Thermal decomposition of barium ferrate(VI): Mechanism and formation of FeIV intermediate and nanocrystalline Fe2O3 and ferrite

Article abstract of DOI:10.1016/j.jallcom.2016.01.185

Simple high-valent iron-oxo species, ferrate(VI) (Fe^VIO₄^2-, Fe(VI)) has applications in energy storage, organic synthesis, and water purification. Of the various salts of Fe(VI), barium ferrate(VI) (BaFeO₄) has also a great potential as a battery material. This paper presents the thermal decomposition of BaFeO₄ in static air and nitrogen atmosphere, monitored by combination of thermal analysis, M?ssbauer spectroscopy, X-ray powder diffraction, and electron-microscopic techniques. The formation of Fe^IV species in the form of BaFeO₃ was found to be the primary decomposition product of BaFeO₄ at temperature around 190 °C under both studied atmospheres. BaFeO₃ was unstable in air reacting with CO₂ to form barium carbonate and speromagnetic amorphous iron(III) oxide nanoparticles (<5 nm). Above 600 °C, a solid state reaction between BaCO₃ and Fe₂O₃ occurred, leading to the formation of barium ferrite nanoparticles, BaFe₂O₄ (20-100 nm).

Full text of DOI:10.1016/j.jallcom.2016.01.185

Journal of Alloys and Compounds 668 (2016) 73e79
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Thermal decomposition of barium ferrate(VI): Mechanism and
formation of FeIV intermediate and nanocrystalline Fe2O3 and ferrite*
,
*
b
c
c
a
Libor Machala a , Virender K. Sharma , Erno Kuzmann , Zoltan Homonnay , Jan Filip ,
€
ꢀ
Radina P. Kralchevska a
a Regional Centre of Advanced Technologies and Materials, Department of Experimental Physics, Faculty of Science, Palacký University, Olomouc, Czech
Republic
b Department of Environmental and Occupational Health, School of Public Health, Texas A&M University, 1266 TAMU, College Station, TX 77843, USA
c
€
€
ꢀ
Institute of Chemistry, Eotvos Lorand University, Budapest, Hungary
a r t i c l e i n f o
a b s t r a c t
Simple high-valent iron-oxo species, ferrate(VI) (FeVIO42ꢀ, Fe(VI)) has applications in energy storage,
organic synthesis, and water purification. Of the various salts of Fe(VI), barium ferrate(VI) (BaFeO4) has
also a great potential as a battery material. This paper presents the thermal decomposition of BaFeO4 in
Article history:
Received 13 November 2015
Received in revised form
8 January 2016
Accepted 23 January 2016
Available online 28 January 2016
€
static air and nitrogen atmosphere, monitored by combination of thermal analysis, Mossbauer spec-
troscopy, X-ray powder diffraction, and electron-microscopic techniques. The formation of FeIV species in
the form of BaFeO3 was found to be the primary decomposition product of BaFeO4 at temperature around
190 ꢁC under both studied atmospheres. BaFeO3 was unstable in air reacting with CO2 to form barium
carbonate and speromagnetic amorphous iron(III) oxide nanoparticles (<5 nm). Above 600 ꢁC, a solid
state reaction between BaCO3 and Fe2O3 occurred, leading to the formation of barium ferrite nano-
particles, BaFe2O4 (20e100 nm).
Keywords:
Solid state reactions
Hyperfine interactions
€
Mossbauer spectroscopy
© 2016 Elsevier B.V. All rights reserved.
Thermal analysis
X-ray diffraction
1. Introduction
In the past few years, we have been interested in mechanistic
studies on the reduction of Fe(VI) to answer whether the reactions
Oxidized iron usually exists in ferrous (Fe(II)) and ferric (Fe(III))
forms, but high-valent iron compounds of Fe(IV), Fe(V), and Fe(VI)
have also been intensively studied in the last decade [1e4]. For
example, several oxoiron(IV) (FeIV]O) and oxoiron(V) (FeV]O)
complexes containing model organic ligands have been synthe-
sized to understand the biological imperative of oxidative trans-
formation in biological environment [5e11]. Another class of high-
valent iron species are simple tetraoxy high-valent iron anions such
as FeIVO24ꢀ (Fe(IV)), FeVO43ꢀ (Fe(V)), and FeVIO24ꢀ (Fe(VI)), commonly
called ferrates. These have also been of great interest due to their
potential in homogeneous water oxidation catalyst, energy storage,
green chemistry oxidations, and detoxification of contaminants and
toxins [12,13]. Examples of the application of Fe(VI) include gen-
eration of oxygen from water, production of super-iron batteries,
selective conversion of alcohol to aldehyde, and oxidative trans-
formation of cyanotoxins and antibiotics [12,14e19].
go through either 1ꢀeꢀ or 2ꢀeꢀ transfer steps with the formation
of Fe(V) and Fe(IV) intermediates, respectively, where Fe(III) and
Fe(II) are final iron reduced species [13,19,20]. Some progress has
been made by understanding relationships between reaction rates
and thermodynamic potentials [21]. A handful studies on experi-
mental evidences suggest both kinds of electron transfer mecha-
nistic pathways [14,20,22]. The recent advancement in oxidative
mechanism was possible due to improvement in analytical
techniques.
A previous study on the thermal decomposition of potassium
ferrate(VI) (K2FeO4) did not observe any intermediate iron species
(i.e. Fe(V) and Fe(IV)), which are highly unstable [23]. However,
recent work with synchrotron radiation using a nuclear forward
scattering experimental technique clearly showed these unstable
intermediate iron species [24]. The present paper focuses on
barium ferrate(VI), BaFeO4, which exhibited a high discharge per-
formance at high current when applied as a battery material [25].
This performance may be related to the formation of a relatively
stable intermediate with perovskite-like structure during decom-
position of BaFeO4. In contrast, the discharge intermediate of
*
€
ESI available: additional Mossbauer and XRD spectra.
* Corresponding author.
E-mail address: libor.machala@upol.cz (L. Machala).
http://dx.doi.org/10.1016/j.jallcom.2016.01.185
0925-8388/© 2016 Elsevier B.V. All rights reserved.

74
L. Machala et al. / Journal of Alloys and Compounds 668 (2016) 73e79
K2FeO4 was very unstable and immediately converted to Fe(III)
oxides [26,27]. Significantly, comprehending the decomposition
mechanism of salts of Fe(VI) is of utmost importance to advancing
the fundamental chemistry of ferrates, which may lead to simple
synthesis of these compounds and an efficient performance of a
super iron discharged battery. The aim of the current paper is the
mechanistic understanding of the thermal decomposition of
BaFeO4.
as at 5 K in an external magnetic field of 5 T, applied parallel to the
direction of the gamma ray propagation. Low temperature and in-
field measurements were conducted using a cryomagnetic system
of Oxford Instruments. TEM images were obtained on JEOL 2010
instrument with LaB6 cathode at accelerating voltage of 160 kV.
SEM images were obtained on the field-emission scanning electron
microscope (SU6600, Hitachi) working at 6 kV.
A few reports on the decomposition of BaFeO4 under thermal
and humid conditions have conflicting findings in terms of inter-
mediate iron oxidation state(s) and final iron oxide phases [26e32].
Our results in the current paper unequivocally demonstrate the
formation of Fe(IV) as intermediate species and final nanoscale-
iron oxide and -ferrite phases. The objectives of the article are to:
(i) provide evidence of the electron transfer steps of decomposition
of BaFeO4 by using thermogravimetry (TG), differential scanning
3. Results and discussion
3.1. Characterization of synthesized BaFeO4
Initially, the as-prepared BaFeO4 sample, labeled as BF, was
€
€
analyzed by Mossbauer spectroscopy. A Mossbauer spectrum
recorded at room temperature (RT), shown in Fig. S1a of the ’ESI0,
had a doublet (84.1% of spectral area) with hyperfine interaction
€
calorimetry (DSC), and Mossbauer spectroscopy techniques, (ii)
parameters dFe ¼ ꢀ0.90 mm/s,
avalent iron atom [34]. The minor doublet (15.9% of spectral area)
with hyperfine interaction parameters dFe 0.31 mm/s,
EQ ¼ 0.61 mm/s was ascribed to (super)paramagnetic iron(III)
oxides or oxyhydroxides.
Next, the XRD pattern of a BF sample was examined (Fig. S1b of
the ’ESI0), which showed diffraction lines corresponding to only two
crystalline phases, orthorhombic BaCO3 and orthorhombic BaFeO4.
The observed weight ratio between BaFeO4 and BaCO3 is 85:15.
Since no additional phase was observed in the XRD pattern, the
D
EQ ¼ 0.17 mm/s; typical for a hex-
distinguish decomposition of BaFeO4 under static air and inert at-
mosphere, and (iii) learn the nature of reduced iron(III) oxide
phases by applying low temperature (5 K)/in-field (5 T) Mossbauer
spectroscopy, variable temperature X-ray diffraction (VT-XRD), and
imaging (scanning electron microscopy (SEM) and transmission
electron microscopy (TEM)) techniques.
¼
€
D
2. Experimental details
€
2.1. Sample preparation
Fe(III) phase identified in the Mossbauer spectroscopy measure-
ment is therefore X-ray amorphous. The presence of BaCO3 and
Fe(III) were considered as impurities present in the initial BF
sample.
Barium ferrate(VI) was prepared by using a method reported
earlier [33]. Briefly, a basic solution of the barium chloride was
allowed reacting with a solution of K2FeO4 at 0 ꢁC. Solutions used in
this procedure were purged with nitrogen in order to minimize the
presence of atmospheric CO2. A rapid filtration of the barium fer-
rate(VI) obtained was carried out in order to increase the purity of
product.
3.2. Thermal decomposition study
3.2.1. Thermal analysis
In this study, thermal decomposition of BaFeO4 was first
monitored in air by TG and DSC techniques. Fig. 1a shows three
main decomposition steps. The first step was within a temperature
range from 25 to 230 ꢁC, which was ascribed to a dehydration of the
sample. The second step started at 230 ꢁC and was related to the
decomposition of BaFeO4. The second step was completed at 310 ꢁC.
It was accompanied by an endothermic effect, and mass loss was
3.0 wt%. The third step occurred within a broad temperature range
from 600 ꢁC to 920 ꢁC; it exhibited two thermal effects on the DSC
curve and overall mass loss was 4.25 wt%. The chemical trans-
formation of barium carbonate describes the third step [35].
The results of thermal analysis of decomposition of the BF
sample under an inert environment (i.e. Ar) are shown in Fig. 1b.
Both the TG and DSC curves of the BF sample were very similar to
those seen under air (Fig. 1b vs. Fig. 1a). The only significant dif-
ference was a slightly higher mass loss (4.0 wt%) observed in an
inert atmosphere during the second decomposition step, which
could be explained by the absence of carbon dioxide. As expected,
the evolution of oxygen was detected within the second decom-
position step (230e300 ꢁC) by mass spectrometry of evolved gasses
(Fig.1c). The evolution of carbon dioxide, from BaCO3, was observed
in the temperature range from 550 to 1000 ꢁC (Fig. 1d).
2.2. Techniques
Thermal analysis was carried out simultaneously in the ther-
mogravimetric (TG) and calorimetric (DSC) analysis device (STA
449 ꢁC, Netzsch). The samples were dynamically heated from 25 ꢁC
to 1000 ꢁC in the dynamic atmospheres of argon and air (both with
the flow of 30 ml/min) with a heating rate of 10 ꢁC/min. Evolved
gasses were analyzed using a mass spectrometry device (QMS
€
403 C, Aeolos).
X-ray powder diffraction (XRD) experiments were performed
with a PANalytical X'Pert PRO instrument (CoKa radiation) equip-
ped with an X'Celerator detector and programmable divergence
and anti-scatter slits. Standard samples were placed on a zero-
background Si slides, gently pressed in order to obtain sample
thickness of about 0.5 mm and scanned in the 2q
range of 10e90ꢁ in
steps of 0.017ꢁ. The in-situ variable-temperature XRD measurement
was performed in an X-ray reaction chamber XRK 900 (Anton Paar
GmbH) under constant nitrogen flow (20 ml/min) and temperature
range from 100 ꢁC to 600 ꢁC. The heating slope was 40 ꢁC/min and
XRD patterns were collected in steps of every 20 ꢁC (2
q range of
20e60ꢁ; 10 min each scan). Therefore, the resulting slope was
approximately 2 ꢁC/min. In the reaction chamber, the powder
sample was placed into the sample holder made of glass ceramics
(Macor).
€
3.3. Mossbauer spectroscopy measurements
In the following parts, thermodynamic decomposition was
examined by isothermal heating of the BF sample at different
temperatures in air, followed by characterization of selected sam-
The transmission 57Fe Mossbauer spectra were measured using
€
€
a Mossbauer spectrometer in a constant acceleration mode with a
57Co(Rh) source. The isomer shift values were related to metallic
alpha iron at room temperature (RT). The measurements were
carried out at 25 and 300 K in a zero external magnetic field as well
ples by Mossbauer spectroscopy. The BF sample was heated at
€
190 ꢁC for 2 h, at 300 ꢁC for 1 h, and at 600 ꢁC for 1 h (each heat
treatment started from the initial BF sample) and labeled as BF190,

L. Machala et al. / Journal of Alloys and Compounds 668 (2016) 73e79
75
Fig. 1. Thermal analysis and evolved gases analysis of the thermal decomposition of BaFeO4. (a) Air, (b) N2, (c) O2 evolution, and (d) CO2 evolution (TG-solid line, DSC- dashed line,
heating rate 10 ꢁC/min).
BF300A, and BF600, respectively. Part of the BF300A sample was
further exposed to open air for six months (labeled as BF300B) and
then it was characterized to learn the long-term stability of
decomposed product in air.
In the BF300B sample, Fe(IV) singlet was not observed (Fig. 2c).
There is only one Fe(III) doublet, with quadrupole splitting slightly
increased in comparison with the evaluated quadrupole splitting of
€
BF300A sample. Mossbauer spectrum of BF600 sample at room
€
The room temperature Mossbauer spectra of different samples
temperature shows a dominant sextet (RA z 60%) (Fig. 2d). The
hyperfine interaction parameters of this sextet, reported in Table 1,
are similar to those reported for barium ferrite(III) (BaFe2O4) with
tetrahedrally coordinated trivalent iron in a spinel structure [37].
Singlet and doublet, which are present in the spectrum along with
the barium ferrite sextet, belong to remaining Fe(IV) and Fe(III)
phases identified in the samples heated at lower temperatures
(compare hyperfine interaction parameters in Table 1).
are shown in Fig. 2 and the evaluated hyperfine interaction pa-
rameters are given in Table 1. The Mossbauer spectrum of the BF190
sample was characterized by three spectral components (Fig. 2a),
€
which include
a doublet corresponding to non-decomposed
barium ferrate(VI), a singlet with an isomer shift of ꢀ0.28 mm/s,
and a doublet with hyperfine interaction parameters typical for
€
octahedrally coordinated Fe(III). A Mossbauer spectrum of BF300A
sample, shown in Fig. 2b, indicates that the decomposition of
BaFeO4 is complete because the respective Fe(VI) doublet with
isomer shift (zꢀ0.9 mm/s) was not present in the spectrum.
The isomer shift of the singlet (zꢀ0.27 mm/s), observed in the
spectra of both BF190 and BF300A samples, was assigned to
tetravalent iron atoms, based on known parameters reported in
literature [36]. This assignment was further confirmed by per-
3.4. X-ray diffraction study
The same samples as characterized in the previous sections
were subjected to X-ray diffraction measurements (Fig. 4). The XRD
pattern of the sample BF300A in Fig. 4a reveals the presence of
orthorhombic (space group Pmcn) BaCO3 and rhombohedral (space
group R-3m) BaFeO3 phases. On the other hand, BF300B sample had
only diffraction lines of barium carbonate in its XRD pattern
(Fig. 4b). No other crystalline phases were identified in the patterns
of BF300A and BF300B samples. Thus, the Fe(III) phases suggested
€
forming Mossbauer measurement of the BF300A sample at low
temperature (T ¼ 25 K) (Fig. 3a). The spectrum contained a
magnetically split Fe(IV) component with isomer shift
of ꢀ0.19 mm/s (a second order Doppler shift has to be taken into
account) and the relative area was almost the same as at room
temperature (Table 1). Another two subspectra, doublet and sextet,
corresponded to octahedrally coordinated Fe3þ atoms within a
(super)paramagnetic and magnetically ordered fractions,
respectively.
€
by Mossbauer spectroscopy are X-ray amorphous, similar to the
initial BF sample. Besides the barium carbonate and barium iron(IV)
oxide phases, previously identified in BF300A sample, the XRD
pattern of BF600 sample had diffraction lines corresponding to
orthorhombic barium ferrite (BaFe2O4) (Fig. 4c).

76
L. Machala et al. / Journal of Alloys and Compounds 668 (2016) 73e79
€
Fig. 2. Room temperature Mossbauer spectra for different samples. (a) BF190, (b) BF300A, (c) BF300B, and (d) BF600.
Table 1
€
Parameters of Mossbauer spectral components for samples BF190, BF300A, BF300B, and BF600.
Sample
BF190
T (K)
Component
dFe (mm/s)
D
EQ(εQ) (mm/s)
Bhf (T)
LW (mm/s)
RA (%)
300
Fe(VI)
Fe(IV)
Fe(III)
Fe(IV)
Fe(III)
Fe(IV)
Fe(III)
Fe(III)
Fe(III)
Fe(IV)
Fe(III)
BaFe2O4
ꢀ0.90
ꢀ0.28
0.28
0.15
0.00
0.70
0.00
0.56
0.04
0.55
e
0.31
0.42
0.54
0.41
0.49
1.01
1.35
1.55
0.52
0.50
0.63
0.31
25.2
8.9
e
e
65.9
18.7
81.3
19.0
41.4
39.6
100
14.1
25.9
59.9
BF300A
300
25
ꢀ0.27
e
0.35
e
ꢀ0.19
0.45
0.46
23.2
e
ꢀ0.02
0.74
0.00
0.59
0.32
47.3
e
BF300B
BF600
300
300
0.35
ꢀ0.25
e
0.36
e
0.18
47.1
dFe, isomer shift (related to metallic iron);
DEQ, quadrupole splitting; εQ, quadrupole shift; Bhf, hyperfine magnetic field; LW, full width at half maximum; RA, relative sub-
spectrum area.
The thermal decomposition of BaFeO4 under an inert environ-
ment was carried out by performing in-situ variable temperature
XRD experiments in nitrogen atmosphere. Two representative XRD
patterns measured at 300 ꢁC and 600 ꢁC are shown in Fig. 5. At
300 ꢁC, two crystalline phases, rhombohedral BaFeO3 and ortho-
rhombic BaCO3 (high temperature modifications) were identified.
In comparison with the sample BF300A (see Fig. 4a), the relative
BaFeO3 content was significantly higher with respect to BaCO3. This
is consistent with the absence of gaseous CO2 in the decomposition
reaction. At 600 ꢁC, in addition to BaFeO3 diffraction peaks, ortho-
rhombic BaFe2O4 was identified, the same as in the XRD pattern of
the sample BF600 (see Fig. 4). However, the relative amount of
BaFe2O4 was considerably lower in comparison with the sample
BF600 due to lower content of BaCO3.
and X-ray powder diffraction unambiguously showed that the
rhombohedral barium iron(IV) oxide, BaFeO3, was the primary
decomposition product of barium ferrate(VI) above 190 ꢁC in air.
The confidence in assigning the valence state of þ4 lies in the low
€
temperature Mossbauer measurements (see Fig. 3a, Table 1). The
€
hyperfine field of z 23 T, observed in the Mossbauer spectrum, is
consistent with earlier reported value for BaFeO3 [29,32]. This hy-
perfine field together with isomer shift of zꢀ0.15 mm/s (consider
second order Doppler shift) is generally typical for tetravalent iron
atoms in barium iron oxides [38,39]. To further confirm that no
other iron bearing intermediates were formed during the decom-
position of BaFeO4 in air, in-situ high temperature (190 ꢁC)
€
Mossbauer spectrum was obtained. The results agreed with the ex-
situ experiments, demonstrating only the Fe(IV) and Fe(III) phases
again with a slightly higher content of BaFeO3 (z 29% of spectral
area).
3.5. Mechanism of BaFeO4 thermal decomposition
Along with BaFeO3, X-ray amorphous Fe(III) phase and barium
carbonate were formed from initial barium ferrate(VI) thermally-
€
The combination of thermal analysis, Mossbauer spectroscopy

L. Machala et al. / Journal of Alloys and Compounds 668 (2016) 73e79
77
€
Fig. 3. a) Low temperature (25 K) Mossbauer spectrum of sample BF300A, b) low
€
temperature (5 K)/in-field (5 T) Mossbauer spectrum of sample BF300B.
treated in air. An Fe(III) phase was identified in all the examined
samples (BF, BF190, BF300A, BF300B, BF600) as
a super-
€
paramagnetic doublet in the RT Mossbauer spectra with hyperfine
interaction parameters typical for iron(III) oxides or oxyhydroxides.
Apparently, the Fe(III) phase present in the initial BF sample
€
showed practically the same RT Mossbauer spectral component as
in the samples obtained by the thermal treatment. To determine
chemical form, magnetic behavior, particle size, and morphology of
the Fe(III) phase, the detailed analysis of the BF300B sample was
€
carried out by low temperature (5 K)/in-field (5T) Mossbauer
spectroscopy in combination with electron microscopy. In-field
€
Mossbauer spectrum of BF300B sample, shown in Fig. 3b, was
evaluated by one sextet component
(
dFe
¼
0.46 mm/s,
εQ ¼ ꢀ0.02 mm/s, Beff ¼ 46.6 T) with the ratios of intensities of
spectral lines very close to 3:2:1:1:2:3, which were also observed in
Fig. 4. XRD patterns of the samples BF300A (a), BF300B (b), BF600 (c). Peak labeling: *
… BaCO3 (PDF 00-005-0378), þ … BaFeO3 (PDF 01-074-0646), # … BaFe2O4 (PDF 00-
025-1191).
€
a zero-field Mossbauer spectrum (data not shown). Such character
€
of Mossbauer spectra was assigned to speromagnetic behavior of
the Fe(III) phase, showing that the application of an external
magnetic field did not induce any long-range ordering in the
structure due to the absence of the principal crystallographic axis in
amorphous particles [40]. Since the formation of iron(III) oxy-
hydroxides or hydrated forms of iron(III) oxides was not likely to
occur at high temperature, the assumption was made that this
phase was an amorphous anhydrous iron(III) oxide. TEM and SEM
images, presented in Fig. 6, confirm the formation of BaCO3 crystals
which were partially covered by ultra-small Fe2O3 nanoparticles
(<5 nm).
1
2
1
4
BaFeO3 þ CO2/BaCO3 þ Fe2O3 þ O2
(2)
Reaction (2) shows the participation of CO2 from the air in the
reaction mechanism of thermal decomposition of BaFeO4 at high
temperature and also in the sample exposed to air after heating at
300 ꢁC at room temperature (i.e. BF300B). The presence of carbon
dioxide, therefore, caused the degradation of BaFeO3. At 600 ꢁC, the
solid state reaction between the Fe2O3 formed in reaction (2) and
BaCO3 resulted in the formation of BaFe2O4 (reaction 3):
The mechanism of thermal decomposition of barium ferrate(VI)
at 300 ꢁC in static air may thus be described by the following
reactions:
Fe2O3 þ BaCO3/BaFe2O4 þ CO2
(3)
1
2
The solid state reaction between
a
-Fe2O3 and BaCO3 leading to
BaFeO4/BaFeO3 þ O2
(1)
BaFe2O4 was also observed in the temperature range of 540e590 ꢁC
by other researchers [41]. The TEM image shows that barium ferrite

78
L. Machala et al. / Journal of Alloys and Compounds 668 (2016) 73e79
Fig. 5. XRD patterns recorded during in-situ variable temperature measurement at
300 ꢁC (up) and 600 ꢁC (down) in nitrogen atmosphere starting from the initial BF
sample. Peak labeling: * … BaCO3 (PDF 00-052-1528), ** … BaCO3 (PDF 00-041-
0373), þ … BaFeO3 (PDF 01-074-0646), # … BaFe2O4 (PDF 00-025-1191).
Fig. 6. TEM (a) and SEM (b) images of BF300B sample.
in sample BF600 formed nanoparticles with a broad particle size
distribution (20e100 nm) (Fig. 7).
The suggested decomposition mechanism of BaFeO4 agrees very
well with thermal analysis of air samples (Fig. 1a). A theoretical
value of mass loss related to the complete decomposition of BaFeO4
to BaFeO3 was estimated to be 5.16 wt%. This calculation considered
the initial impurity of the BF sample as 15 wt% of Fe2O3 and
consequently the corresponding amount of BaCO3. The significantly
lower value of mass loss observed (3.0 wt%) within the second
decomposition step (between 230 and 310 ꢁC) in air can be
explained by a participation of gaseous CO2 in the reaction,
resulting in the formation of additional barium carbonate in the
sample. A broad endothermic effect on DSC curve with the peak at
730 ꢁC and corresponding mass loss of z 1.5 wt% were related to
the solid-state reaction of Fe2O3 with BaCO3 (reaction 3). The
decomposition of barium carbonate to barium oxide and carbon
dioxide takes place as a competitive process above 730 ꢁC (Fig. 1a),
which was seen in a broad exothermal effect and mass loss of
2.75 wt% on DSC/TG curves.
Fig. 7. TEM image of BF600 sample.
4. Conclusions
as amorphous Fe2O3 in the form of nanoparticles. Above 600 ꢁC,
carbon dioxide was evolved in the two competitive processes
including the solid state reaction of barium carbonate with iron(III)
oxide and the decomposition of barium carbonate. Future work
The presented results clearly demonstrate that the first step of
thermal decomposition of BaFeO4 is the formation of FeIV species in
the form of BaFeO3 under both static air and inert environment. The
subsequent reaction of BaFeO3 with CO2 in air at high temperature
results in the formation of Fe(III) and barium carbonate. This re-
action also occurred during the aging of BaFeO3. The Fe(III) phase,
which was formed by the transformation of BaFeO3, was identified
€
may include in-situ high temperature Mossbauer and XRD mea-
surements under rigorously inert atmosphere with pure BaFeO4
sample without any presence of CO2. This will further clarify the
role of CO2 in thermal decomposition of BaFeO4. Similar studies can

L. Machala et al. / Journal of Alloys and Compounds 668 (2016) 73e79
182e191, http://dx.doi.org/10.1021/ar5004219 pMID: 25668700.
79
be extended to other salts of Fe(VI) to learn the role of cations in
finding the intermediate iron species. Furthermore, decomposition
studies of salts of Fe(V) and Fe(IV) will advance the understanding
of the chemistry of these high-valent iron compounds. Last but not
least, understanding of reversibility of the proposed transformation
reactions can be important in electrochemical applications of
barium ferrate(VI).
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10380e10389, http://dx.doi.org/10.1021/es5028426 pMID: 25073066.
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Financial supports from the Ministry of Education of the Czech
Republic (projects LO1305, MEB040806) are gratefully acknowl-
edged. This work was also supported by the Operational Program
Education for Competitiveness e European Social Fund (projects
CZ.1.07/2.3.00/30.0004 and CZ.1.07/2.3.00/20.0058 of the Ministry
of Education, Youth and Sports of the Czech Republic), and by
ꢀ
Czech-Hungarian research cooperation project TeT CZ-11/2007. V.
K. Sharma acknowledges the support of the United States National
Science Foundation (CBET-1439314) for this research. The authors
[21] V.K. Sharma, Oxidation of inorganic compounds by ferrate (VI) and ferrate (V):
one-electron and two-electron transfer steps, Environ. Sci. Technol. 44 (13)
(2010) 5148e5152.
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ꢁ
thank to Petr Novak and Josef Kaslík for technical assistance.
[22] V.K. Sharma, G.W. Luther, F.J. Millero, Mechanisms of oxidation of organo-
sulfur compounds by ferrate (VI), Chemosphere 82 (8) (2011) 1083e1089.
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Appendix A. Supplementary data
€
Mossbauer characterization and in situ monitoring of thermal decomposition
of potassium ferrate (VI), K2FeO4 in static air conditions, J. Phys. Chem. B 111
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jallcom.2016.01.185.
(16) (2007) 4280e4286.
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[24] L. Machala, V. Prochazka, M. Miglierini, V.K. Sharma, Z. Marusak, H.-C. Wille,
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R. Zboril, Direct evidence of Fe(V) and Fe(IV) intermediates during reduction
of Fe(VI) to Fe(III): a nuclear forward scattering of synchrotron radiation
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