) and differential thermal
analysis (DTA) indicate that Fe[Fe(CN)5N0] jcH20
begins to lose water around 50 °C and is endother-
mically dehydrated at 144 °C. The water con-
tent calculated from the weight loss of the TG
curve corresponds to x = 5.6. The water content
in Fe[Fe(CN)5N0]-xH20 varied in the range from
5 to 6 depending on the atmosphere to which the
compound is exposed.
Structural change with therm al decomposition
The starting material Fe[Fe(CN)5N0]jtH20
After the endothermic dehydration,
(jc = 5~6) crystallizes in space group Fm
3
m, a =
Fe[Fe(CN) NO] decomposes gradually between
5
1042 pm , Z = 4 [4, 7]. The face-centered cubic
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687
H. Inoue et al. •Thermal Decomposition of Prussian Blue Analogues
Table 1. IR data of heat-treated samples in the CN and
NO stretching reagion.
Assignment
Sample Heat-treatment
temp. (°C)
i' cn (cm-1)
f^No (cm-1)
not treated3 2184(vs)b
1934(vs)
1949(w)
a
b
c
d
2186(w), 2076(vs)
200
300
350
2076(vs)
2076(sh), 2033(vs)
a Starting material;b vs = very strong; w = weak;
sh = shoulder.
300 °C, the X-ray diffraction peaks (Fig. 2) of the
starting material disappear. The sample heated un-
der vacuum at 350 °C for 24 h exhibits a new well-
resolved X-ray diffraction pattern, indicating that a
structural rearrangement of the cyano ligands or a
crystallization of amorphous intermediates occurs
between 300 and 350 °C. This is supported by the
fact that an exothermic peak without weight loss is
observed at 320 °C in the DTA curve (cf Fig. 1).
However, many efforts failed to index the diffrac-
tion peaks to a plausible crystal system probably
due to the presence of impurities.
The IR spectral data of the solid residue obtained
by heating under vacuum at 200, 300, and 350 °C
are listed together with those of the starting mate-
rial in Table 1. Besides the CN stretching band at
2184 cm-1 of the starting material, a new broad CN
stretching band appears around 2076 cm-1 in the
sample treated under vacuum at 200 °C. The NO
stretching band observed around 1934 cm-1 disap-
pears almost completely after heating the sample
to 250 °C for 24 h. The intermediate produced at
2 0 / degree
Fig. 2. X-ray diffraction powder patterns of starting ma-
terial (a) and the samples heated at 200 °C (b), at 300 °C
(c) and at 350 °C (d).
unit cell forms the three-dimensional framework 300 °C under vacuum exhibited u(C=N) around
Fen-C =N -Fen in which Fe[Fe(CN)5N0]-xH20 2076 cm-1 which is shifted to lower frequencies by
(.x = 5~6) consists of the high-spin Fe11coordinated about 108 cm“ 1as compared with the starting mate-
by the nitrogen ends of the cyano ligands and the rial and similar to that of Prussian blue, i. e. iron(III)
oxygen end of the nitrosyl ligand and the low-spin
Fe11 coordinated by the carbon ends of the cyano to that observed on going from ferricyanide to fer-
hexacyanoferrate(II). This large shift is comparable
ligands and the nitrogen end of the nitrosyl ligand.
As Fig. 2 shows, the X-ray powder diffraction pat-
rocyanide [13], suggesting that upon heating above
250 °C the reduction of the central iron(III) ion to
tern of the sample heated under vacuum at 200 °C the iron(II) ion occurs with loss of cyanogen gas.
for 24 h is much the same as that of the starting ma- The i'(CN) of the sample heated to 350 °C appeared
terial, suggesting that the three-dimensional frame- at 2033 cm-1 with a shoulder around 2076 cm-1,
work characteristic of Prussian blue analogues is
dramatically shifted to lower frequencies by about
still maintained although a few peaks begin to ap- 43 cm-1 compared with that observed for the sam-
pear in the heat-treated sample. At this stage the ple heated to 300 °C. This large shift in the CN
color of the product is quite similar to that of Prus- stretching band is consistent with the conclusion
sian blue. After 24 h of heating under vacuum at drawn from the X-ray powder diffraction of the ther-
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688
H. Inoue et al. • Thermal Decomposition of Prussian Blue Analogues
Table 2. Mössbauer parame-
ters obtained at 293 K and
peak assignment.
Mössbauer parameter
b (mms-1) A E q (mms-
Sample Heat-treatment
temp. (°C)
Assignment
x)
a
-0.32
1.11
-0.33
1.10
-0.19
0.36
-0.14
0.31
not treated3
200
1.89
1.49
1.92
1.51
-
Fe(CN)5N 0 2- (low-spin)
Fe2+ (high-spin)
a Starting material.
b
Fe(CN)5NO
(low-spin)
Fe + (high-spin)
Fe(CN)5 - (low-spin)
Fe3+ (high-spin)
Fe(CN)5 - (low-spin)
Fe3+ (high-spin)
Fe2+ (high-spin)
0.68
-
c
300
350
0.55
0.90
-
1.29
-0.14
1.07
d
Fe(CN)4 - (low-spin)
Fe + (high-spin)
-
mal decomposition product, indicating a structural
rearrangement or a crystallization of amorphous in-
termediates between 300 and 350 °C.
Electronic structure o f therm al decom position
products
The 57Fe Mössbauer spectra of the samples are
shown in Fig. 3 together with that of the starting ma-
terial. The Mössbauer parameters obtained at 293 °C
are listed in Table 2. The Mössbauer spectrum of the
starting material Fe[Fe(CN)5N0] jcH20 consists of
a pair of quadrupole doublets. One is assigned to the
low-spin iron(II) ion of the Fe(CN)5N 0 2- moiety
and the other is attributed to the high-spin iron(II)
ion [14]. The sample heated to 200 °C for 24 h ex-
hibits a singlet and a quadrupole doublet other than
a pair of quadrupole doublets originating from the
starting material. The new doublet is characteristic
of high-spin Fe(III) complexes, while the new sin-
glet is typical of low-spin Fe(II) complexes. These
peaks become predominant in the sample heated to
250 °C. Clearly, at 270 °C almost all of the starting
material is converted to a new material tentatively
assigned to Fe[Fe(CN) ]. This mixed-valence com-
5
pound is probably the reason why the area ratio of
the singlet absorption peak to the quadrupole dou-
blet peak is not unity (cf Fig. 3 (b) and (c)).
In the Mössbauer spectrum of the sample heated
to 300 °C, a new quadrupole doublet with 6 =
1.29 mms"1and ^\Eq = 0.90 mms-1 appears. This
doublet is assigned to a high-spin iron(II) species
which is produced by evolution of cyanogen gas
from Fe[Fe(CN)5]. It is noteworthy that the com-
bination of the singlet and the quadrupole doublet
is quite similar to that observed for Prussian blue.
As Fig. 3(d) shows, the sample heated to 350 °C
-4.5 -3.5 -2.5 -1.5 -0.5 0.5 15 2J5 3.5 4.5
Velocity / mms'1
Fig. 3. 57Fe Mössbauer spectra of starting material (a)
and the samples heated at 200 °C (b), at 300 °C (c) and at
350 °C (d).
for 24 h exhibits several singlets which can be as-
signed to the low-spin and high-spin iron(II) species
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H. Inoue et al. •Thermal Decomposition of Prussian Blue Analogues
689
1/T(K) x io3
1/T(K) x 103
Fig. 4. Plots of the logarithm of electrical conductivity v
300 °C (b).
1/T for starting material (a) and the sample heated at
probably corresponding to a product of the formula Table 3. Specific conductivity and activation energy of
heat-treated samples.
Fe(II)[Fe(II)(CN)4].
Sample Heat-treatment Specific conductivity Activation
Electrical conductivity o f therm al decom position
products
temp. (°C) at 293 K (Scm-1)
energy (eV)
0.51 (0.59)b
0.49 (0.59)
0.44 (0.57)
0.38 (0.45)
a
b
c
d
not treated3 5.8 x 10“ 13
2 0 0
300
350
1 . 1
x
1 0 " 9
Plots of the logarithm of electrical conductivity
a versus 1/T are shown for the starting material
and the sample heat-treated at 300 °C in Fig. 4. As
expected, the temperature-dependence of the con-
ductivity is characteristic of semiconductors and
obeys the law a = <7oexp(-Ea/&T), where do is
a constant, Ea the activation energy of electrical
conduction, k the Boltzmann constant, respectively.
The specific conductivity of the starting material
Fe[Fe(CN)5N0] xH20 is small compared to that
of Prussian blue Fe4 [Fe(CN)6 ] 3 -jcH20[10,ll]. This
is probably due to the absence of the mixed-va-
4.1 x 10“
8.5 x 10“
8
8
a Starting material; b activation energy in the higher tem-
perature region.
after heat-treatment at 350 °C for 24 h, even though
the compound turns crystalline in this temperature
range.
The activation energy of electrical conduction
was calculated from the slope of the log a ver-
sus 1/T plots and listed together with the specific
conductivity in Table 3. It is interesting to note that
lence state as in Prussian blue. The conductivity except for the starting material the activation ener-
of the sample heated at 200 °C is significantly im- gies in the impurity and intrinsic regions decrease as
proved and further increased for the sample heated the temperature of the heat-treatment is raised. The
at 300 °C. This conductivity does not change much lowering of the activation energies is ascribed to the
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690______________________________________ H. Inoue et al. • Thermal Decomposition of Prussian Blue Analogues
mixed-valence state created by the heat-treatment
of Fe[Fe(CN)5N 0] xH20. The conduction mecha-
nism in Prussian blue and its analogs has been de-
scribed in terms of the band model [10] and elec-
tron hopping [11]. These two models might be also
applicable to the interpretation of the conduction
mechanism in the thermal decomposition products
of Fe[Fe(CN)5N0]jcH20 .
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