Molecular Design for Tailoring a Single-Source Precursor for Bismuth Ferrite

Article abstract of DOI:10.1021/acs.inorgchem.6b00951

Nearly phase-pure bismuth ferrite particles were formed by thermolysis of the single-source precursor [Cp(CO)₂FeBi(OAc)₂] (1) in octadecene at 245 °C, followed by subsequent calcination at 600 °C for 3 h. In contrast, the slightly modified compound [Cp(CO)₂FeBi(O₂C^tBu)₂] (2) yielded only mixtures of different bismuth oxide phases, revealing the distinctive influence of molecular design in material synthesis. The chemical composition, morphology, and crystallinity of the resulting materials were investigated by X-ray diffraction, transmission electron microscopy, and energy-dispersive X-ray spectroscopy. In addition, the optical properties were investigated by Fourier transform infrared and UV-vis spectroscopies, showing a strong band gap absorption in the visible range at 590 nm (2.2 eV). The magnetic behavior was probed by vibrating-sample and superconducting quantum interference device magnetometry, as well as ⁵⁷Fe M?ssbauer spectroscopy.

Full text of DOI:10.1021/acs.inorgchem.6b00951

Article
pubs.acs.org/IC
Molecular Design for Tailoring a Single-Source Precursor for Bismuth
Ferrite
Georg Bendt,† Rafael Schiwon,‡ Soma Salamon,§ Joachim Landers,§ Ulrich Hagemann,∥
Christian Limberg,*,‡ Heiko Wende,§ and Stephan Schulz*,†
†Faculty of Chemistry, Inorganic Chemistry, and Center for Nanointegration Duisburg-Essen (CENIDE), University of
Duisburg-Essen, Universitatsstrasse 7, 45114 Essen, Germany
̈
§Faculty of Physics and Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, Lotharstrasse 1, 47048
Duisburg, Germany
‡Institut fur Chemie, Humboldt-Universitat zu Berlin, Brook-Taylor-Straße 2, 12489 Berlin, Germany
̈
̈
∥Interdisciplinary Center for Analytics on the Nanoscale, Nano Energie Technik Zentrum, Carl-Benz-Strasse 199, 47047 Duisburg,
Germany
S
* Supporting Information
ABSTRACT: Nearly phase-pure bismuth ferrite particles
were formed by thermolysis of the single-source precursor
[Cp(CO)2FeBi(OAc)2] (1) in octadecene at 245 °C, followed
by subsequent calcination at 600 °C for 3 h. In contrast, the
slightly modified compound [Cp(CO)2FeBi(O2CtBu)2] (2)
yielded only mixtures of different bismuth oxide phases,
revealing the distinctive influence of molecular design in
material synthesis. The chemical composition, morphology,
and crystallinity of the resulting materials were investigated by
X-ray diffraction, transmission electron microscopy, and
energy-dispersive X-ray spectroscopy. In addition, the optical properties were investigated by Fourier transform infrared and
UV−vis spectroscopies, showing a strong band gap absorption in the visible range at 590 nm (2.2 eV). The magnetic behavior
was probed by vibrating-sample and superconducting quantum interference device magnetometry, as well as 57Fe Mossbauer
̈
spectroscopy.
catalytic applications, i.e. the degradation of organic mole-
cules8−10 and also water splitting.11,12
INTRODUCTION
■
The use of single-source precursors (SSPs) as starting
compounds for the formation of nanoparticles and thin films
is a promising alternative approach for the synthesis of high-
quality materials.1−3 SSPs already contain the elements of the
desired material in a single molecule, and their thermal
properties and decomposition pathways can be modified by
the precursor design; in particular, the loss of highly volatile
elements during synthesis resulting in off-stoichiometric
products can be avoided. BiFeO3 combines antiferromagnetic
and ferroelectric behavior and belongs to the family of so-called
multiferroics.4 To the best of the authors’ knowledge, BiFeO3 is
the only multiferroic material that retains its properties above
The synthesis of BiFeO3 is afflicted with several problems;
the ternary bismuth−iron−oxygen phase diagram shows
multiple bismuth-rich phases including Bi2Fe4O9 and
Bi25FeO40, which are often seen as parasitic phases.
Furthermore, a high leak current due to defects, mostly oxygen
vacancies and impurities (Fe2+ ions), limits the technical
application.13,14 Several synthetic strategies have been devel-
oped for the formation of BiFeO3, including solid-state,15 sol−
gel,16 hydrothermal as well as microwave-assisted hydro-
thermal,17 self-combustion,18 and electrospinning19 reactions.
The most common precursors are bismuth and iron nitrates.
However, while SSPs for the solution-based synthesis of
BiFeO3 are unknown, to date, Blackman et al. investigated the
heterobimetallic compound [Cp(CO)2FeBiCl2] (Cp = η5-
C5H5) as the aerosol-assisted chemical vapor deposition
precursor for the deposition of BiFeO3 onto different substrates
at 300 °C with subsequent annealing at 500−700 °C.20 The
phase-pure films obtained at 700 °C exhibit a direct band gap at
́
room temperature with an antiferromagnetic Neel temperature
TN = 370 °C and a ferroelectric Curie temperature TC = 830
°C. BiFeO3 crystallizes in the rhombohedrally distorted
perovskite structure with lattice constants a = 5.58 Å and c =
13.87 Å in the hexagonal representation. The multiferroic
nature can be explained by the displacement of Fe3+ and Bi3+
along the c axis and a canted ordering of the Fe3+ spins, leading
to a cycloidal spin ordering with a period length of 62 nm.5−7
Moreover, BiFeO3 exhibits a band gap (2.1−2.8 eV) in the
visible range, making it also an ideal candidate for photo-
Received: April 16, 2016
© XXXX American Chemical Society
A
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2.1 eV and high activity for water oxidation under visible-light
irradiation.
Recently, we reported a precedent case, proving that
molecular Fe−Bi entities can be constructed by the reactions
of complexes with Fe−Fe bonds and bismuth carboxylates: The
reaction between [Cp(CO)2Fe]2 and [Bi(OAc)3] yielded
[Cp(CO)2FeBi(OAc)2] (1; Figure 1).21 Herein we describe
our results in testing 1 as well as the newly prepared derivative
[Cp(CO)2FeBi(O2CtBu)2] (2), varying in the carboxylate
residue, as SSPs.
Figure 1. Molecular structure of iron−bismuth compound 1.22
Figure 2. Synthesis of 2 by the reaction of [Fp]2 with [Bi(O2CtBu)3]
and molecular structure of 2. Displacement ellipsoids are shown at the
50% probability level. Hydrogen atoms are omitted for clarity. Selected
bond lengths (Å) and angles (deg): Fe−Bi 2.6072(4), Bi−O2
2.2978(18), Bi−O3 2.3277(18), Bi···O1 2.6298(17), Bi···O4
2.6312(17), Bi···O4′ 2.7971(18), C1−O1 1.249(3), C1−O2
1.285(3), C6−O4 1.251(3), C6−O3 1.286(3), Bi···Bi′ 4.4989(4);
Cp−Fe−Bi 122.919(16), O2−Bi−O3 79.73(6), O2−Bi−Fe 95.09(5),
O3−Bi−Fe 95.23(5), O2−Bi···O4′ 157.455(60), O1···Bi···O4
171.07(6).
RESULTS AND DISCUSSION
■
After the successful preparation of 1, we have been able to show
that, setting out from [Cp(CO)2Fe]2 ([Fp]2), also [FpBi-
(O2CH)2]n is accessible via a reaction with bismuth formate;21b
because of aggregation to a coordination polymer, the product
was, however, only sparingly soluble in common organic
solvents. Aiming at soluble derivatives and further support that
indeed reactions of [Fp]2 with bismuth carboxylates provide
more general access to molecular bismuth−iron compounds,
we have thus treated [Cp(CO)2Fe]2 with bismuth pivalate
instead of bismuth acetate. The stirring of an equimolar [Fp]2/
[Bi(O2CtBu)3] suspension in dichloromethane at room
temperature for a period of 9 days resulted in a red solution,
which was evaporated to dryness to yield a brown powder.
Washing the residue with small portions of hexane and
crystallization from a dichloromethane/hexane mixture led to
red crystals of 2 in 53% yield. A single-crystal X-ray diffraction
(XRD) analysis revealed that 2 crystallizes as a dimer (with a
crystallographic inversion center) similar to that of the acetate 1
(Figure 2).
Figure 3 displays the results obtained from thermogravi-
metric analysis (TGA) investigations (see also Figures S1 and
Each bismuth atom is bound to one organometallic iron
fragment via a Fe−Bi metal bond [2.6072(4) Å], which is
longer than the corresponding distance in 1 [Fe−Bi 2.585(2)
Å]. Additionally, two covalent bonds to two pivalate ligands are
formed [Bi−O2:2.2978(18) Å; Bi−O3:2.3277(18) Å] with
distances that are also longer than the corresponding distances
in 1 [d(Bi−O) = 2.342−2.353 Å]. The angle between these
oxygen atoms [O2−Bi−O3 79.73(6) Å] and also all of the Fe−
Bi−O angles [92.56(4)−95.23(5)°] indicate that these covalent
bonds are mainly established through p orbitals at the bismuth
atom. The corresponding carbonyl units of the carboxylate
ligands also interact with the respective bismuth centers, and
one of them per molecule is further involved in a secondary
Bi···O−Bi′ contact with the bismuth atom of a second molecule
[O4′···Bi and O4···Bi′ 2.7971(18) Å]. As the second molecule
bonds back via an analogous interaction, a dimer is thus created
featuring a [Bi2O2] diamond core motif with larger Bi−O′ and
Bi···Bi′ distances [4.4989(4) Å] compared to those in 1 [d(Bi−
O′) = 2.711(4) Å; d(Bi−Bi′) = 4.3485(6) Å].
Figure 3. Simultaneous thermal analysis measurement (argon purge
gas, 60 mL/min, heating rate 10 K/min) of 1 (dashed lines) and 2
(solid lines). TGA is shown in green, and differential thermal analysis
(DTA) is shown in red.
S2), showing that compounds 1 and 2 cannot be transferred
into the gas phase under normal pressure without decom-
position, so that determination of their potential as SSPs for
chemical vapor deposition requires more studies under
authentic conditions. We have therefore examined their
behavior as SSPs in solution, as described below.
Thermolysis reactions were performed first with 200 mg of 1,
which was suspended in 10 mL of octadecene, slowly heated to
215, 230, and 245 °C in order to investigate the role of the
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reaction temperature, and stirred for 1 h. The orange
suspension turned to a black suspension around 200 °C.
After cooling to ambient temperature, the decomposition
products were isolated by centrifugation and repeatedly washed
with chloroform. The chemical composition of the resulting
materials was analyzed by energy-dispersive X-ray spectroscopy
(EDX). The product obtained at 245 °C shows a composition
of 20 atom % bismuth, 20 atom % iron, and 60 atom % oxygen,
corresponding to a bismuth-to-iron atomic ratio of close to 1,
indicating complete decomposition of the precursor, while the
products obtained at lower temperatures are bismuth-rich. In
addition, the EDX analyses of all three samples reveal high
carbon contents, which are not taken into account, indicating
the presence of organic groups.
The samples were further investigated by IR spectroscopy. In
Figure 4, the Fourier transform infrared (FTIR) spectra of
Figure 5. Powder XRD patterns of BiFeO3 particles obtained by
calcination of the decomposition product from precursor 1 at different
temperatures (red: BiFeO3, PDF 71-2494) in octadecene: (a) 215 °C;
(b) 230 °C; (c) 245 °C.
Table 1. Bismuth-to-Iron Ratios for Different Samples As
Determined by EDX Measurements and the Results of the
Quantitative Rietveld Refinement for the Samples
bismuth-to-
a
bismuth-to-
b
c
sample precursor Tdecomp
iron ratio
iron ratio
phase
I
1
215
62:38
58:42
93.5 wt %
BiFeO3
6.5 wt %
Bi2Fe4O9
II
1
230
57:43
54:46
87.7 wt %
BiFeO3
8.6 wt %
Bi2Fe4O9
3.4 wt %
Bi25FeO39
Figure 4. FTIR spectra of precursor 1 (black) and the decomposition
product obtained at 245 °C after workup (red).
III
1
245
53:47
51:49
100:0
82.0 wt %
BiFeO3
12.6 wt %
Bi2Fe4O9
precursor 1 (black) and the as-obtained decomposition product
at 245 °C (red) are displayed. Compared to the precursor, the
black powder shows the loss of characteristic absorption bands
around 2000 cm−1 for the carbonyl ligands, whereas the bands
for the carboxylate group around 1500 cm−1 are still present.
In order to obtain crystalline BiFeO3, the amorphous black
powders were annealed at 600 °C for 3 h in air. The resulting
products showed the characteristic orange color of BiFeO3.
The XRD patterns of the annealed powders are displayed in
Figure 5. The powders obtained at 215 and 230 °C after
annealing consist of different phases with BiFeO3 (PDF 71-
2494) as the major phase and Bi2Fe4O9 (PDF 71-2494) and
Bi25FeO40 (PDF 71-2494) as minor phases. The results of
quantitative phase analysis are displayed in Table 1. In contrast,
the pattern of the annealed powder obtained at 245 °C can be
indexed on the basis of a rhombohedrally distorted perovskite
structure, which indicates the formation of nearly phase-pure
BiFeO3. The origin of the formation of bismuth-rich phases can
be found in the high bismuth content of the decomposition
product. The bismuth content is lower than that in the as-
decomposed products, indicating the loss of bismuth or Bi2O3
due to their high vapor pressure.
5.4 wt %
Bi25FeO39
IV
V
2
2
215
245
100:0
100:0
Bi2O3
Bi2O3
100:0
a
b
c
EDX before calcination. EDX after calcination. As determined by
XRD.
cm−1 related to the characteristic stretching and bending of the
Fe−O bonds in the FeO6 octahedron, the main building block
of the perovskite structure. In order to determine the band gap
of the BiFeO3 powder, UV−vis diffuse-reflectance spectra were
recorded. Applying the Kubelka−Munk function leads to a
band gap energy of 2.2 eV (Figure 6b).
Figure 7 displays a typical transmission electron microscopy
(TEM) image obtained from the phase-pure BiFeO3
crystallites, showing a relatively broad size distribution between
100 and 200 nm. The crystalline particles, which are electron
transparent, as can be seen in Figure 7c, are sintered together
by thermal treatment (annealing). High-resolution TEM and
selected-area electron diffraction (SAED) of a single particle
demonstrate their highly crystalline nature, as was expected
from the powder XRD studies.
The FTIR spectrum of the phase-pure BiFeO3 (sample III,
Figure 6a) shows strong absorption bands around 530 and 430
C
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Figure 6. Typical FTIR spectrum of BiFeO3 powder (sample III, a) and Tauc plot for determining the optical band gap (b).
1 h. The red suspension turned black around 200 °C. After
cooling to ambient temperature, the decomposition products
were isolated by centrifugation and repeatedly washed with
chloroform.
The chemical composition of the resulting materials was
analyzed by EDX (Table 1). Both products, obtained at 215
and 245 °C, showed only the presence of bismuth and carbon.
The IR spectrum of the washed powder shows weak absorption
bands, indicating the presence of traces of intact precursor
molecules (Figure 9). After thermal treatment at 600 °C for 3
h, yellow powders were obtained. As expected from the EDX
results of the black powder, the XRD pattern showed the
presence of Bi2O3 (PDF 41-1449) as the major product and
traces of the nonstoichiometric phase Bi2O2.33 (PDF 27-0051;
Figure 10).
The magnetic properties of sample III were investigated by
vibrating-sample (VSM) and superconducting quantum inter-
ference device (SQUID) magnetometry. The 300 K M(H)
magnetization curve displayed in Figure 11 shows a linear
response in the high-field range, often seen in antiferromagnetic
samples. In the low-field range, a weak ferromagnetic signal due
to canted antiferromagnetically coupled spins is visible. The
maximum magnetization at room temperature was determined
to be 0.73 emu/g at 9 T, which is large compared to that of
bulk BiFeO3, presumably because of uncompensated surface
spins and size effects.23 The 5 K magnetization curve shows an
open hysteresis that was not visible in the 300 K curve,
indicating superparamagnetic behavior also visible in the
temperature-dependent magnetization: The M(T) data shown
in Figure 11 reveal a clear splitting between field-cooled (FC)
and zero-field-cooled (ZFC) magnetization at low temper-
Figure 7. TEM images (a−c) and SAED pattern (d) of sample III.
Scale bars: 0.5 μm (a); 50 nm (b); 5 nm (c); 5 nm (d).
X-ray photoelectron spectroscopy (XPS) is very sensitive to
the oxidation state of metals. The XPS measurement of sample
III confirms the presence of bismuth, iron, and oxygen. In
addition a small amount of carbon is detected. Soft Ion
sputtering ,using 500V Ar+ ions, shows that the carbon is not
only due to surface contamination but also originiates from the
material itself, indicating an incomplete decomposition of the
precursor. The high-resolution spectrum of the Bi 4f peaks
shows the characteristic peaks at 164 and 159 eV corresponding
to Bi3+. For iron, the Fe 2p peaks at 724 and 710 eV, and
especially the existence and position of the Fe 2p satellite at 719
eV, indicate only the presence of Fe3+ cations; no peaks
corresponding to Fe2+ are found (Figure 8).
In order to investigate the influence of the precursor design
on the decomposition pathway, 2 was also investigated as a
potential precursor for BiFeO3, and the results were compared
to those obtained with precursor 1, which contains acetate
instead of pivalate groups. The thermolysis reaction was
performed with 200 mg of 2 suspended in 10 mL of
octadecene, slowly heated to 215 and 245 °C, and stirred for
́
atures, corresponding to Neel relaxation behavior of the smaller
particles within the sample, in agreement with the beginning
superparamagnetic doublet visible in the room temperature
Mossbauer spectrum. Furthermore, a clear cusp is visible in
̈
both the ZFC and FC curves at a temperature of 236 3 K,
marked by arrows. This indicates the presence of a minor
Bi2Fe4O9 phase; this material is reported to show an
antiferromagnetic-to-paramagnetic transition in the range of
about 250−260 K,24,25 presumably decreased here by size
effects. Because of its antiferromagnetic behavior, it has no
noticeable influence on the M(H) curves shown earlier. The
presence of a small amount of Bi2Fe4O9 is also verified by
Mossbauer spectroscopy, as described in detail below.
̈
Mossbauer spectra were recorded at room temperature (300
̈
K), 80 K, and 4.3 K, and hyperfine parameters of the spectrum
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Figure 8. C 1s, O 1s, Fe 2p, and Bi 4f XPS spectra of BiFeO3 (sample III): experimental data (black), envelope (red), and deconvoluted (gray)
peaks.
Figure 10. Powder XRD patterns of Bi2O3 particles after calcination of
the decomposition product from precursor 2 at different temperatures
(red: Bi2O3, PDF 41-1449): (a) 215 °C; (b) 245 °C.
product obtained at 245 °C after workup (red).
Figure 9. FTIR spectra of precursor 2 (black) and the decomposition
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Figure 11. M(H) loops of sample III at 300 and 5 K (left). The inset shows a magnification of the low-field regime. ZFC−FC M(T) measurement
performed at 10 mT (right).
at 80 K are shown representatively in Table 2, displaying good
agreement with data previously reported for BiFeO3 (full details
are given in Table S1).23,26,27 As is visible in Figure 12a, the
room temperature spectrum is composed of a sextet
corresponding to an antiferromagnetically coupled BiFeO3
phase and several (super)paramagnetic doublets. The sextet
displays a clearly visible asymmetry between opposite lines such
as 1 and 6, which are often explained by an anisotropic
hyperfine magnetic field, caused by the complex spin
structure.28 The latter leads to a distribution of hyperfine
magnetic fields, which prevents the successful fitting of the
spectrum using a single sextet. Therefore, a distribution of
hyperfine magnetic fields with 40 equidistant subspectra was
used to achieve a precise congruence between the measurement
Table 2. Hyperfine Parameters of Sample III at 80 K:
Hyperfine Magnetic Field Bhf, Isomer Shift δ Relative to α-
Iron at Room Temperature, Nuclear Quadrupole Level Shift
2ε, Respectively, Quadrupole Splitting ΔEQ, and Relative
Spectral Area A
2ε/ΔEQ
Bhf (T)
δ (mm/s)
(mm/s)
A (%)
BiFeO3
54.07 0.03
0.50 0.01
0.18 0.01
0.32 0.01
0.44 0.01
0.14 0.01
0.45 0.01
0.12 0.01
0.31 0.02
86.0 0.6
5.2 0.1
4.7 0.1
4.7 0.2
BiFeO3 sp.
Bi2Fe4O9 I
́
data and fit function. Because of the high Neel temperature of
44.15 0.05
BiFeO3, it is reasonable to assume that the dominant doublet is
mainly caused by the superparamagnetic behavior of the
smallest BiFeO3 particles contained in the sample.26,29 This is
supported by spectra at 4.3 (Figure 12c) and 80 K (Figure
12b), which reveal a decrease in the doublet spectral area from
ca. 13% at room temperature down to 5% at 80 K. However,
the isomer shift shows a strong decrease from 300 to 80 K and
little change from 80 to 4.3 K (see Table S1). We therefore
assume that the small residual contribution at low temperatures
is caused by a minute paramagnetic iron-bearing phase. The
change in the isomer shift means that the majority of the
doublet at 300 K is caused by superparamagnetic BiFeO3,
which becomes magnetically blocked at 80 K. When closely
examining the low velocity range at the center of the room
temperature spectrum, one can identify two additional doublets
not belonging to the main BiFeO3 doublet, matching hyperfine
parameters of Bi2Fe4O9 at room temperature.30 This is further
supported by the observation of two minor sextets of equal
spectral area and with lower hyperfine magnetic fields
compared to BiFeO3 at 80 and 4.3 K, with hyperfine
parameters, listed in Table 2 and SI, matching those of
Bi2Fe4O9.31−33 Because Bi2Fe4O9 has a higher iron content than
BiFeO3, the combined relative spectral area (proportional to
the number of iron atoms) of ca. 9% corresponds to only 5−6
wt %.
Bi2Fe4O9 II 47.65 0.05
CONCLUSIONS
■
First we report further precedence demonstrating that
heterobimetallic iron−bismuth compounds can be obtained
readily through reactions between [Fp]2 and bismuth
carboxylates. We have then provided evidence that such
complexes can indeed serve as SSPs for the preparation of
BiFeO3 in the course of procedures involving thermal
Figure 12. Mossbauer spectra recorded at 300 K (a), 80 K (b), and 4.3
̈
K (c): experimental data (black symbols), sites I (tetrahedral) and II
(octahedral) of Bi2Fe4O9 (green and blue), magnetically coupled
BiFeO3 (brown), and superparamagnetic BiFeO3 (orange), with the
red line showing the overall theoretical spectrum.
F
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crucible); constant purge gas flow of 100 mL/min argon 5.0
(AIRLIQUIDE); constant heating rate 10 K/min; raw data evaluation
with manufacturer’s software PROTEUS (version 4.3) without further
data treatment, e.g., such as smoothing.
Powder XRD. XRD patterns were obtained at ambient temperature
(25 2 °C) using a Bruker D8 Advance powder diffractometer in
Bragg−Brentano mode with Cu Kα radiation (λ = 1.5418 Å, 40 kV,
and 40 mA). The powder samples were investigated in the range of 2θ
= 5−90° with a step size of 2θ = 0.01° and a counting time of 0.3 s.
Rietveld refinement was performed with the program package TOPAS
4.2 from Bruker.
treatment. Notably, our results also show that subtle changes
within the precursor molecules have pronounced effects on the
BiFeO3 materials obtained; that is, the properties of the
material generated are sensitively determined by the SSP, which
motivates further efforts with respect to the modification of
such systems.
Magnetometry indicates partial superparamagnetic behavior
and the presence of a minor Bi2Fe4O9 phase, while the
saturation magnetization is slightly higher than that of bulk
BiFeO3, presumably due to size/surface effects. These findings
EDX Analysis. EDX studies were performed with a Jeol JSM 6510
scanning electron microscope equipped with an EDX device (Bruker
Quantax 400).
support the results from Mossbauer spectroscopy and allow us
̈
to quantify the amount of Bi2Fe4O9 to 5−6 wt %, while the
BiFeO3 parameters are in agreement with literature values. The
determined amount of Bi2Fe4O9 is in good agreement with the
result of the quantitative Rietveld refinement.
TEM Analysis. TEM and SAED measurements were carried out on a
Philips CM200 LaB6 microscope operated at 200 kV. Samples for
TEM observations were prepared by depositing a drop of a suspension
of the material in ethanol onto a carbon-coated copper grid.
Magnetometry. Temperature-dependent magnetic measurements
between 5 and 350 K were performed using a Quantum Design
MPMS-5S, while the field-dependent M(H) loops up to 9 T were
recorded by a Quantum Design PPMS DynaCool using powder
samples.
EXPERIMENTAL SECTION
■
Synthetic Procedures, Materials, and Methods. Octadecene
(Sigma-Aldrich) was carefully dried over a sodium/potassium alloy
and degassed prior to use. [Cp(CO)2FeBi(OAc)2] (1)2 and
[Bi(O2CtBu)3]3 were synthesized according to literature methods,
while [Fp]2 (99%, Acros Organics) was used as received. All synthetic
steps were performed under an argon atmosphere using standard
Schlenk techniques.
̈
Mossbauer Spectroscopy. Mossbauer spectra were recorded in
transmission geometry using a constant-acceleration spectrometer with
̈
a
57Co source embedded in a rhodium matrix. BiFeO3 powder was
η5-Cyclopentadienyldicarbonyliron−bismuth Di-η2-pivalate (2).
Solid [Bi(O2CtBu)3] (289.60 mg, 0.566 mmol, 1 equiv) was added to a
suspension of [Fp]2 (200 mg, 0.566 mmol, 1 equiv) in dichloro-
methane (10 mL) to give a red solution within 9 days at ambient
temperature. Removing all volatiles at reduced pressure and washing
the brown residue with three portions of 2 mL of hexane led to an
orange powder, which was extracted with 9 mL of dichloromethane in
small portions. The concentration of the combined extracts to 3 mL
and layering of the solution with 9 mL of hexane resulted in the
formation of red crystals of 2 within 7 days upon storing at −30 °C
(yield: 177 mg, 0.301 mmol, 53%). 1H NMR (C6D6, 300.13 MHz, 25
°C): δ 4.15 (s, 5 H, C5H5), 1.33 (s, 18 H, C(CH3)3). 13C{1H} NMR
(C6D6, 75.47 MHz): δ 196.9 (2 C, CO), 187.5 (2 C, O(O)
CC(CH3)3), 83.6 (5 C, C5H5), 41.1 (2 C, O(O)CC(CH3)3), 28.0 (2
C, CH3). IR (KBr; cm−1): ν 3926 s, 3107 s, 3098 s, 3089 s, 2965 m,
2931 s, 2906 s, 2870 s, 2015 sst, 1987 m, 1964 st, 1942 sst, 1920 m,
1582 s, 1547 m, 1525 m, 1507 s, 1480 m, 1460 s, 1448 s, 1430 s, 1398
m, 1370 m, 1350 m, 1351 m, 1219 m, 1088 s, 1066 s, 1030 s, 1013 s,
1001 s, 940 s, 901 s, 896 m, 876 s, 855 m, 839 s, 835 s, 804 m, 789 s,
634 m, 606 m, 579 m, 572 m, 537 s. 505 s, 454 s. Anal. Calcd for
C17H23BiFeO6 (588.19): C, 34.71; H, 3.94. Found: C, 35.02; H, 3.90.
Single-Crystal XRD. The data collection for 2 was performed with a
Bruker D8 VENTURE area detector using Mo Kα radiation (λ =
0.71073 Å), and its crystallographic data are summerized in Table S1.
Multiscan absorption corrections implemented in SADABS34 were
applied to the data. The structures were solved by intrinsic phasing
(SHELXS-97)35 and refined by full-matrix least-squares procedures on
F2 with all measured reflections (SHELXL-2014)36 with anisotropic
temperature factors for all non-hydrogen atoms. All hydrogen atoms
were added geometrically and refined by using a riding model.
Thermolysis of 1 and 2. A total of 200 mg (0.3 mmol) of the
precursor was suspended in 10 mL of octadecene, and the suspension
was heated to 215 °C (1 and 2), 230 °C (1 and 2), and 245 °C (1) for
1 h. After cooling to ambient temperature, the black precipitate was
isolated by centrifugation and purified by repeated washing with n-
hexane (two or three times). The dried solid was loaded in a ceramic
boat and heated in a tube furnace to 600 °C with a heating rate of 10
K/min for 3 h.
mixed with chemically inert boron nitride to obtain a sample with ca.
20 mg/cm2 BiFeO3. Measurements below room temperature were
performed in a liquid-helium bath cryostat, while all spectrometers
were calibrated using α-iron foil at room temperature. The
experimental spectra were least-squares-fitted using the software
package “Pi” by Ulrich von Horsten.37
̈
CCDC 1446539 (2) contains the supplementary crystallographic
data for this paper. These data can also be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b00951.
Simultaneous thermal analysis measurement of 1 and 2
and Mossbauer parameters for 300, 80, and 4.3 K for
̈
BiFeO3, superparamagnetic BiFeO3, and the tetrahedral
(I) and octahedral (II) iron sites in Bi2Fe4O9 (PDF)
CCDC 1446539 (2) (CIF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: christian.limberg@chemie.hu-berlin.de.
*E-mail: stephan.schulz@uni-due.de.
■
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Characterization. Thermal Analysis. A NETZSCH thermoana-
lyzer STA 409 C Skimmer system, equipped with a BALZERS QMG
421 spectrometer, was used to record the thermoanalytical curves.
Further experimental details were as follows: DTA−TGA sample
carrier system; Pt/PtRh10 thermocouples; platinum crucibles (beaker,
0.8 mL); sample mass 20−30 mg (measured versus empty reference
This work was supported by the DFG (Grants SPP1681,
WE2623/7-1, and WE2623/13-2) and by Stiftung Mercator
(MERCUR). Financial support by the University of Duisburg-
Essen is acknowledged by S.S. and H.W., while C.L. is grateful
to the Humboldt-Universitat zu Berlin for core support. We
̈
G
DOI: 10.1021/acs.inorgchem.6b00951
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
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