Charge-transfer phase transition and zero thermal expansion in caesium manganese hexacyanoferrates

Article abstract of DOI:10.1039/b605269j

A series of caesium manganese hexacyanoferrates is prepared; Cs ^I_1.78Mn^II[Fe^II(CN)₆] _0.78[Fe^III(CN)₆]_0.22 (1), Cs ^I_1.57Mn^II[Fe^II(CN)₆] _0.57[Fe^III(CN)₆]_0.43 (2), Cs ^I_1.51Mn^II[Fe^II(CN)₆] _0.51[Fe^III(CN)₆]_0.49 (3), and Cs^I_0.94Mn^II[Fe^II(CN) ₆]_0.21[Fe^III(CN)₆] _0.70·0.8H₂O (4). 1-3 show charge-transfer phase transitions between the high-temperature (HT) and low-temperature (LT) phases with transition temperatures (T_1/2↓, T_1/2↑) of (207 K, 225 K) (1), (190 K, 231 K) (2), and (175 K, 233 K) (3) at a cooling and warming rates of 0.5 K min^-1. Variable temperature IR spectra indicate that the valence states of the LT phases of 1-3 are Cs^I _1.78Mn^II_0.78Mn^III _0.22[Fe^II(CN)₆], Cs^I _1.57Mn^II_0.57Mn^III _0.43[Fe^II(CN)₆], and Cs^I _1.51Mn^II_0.51Mn^III_0.49 [Fe^II(CN)₆], respectively. The XRD measurements for 1-3 show that crystal structures of the HT and LT phases are cubic structures (Fm3m), but the lattice constants decrease from the HT phase to the LT phase; a = 10.5446(17) → 10.4280(7) A (1), 10.5589(17) → 10.3421(24) A (2), and 10.5627(11) → 10.3268(23) A (3). The magnetization vs. temperature curves and the magnetization vs. external magnetic field curves show that the LT phases are ferromagnetic with Curie temperatures of 4.3 (1), 5.0 (2), and 5.6 K (3). At a cooling rate of -0.5 K min^-1, 4 does not show the charge-transfer phase transition, but does show a behavior of zero thermal expansion with a thermal expansivity of +0.2 × 10^-6 K^-1 throughout the temperature range 300 and 20 K. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b605269j

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PAPER
www.rsc.org/dalton | Dalton Transactions
Charge-transfer phase transition and zero thermal expansion in caesium
manganese hexacyanoferrates†
Tomoyuki Matsuda,a,b Hiroko Tokoro,a,b Kazuhito Hashimotoa and Shin-ichi Ohkoshi*a,b
Received 12th April 2006, Accepted 25th August 2006
First published as an Advance Article on the web 7th September 2006
DOI: 10.1039/b605269j
A series of caesium manganese hexacyanoferrates is prepared; CsI1.78MnII[FeII(CN)6]0.78[FeIII(CN)6]0.22
(1), CsI1.57MnII[FeII(CN)6]0.57[FeIII(CN)6]0.43 (2), CsI1.51MnII[FeII(CN)6]0.51[FeIII(CN)6]0.49 (3), and
CsI0.94MnII[FeII(CN)6]0.21[FeIII(CN)6]0.70·0.8H2O (4). 1–3 show charge-transfer phase transitions between
the high-temperature (HT) and low-temperature (LT) phases with transition temperatures (T1/2↓, T1/2↑
of (207 K, 225 K) (1), (190 K, 231 K) (2), and (175 K, 233 K) (3) at a cooling and warming rates of
)
0.5 K min−1. Variable temperature IR spectra indicate that the valence states of the LT phases of 1–3 are
CsI1.78MnII0.78MnIII0.22[FeII(CN)6], CsI1.57MnII0.57MnIII0.43[FeII(CN)6], and CsI1.51MnII0.51MnIII [FeII(CN)6],
0.49
respectively. The XRD measurements for 1–3 show that crystal structures of the HT and LT phases are
¯
cubic structures (Fm3m), but the lattice constants decrease from the HT phase to the LT phase; a =
˚
˚
˚
10.5446(17) → 10.4280(7) A (1), 10.5589(17) → 10.3421(24) A (2), and 10.5627(11) → 10.3268(23) A
(3). The magnetization vs. temperature curves and the magnetization vs. external magnetic field curves
show that the LT phases are ferromagnetic with Curie temperatures of 4.3 (1), 5.0 (2), and 5.6 K (3). At
a cooling rate of −0.5 K min−1, 4 does not show the charge-transfer phase transition, but does show a
behavior of zero thermal expansion with a thermal expansivity of +0.2 × 10−6 K−1 throughout the
temperature range 300 and 20 K.
using the Rb+ ion, a series of CsxMn[Fe(CN)6]y·zH2O (x = 0.94–
1. Introduction
1.78, y = 0.92–1) was prepared. This article describes the synthesis,
electronic states, crystal structures, and magnetic properties of this
series.
Prussian Blue analogues1,2 are condensed matter with a strong
cooperativity since the transition metal ions are bridged by ligands
with a 3-D network structure. Various interesting properties
have been reported with Prussian Blue analogues,3–11 e.g., high
Curie temperatures (Tc),3 humidity responses,4 photomagnetism,5
pressure-induced CN flip,6 and zero thermal expansion.7,8 Prussian
Blue analogues have several types of crystal structure such
2. Experimental
The target materials were prepared by reacting an aqueous
solution of MnCl2 (a mol dm−3) and CsCl (b mol dm−3) with
a mixed aqueous solution of K3[Fe(CN)6] (a mol dm−3) and
CsCl (b mol dm−3): (a, b) = (0.01, 1) (condition I), (0.02,
1) (condition II), (0.025, 5) (condition III), and (0.02, 0.03)
(condition IV).12 The precipitates were filtered and dried, yielding
powders. Caesium(I) manganese(II) hexacyanoferrate(II)13 was
also prepared as a reference by reacting an aqueous solution of
MnCl2 (0.025 mol dm−3) and CsCl (1 mol dm−3) with a mixed
aqueous solution of K4[Fe(CN)6] (0.025 mol dm−3) and CsCl
(1 mol dm−3).
Elemental analyses of the synthesized samples were performed
by inductively coupled plasma mass spectrometry for Cs, Mn,
and Fe and standard microanalytical methods for C and N. The
infrared (IR) spectra were recorded on a Shimadzu FT-IR
8200PC spectrometer. The X-ray photoelectron spectroscopy
(XPS) measurements were conducted by a Physical Electronics
(PHI) model 5600 spectrometer equipped with a hemispherical
capacitor analyzer. The X-ray powder diffraction (XRD) patterns
were measured with a Rigaku RINT2100 instrument (Cu Ka).
The magnetic properties were investigated using a Quantum
Design MPMS-5S superconducting quantum interference device
magnetometer.
¯
as MA[MB(CN)6]2/3·zH2O (space group = Fm3m), AMA[MB(CN)6]
¯
¯
(F43m), and A2MA[MB(CN)6] (Fm3m) where MA and MB are
transition metal ions, and A is an alkali metal ion in the
interstitial site of the lattice.2 The physical properties and functions
of Prussian Blue analogues are influenced by the type of AI
incorporated. For example, Bleuzen et al. have reported that the
position of the Cs+ ion in Cs0.7Co4[Fe(CN)6]2.9·16H2O is displaced
from the center of the interstitial site and the CoII–NC–FeIII
entities are bent.10 In addition, we recently reported a charge-
transfer phase transition with RbMn[Fe(CN)6], which is caused
by a metal-to-metal charge-transfer from MnII to FeIII and a Jahn–
Teller distortion of the produced MnIII.11 In this work, instead of
aDepartment of Applied Chemistry, School of Engineering, The University
of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan. E-mail:
ohkoshi@chem.s.u-tokyo.ac.jp
bDepartment of Chemistry, School of Science, The University of Tokyo, 7-3-1
Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan
† Electronic supplementary information (ESI) available: The phase tran-
sition temperatures of CsxMn[Fe(CN)6]y·zH2O, O 1s XPS spectra for
1–4, IR spectra, magnetic susceptibility, and XRD patterns for 5,
Weiss temperatures for 1–4, and magnetic properties of 4. See DOI:
10.1039/b605269j
5046 | Dalton Trans., 2006, 5046–5050
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©

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found; Cs, 32.5; Mn, 14.7; Fe, 13.1; C, 16.5; N, 19.5%. The mixing
of FeII(CN)6 in 1–4 is due to the reduction of FeIII(CN)6 in the
prepared solution. The observed densities of the samples were
2.8 (1), 2.6 (2), 2.5 (3), and 2.2 g cm−3 (4). These densities are
almost consistent with the calculated densities of 2.93 (1), 2.74 (2),
2.68 (3), and 2.17 g cm−3 (4), considering the contamination by
MnO2. The reference sample, which was caesium(I) manganese(II)
hexacyanoferrate(II), was pale green. The CN stretching peak of
FeII–CN–MnII was observed at 2082 cm−1 (see ESI†). The formula
was CsI2MnII[FeII(CN)6]·3.5H2O (5): calculated: Cs, 44.6; Mn, 9.2;
Fe, 9.4; C, 12.1; N, 14.1%: found; Cs, 44.3; Mn, 9.2; Fe, 9.7; C,
11.9; N, 13.8%. Fig. 2 schematically illustrates the crystal structure
of the obtained compounds.
3. Results and discussion
3.1. Materials
The precipitates under conditions I–III were dark brown
powders. At 300 K, the CN stretching frequencies of FeIII–
CN–MnII and FeII–CN–MnII were observed in the IR spec-
tra (Fig. 1, left). Elemental analyses showed that the for-
mulas of the obtained compounds under conditions I–III
were CsI1.78MnII[FeII(CN)6]0.78[FeIII(CN)6]0.22 (1) + 0.35MnO2,
CsI1.57MnII[FeII(CN)6]0.57[FeIII(CN)6]0.43 (2)
+ 0.24MnO2, and
CsI1.51MnII[FeII(CN)6]0.51[FeIII(CN)6]0.49 (3) + 0.19MnO2, respec-
tively: calculated wt% for the sum of 1 and 0.35MnO2: Cs, 44.3;
Mn, 13.9; Fe, 10.5; C, 13.5; N, 15.7%: found; Cs, 44.5; Mn, 14.1;
Fe, 10.6; C, 13.5; N, 15.7%, calculated wt% for the sum of 2 and
0.24MnO2: Cs, 42.0; Mn, 13.7; Fe, 11.3; C, 14.5; N, 16.9%: found;
Cs, 42.0; Mn, 13.5; Fe, 11.0; C, 14.2; N, 16.7%, calculated wt% for
the sum of 3 and 0.19MnO2: Cs, 41.5; Mn, 13.5; Fe, 11.5; C, 14.9;
N, 17.4%: found; Cs, 41.9; Mn, 13.5; Fe, 11.2; C, 14.4; N, 17.0%.
To confirm the existence of MnO2, XPS spectra were measured.
The O 1s XPS spectra showed peaks at 533.3 eV and 529.9 eV
(see ESI†). The peak at 533.3 eV is assigned to be oxygen of the
water molecule in CsxMn[Fe(CN)6]y·zH2O and the substrate. The
peak at 529.9 eV is assigned to MnO2, which is consistent with
previous reports.14 The precipitate obtained under condition IV
was a light brown powder. The observed CN stretching peaks in
the IR spectrum at 2151 and 2078 cm−1 were assigned to the
CN groups of FeIII–CN–MnII and FeII–CN–MnII, respectively.
The formula was CsI0.94MnII[FeII(CN)6]0.21[FeIII(CN)6]0.70·0.8H2O
(4): calculated: Cs, 32.3; Mn, 14.2; Fe, 13.1 C, 16.9; N, 19.8%:
Fig. 2 Schematic illustration of the crystal structures of CsxMn[Fe(CN)6]y
for 1–3, 4, and 5.
3.2. Charge-transfer phase transition in 1–4
Fig. 3 shows the product of the molar magnetic susceptibility (vM)
and temperature (T) vs. T plots. The vMT value of 1 decreased
at 207 K (T1/2↓) as the temperature decreased at a cooling rate
of −0.5 K min−1, suggesting that 1 showed a temperature-induced
phase transition. As the sample in the low-temperature (LT) phase
was warmed, the vMT value increased around 225 K (T1/2↑) and
reached the high-temperature (HT) phase value (Fig. 3a). The
width of the thermal hysteresis loop DT (= T1/2↑ − T1/2↓) was
18 K. Like 1, 2 and 3 also exhibited thermal phase transitions.
The vMT vs. T plots for 2 showed T1/2↓ = 190 K, T1/2↑ = 231 K,
and DT = 41 K (Fig. 3b), while the vMT vs. T plots for 3 showed
T1/2↓ = 175 K, T1/2↑ = 233 K, and DT = 58 K (Fig. 3c). 4 did not
exhibit a phase transition over the entire temperature region at a
cooling rate of −0.5 K min−1 (Fig. 3d). Conversely, only at a very
slow cooling rate of −0.01 K min−1, 4 shows a small decrease in
the vMT value. Although the phase transition is not enough under
the present condition, T1/2↓ and T1/2↑ are roughly estimated to be
140 and 230 K, respectively (Fig. 3d). However, 5 did not exhibit
a phase transition over the entire temperature region (see ESI†).
The right hand side of Fig. 1 shows the CN stretching
frequencies in the IR spectra for 1–3 at 100 K. The intensity
Fig. 1 IR spectra for 1 (a), 2 (b), and 3 (c) at 300 (left) and 100 K (right).
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by 3.3%. The crystal structure returned to the original one upon
warming. Similarly, the HT and LT phases of 2 showed a face-
˚
centred cubic structure with a = 10.5589(17) and 10.3421(24) A,
respectively, and the volume reduction was 6.0%. The HT and LT
phases of 3 were face-centred cubic structures with a = 10.5627(11)
˚
and 10.3268(23) A, respectively, and the volume reduction was
6.6%. In the light of the valence states of the HT and LT phases in
1–3, the increased volume reduction is due to the increased charge-
transfer sites of FeIII–CN–MnII. The origins of the temperature-
induced phase transition with a thermal hysteresis loop is a
metal-to-metal charge-transfer from MnII to FeIII and a strong
cooperativity in the cyano-bridged 3-D structure.
3.3. Zero thermal expansion of 4
The vMT value of 4 at a cooling rate of −0.5 K min−1 did not
exhibit a phase transition as shown in Fig. 3d. The ratio between
the IR intensities of FeIII–CN–MnII and FeII–CN–MnII for 4 was
constant over the entire temperature range. The lattice constants
of 4 were nearly constant between a = 10.5764(18) (300 K) and
˚
10.5759(25) A (20 K) as the temperature changed, i.e., the thermal
expansivity of aa (= d(ln a)/dT) was +0.2 × 10−6 K−1, suggesting
that 4 is a zero thermal expansion material (Fig. 4).15 Recently,
Margadonna et al.7 reported that FeIII[CoIII(CN)6] shows zero
thermal expansion behavior of aa = −1.47(1) × 10−6 K−1 between
300 and 4.2 K.7 4 is also a good example of a zero thermal
expansion material. The origin of the zero thermal expansion
observed in 4 is considered to be the transverse vibrations which
contract the lattice on heating. This explanation is used for the
negative thermal expansion observed in the other Prussian Blue
analogues.7,8
Fig. 3 Observed vMT vs. T plots for 1 (a), 2 (b), 3 (c), and 4 (d) in the
cooling (᭹) and warming (᭺) processes at a cooling and warming rate of
0.5 K min−1. The dotted line in (d) shows the vMT value for 4 measured at
a very slow cooling rate, i.e., −0.01 K min−1 (165–120 K), −0.1 K min−1
(200–165 K), −0.5 K min−1 (300–200 K, 120–100 K), and +0.5 K min−1
(100–300 K).
of the FeIII–CN–MnII peak in 1–3 decreased as the temperature
decreased and a broad FeII–CN–MnIII peak appeared in the region
2020–2130 cm−1 at 100 K. From the analyses of the IR changes, the
electronic states of the LT phases for 1, 2, and 3 were assigned to
CsI1.78MnII0.78MnIII0.22[FeII(CN)6], CsI1.57MnII0.57MnIII0.43[FeII(CN)6],
and CsI1.51MnII0.51MnIII0.49[FeII(CN)6], respectively.
The XRD patterns for 1–3 at 300 and 100 K were examined.
Table 1 lists the d-values (lattice spacing) of the observed reflec-
tions. The XRD patterns in the HT and LT phases for 1 showed
¯
face-centred cubic structures (Fm3m) with a = 10.5446(17) and
Fig. 4 (a) XRD patterns for 4 at 300 (upper) and 20 K (lower). (b) Lattice
constant vs. temperature plots for 4.
˚
10.4280(7) A, respectively, and the unit cell volume was reduced
5048 | Dalton Trans., 2006, 5046–5050
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˚
Table 1 The d values (A) of the observed reflections for the HT (300 K) and LT (100 K) phases of 1–3
1
2
3
HT
d (cal)
LT
HT
d (cal)
LT
HT
d (cal)
LT
hkl
d (obs)
d (cal)
d (obs)
d (obs)
d (cal)
d (obs)
d (obs)
d (cal)
d (obs)
220
222
400
422
440
620
3.7281
3.0440
2.6361
2.1524
1.8640
1.6672
3.7265
3.0444
2.6360
2.1538
1.8651
1.6675
3.6869
3.0103
2.6070
2.1286
1.8434
1.6488
3.6859
3.0114
2.6058
2.1287
1.8425
1.6491
3.7331
3.0481
2.6397
2.1553
1.8666
1.6695
3.7312
3.0481
2.6396
2.1563
1.8661
1.6705
3.6565
2.9855
2.5855
2.1111
1.8282
1.6352
3.6581
2.9852
2.5849
3.7345
3.0492
2.6407
2.1561
1.8672
1.6701
3.7330
3.0488
2.6406
2.1569
1.8681
1.6701
3.6511
2.9811
2.5817
2.1080
1.8255
1.6328
3.6514
2.9804
2.5815
a
a
—
—
—
—
a
a
1.6339
1.6333
a Peaks are hidden by the reflection due to Cu of the sample holder.
3.4. Ferromagnetic ordering of the LT phase in 1–3
saturated magnetization (Ms) values were 4.5 (1), 4.2 (2), and 4.0
lB (3) (Fig. 5, right side), which are close to the expected values
of 4.8 (1), 4.6 (2), and 4.5 lB (3) when the all magnetic spins
on MnII and MnIII have parallel ordering. In addition, the Weiss
The left side of Fig. 5 shows the magnetization vs. temperature
plots of the LT phase of 1–3 at 10 G. These phases exhibited
spontaneous magnetization with magnetic phase transition tem-
peratures of 4.3 (1), 5.0 (2), and 5.6 K (3). The magnetization as
a function of the external magnetic field at 2 K showed that the
−1
temperature (h) values where the extrapolation of the vM vs. T
curves intersects the abscissa were positive values of +2.3 (1), +3.9
(2), and +4.0 K (3) (see ESI†). The magnetic data indicated that
the LT phases are ferromagnetic.
4. Conclusion
Cs1.78Mn[Fe(CN)6], Cs1.57Mn[Fe(CN)6], and Cs1.51Mn[Fe(CN)6]
exhibit temperature-induced phase transitions with thermal hys-
teresis loops. These phase transitions are due to a metal-to-
metal charge-transfer from MnII to FeIII, and a strong co-
operativity in the 3-D structure. A material that exhibits a
thermal phase transition with a thermal hysteresis loop is
useful for applications such as memory devices. In addition,
Cs0.94Mn[Fe(CN)6]0.91·0.8H2O exhibits a zero thermal expansion
behavior. Recently, FeIII[CoIII(CN)6] and MII[PtIV(CN)6]·xH2O
have been reported to show a zero (or negative) thermal expansion
behavior.7,8 Cs0.94Mn[Fe(CN)6]0.91·0.8H2O is also a good zero
thermal expansion material, which is useful for resistance to
thermal shock, etc.
Acknowledgements
The present research is supported in part by a Grant-in-Aid for
21st Century COE Program for Frontiers in Fundamental Chem-
istry and for Human-Friendly Materials based on Chemistry,
and a Grand-in-Aid for Scientific Research from the Ministry
of Education, Culture, Sports, Science, and Technology of Japan.
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15 The Rietveld analysis of Cs0.94Mn[Fe(CN)6]0.91·0.8H2O shows that the
+
¯
space group is Fm3m, i.e., the Cs ion randomly occupies the interstitial
site. The displacement of Cs+ ion from the center of the interstitial site
as reported in Cs0.7Co4[Fe(CN)6]2.9·16H2O is not observed.10.
5050 | Dalton Trans., 2006, 5046–5050
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