Reduction properties of phases in the system La0.5Sr 0.5MO3 (M=Fe, Co)

Article abstract of DOI:10.1016/j.jssc.2004.10.003

Phases formed by the reduction of compounds of the type La _0.5Sr_0.5MO₃ (M=Fe, Co) have been characterized by means of temperature programmed reduction, X-ray powder diffraction, ⁵⁷Fe Moessbauer spectroscopy and Fe K-, Co K-, Sr K-, and La L_III-edge X-ray absorption spectroscopy. The results show that treatment of the material of composition La_0.5Sr _0.5FeO₃ (which contains 50% Fe⁴⁺ and 50% Fe³⁺) at 650 °C in a flowing 90% hydrogen/10% nitrogen atmosphere results in the formation of an oxygen-deficient perovskite-related phase containing only trivalent iron. Further heating in the gaseous reducing environment at 1150 °C results in the formation of the Fe ³⁺-containing phase SrLaFeO₄, which has a K ₂NiF₄-type structure, and metallic iron. The material of composition La_0.5Sr_0.5CoO₃ is more susceptible to reduction than the compound La_0.5Sr_0.5FeO₃ since, after heating at 520 °C in the hydrogen/nitrogen mixture, all the Co⁴⁺ and Co³⁺ are reduced to metallic cobalt with the concomitant formation of strontium- and lanthanum-oxides.

Full text of DOI:10.1016/j.jssc.2004.10.003

ARTICLE IN PRESS
Journal of Solid State Chemistry 178 (2005) 961–969
www.elsevier.com/locate/jssc
Reduction properties of phases in the system La0.5Sr0.5MO3
(M=Fe, Co)
Ã
Frank J. Berrya, Jose F. Marcob, , Xiaolin Rena
aDepartment of Chemistry, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
´ ´ ´
Instituto de Quımica-Fısica ‘‘Rocasolano’’, Consejo Superior de Investigaciones Cientıficas, c/ Serrano 119, ES-28006 Madrid, Spain
b
Received 5 May 2004; received in revised form 14 September 2004; accepted 4 October 2004
Abstract
Phases formed by the reduction of compounds of the type La0.5Sr0.5MO3 (M=Fe, Co) have been characterized by means of
temperature programmed reduction, X-ray powder diffraction, 57Fe Mossbauer spectroscopy and Fe K-, Co K-, Sr K-, and La LIII
-
edge X-ray absorption spectroscopy. The results show that treatment of the material of composition La0.5Sr0.5FeO3 (which contains
50% Fe4+ and 50% Fe3+) at 650 1C in a flowing 90% hydrogen/10% nitrogen atmosphere results in the formation of an oxygen-
deficient perovskite-related phase containing only trivalent iron. Further heating in the gaseous reducing environment at 1150 1C
results in the formation of the Fe3+-containing phase SrLaFeO4, which has a K2NiF4-type structure, and metallic iron. The material
of composition La0.5Sr0.5CoO3 is more susceptible to reduction than the compound La0.5Sr0.5FeO3 since, after heating at 520 1C in
the hydrogen/nitrogen mixture, all the Co4+ and Co3+ are reduced to metallic cobalt with the concomitant formation of strontium-
and lanthanum-oxides.
r 2004 Elsevier Inc. All rights reserved.
Keywords: Perovskite-related oxides; Reduction phases; Mossbauer; XAS
1. Introduction
characterization by 57Fe Mossbauer spectroscopy and
by X-ray absorption spectroscopy.
Perovskite-related oxygen-deficient materials of the
type La1ÀySryMO3 (M=Fe, Co) have been the subject
of interest for some time. For example, the cobalt-
containing materials have potential applications as
electrode materials for high temperature fuel cells, as
catalyst in, for example, automobile exhaust catalyst
systems, and as gas sensors [1–3]. There is also interest in
the unusual magnetic and electronic properties of the
materials [4]. Given that some of these applications
involve operation under reducing conditions and that
the nature of reduced phases in these systems is a matter
of sparse attention, we have investigated the nature of
materials formed by temperature programmed reduc-
tion (TPR) in hydrogen and we report here on their
2. Experimental
Compounds of the type La0.5Sr0.5MO3 (M=Fe, Co)
were prepared by grinding appropriate quantities of
lanthanum(III) oxide, strontium(II) carbonate, and
either a-iron(III) oxide or cobalt(II) oxide in an agate
pestle and mortar and heating in air at 1200 1C (12 h).
X-ray powder diffraction (XRD) patterns were
recorded at 298 K with a Siemens D5000 diffractometer
using Cu Ka radiation. TPR profiles were recorded from
ca. 150 mg samples in flowing 10% hydrogen/90%
nitrogen (15–20 ml/min) with the temperature
being increased by 5 1C/min. Thermal analysis was
performed with a Rheometric Scientific STA 1500
system in a 10% hydrogen/90% nitrogen gas mixture
flowing at 20 ml/min.
Ã
Corresponding author. Fax: +34 915 642 431.
E-mail address: jfmarco@iqfr.csic.es (J.F. Marco).
0022-4596/$ - see front matter r 2004 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2004.10.003

ARTICLE IN PRESS
F.J. Berry et al. / Journal of Solid State Chemistry 178 (2005) 961–969
962
57Fe Mossbauer spectra were recorded at 298 K with a
x
Fe
La2O3
microprocessor-controlled Mossbauer spectrometer
using a ca. 25 mCi 57Co/Rh source. All the spectra were
computer fitted and the isomer shift data are reported
relative to that of metallic iron at room temperature.
X-ray absorption near edge structure (XANES) and
extended X-ray absorption fine structure (EXAFS)
measurements were performed at Station 8.1 (Fe K-,
Co K-, and La LIII-edge) and at Station 9.2 (Sr K-edge)
at the Synchrotron Radiation Source at Daresbury
Laboratory operating at an energy of 2.0 GeV and an
average current of 200 mA. Data were collected at
298 K. The energy scale was calibrated using 6 mm iron
and cobalt foils, lanthanum(III) oxide and strontium(II)
carbonate. The position of the first inflection point on
the iron foil edge was taken at 7112.3 eV, that of the
cobalt foil edge at 7708.9 eV, that of lanthanum(III)
oxide at 5480.6 eV and that of strontium(II) carbonate
at 16,110.2 eV. All the iron-, cobalt-, lanthanum-, and
strontium-XANES data are reported here relative to
these values. The edge profiles were separated from the
EXAFS data and, after subtraction of linear pre-edge
background, normalized to the edge step. The position
of the absorption edge was obtained from the first most
intense maximum of the first derivative of the edge
profile. However, since the position of the edge is
sometimes defined as the energy at which the normalized
absorption is 0.5 (i.e., the absorption at half-height of
the edge step) we also quote such values to allow
comparison with published data. The EXAFS oscilla-
tions were isolated after background subtraction of the
raw data using the Daresbury program EXBACK and
converted into k space. The data were weighted by k3,
where k is the photoelectron wave vector, to compensate
for the diminishing amplitude of EXAFS at high k.
The data were fitted using the Daresbury program
EXCURV90.
o SrFeLaO4
o
LaFeO3-type
SrFeO3-type
+
o
x
x
o
*
o
o
o
o
o
.
o
o
o
o
o
.
.
(d)
*
*
*
*
*
*
(c)
(b)
+
+
+
+
+
+
*
*
*
*
*
*
(a)
10
20
30
40
50
60
70
80
2θ / degrees
Fig. 1. XRD patterns recorded from (a) La0.5Sr0.5FeO3 and following
sequential treatment in (b) 90% hydrogen/10% nitrogen at 650 1C,
(c) air at 450 1C, and (d) 90% hydrogen/10% nitrogen at 1150 1C.
La0.5Sr0.5Co0.5O3
La0.5Sr0.5FeO3
0
200
400
600
800
1000
1200
Temperature / °C
Fig. 2. TPR profiles recorded from La0.5Sr0.5FeO3 and La0.5Sr0.5
CoO3.
3. Results and discussion
in the TPR profile (Fig. 2), showed all the XRD peaks to
shift to lower angle and was more similar to that of the
orthorhombic LaFeO3-type phase. No evidence could
be found for the presence of strontium oxide. The XRD
pattern recorded from the material after reheating in air
at 450 1C for 12 h (Fig. 1c) showed the product to return
to the cubic SrFeO3-type structure. The XRD pattern
recorded from the material following treatment by TPR
at 1150 1C (Fig. 1d) showed the formation of SrFeLaO4,
which adopts the K2NiF4-type structure [6], La2O3, and
metallic iron.
The 57Fe Mossbauer spectra recorded at 298 K are
shown in Fig. 3. The spectrum recorded from the pure
material (Fig. 3a) showed a broad absorption line which
was best fitted to two singlet components. The results of
the fit are collected in Table 1. The isomer shift of the
3.1. La0.5Sr0.5FeO3
The XRD pattern recorded from the compound
La0.5Sr0.5FeO3 is shown in Fig. 1a. The pattern is
characteristic of the cubic SrFeO3-type structure. The
TPR profile recorded from La0.5Sr0.5FeO3 is shown in
Fig. 2 and shows an initial narrow peak at ca. 570 1C
followed by a broad peak spanning the temperature
range ca. 800–1200 1C.
The first reduction peak in the TPR profile recorded
from La0.5Sr0.5FeO3 (Fig. 2) was located at a higher
temperature than that observed by TPR in LaFeO3
(475 1C) [5]. The XRD pattern (Fig. 1b) recorded from
La0.5Sr0.5FeO3 following treatment in the reducing
atmosphere at 650 1C, i.e., after the first reduction peak

ARTICLE IN PRESS
F.J. Berry et al. / Journal of Solid State Chemistry 178 (2005) 961–969
963
singlet appearing at lower velocity is characteristic of
Fe4+ in octahedral oxygen coordination [7] whilst the
parameters of the other singlet correspond to Fe3+. A
comparison of the spectral areas of both components
(Table 1) indicates that the material contains equal
amounts of Fe4+ and Fe3+
.
The spectrum recorded following treatment in flowing
hydrogen and nitrogen (Fig. 3b) at ca. 650 1C (i.e., after
the first peak in the TPR profile, Fig. 2) showed the
superposition of two magnetically split sextet compo-
nents both characteristic of Fe3+ (Table 1) and
demonstrating that low temperature treatment induces
reduction of Fe4+–Fe3+. The shift in the XRD peak
positions to lower angle (Fig. 2b) indicated an increase
in unit cell parameters from a=5.490(1) A,
c=13.38(1) A for La0.5Sr0.5FeO3 to a=5.533(1) A,
c=13.50(1) A for the partially reduced sample consistent
with the presence of an increasing amount of Fe3+ with
larger ionic radius. The results of thermal analysis of
La0.5Sr0.5FeO3 in 10% hydrogen/90% nitrogen are
shown in Fig. 4. A weight decrease of ca. 2%
corresponding to a loss of lattice oxygen and the
formation of a material of a composition La0.5Sr0.5
FeO2.72 was observed from the thermogravimetric
analysis (TGA) curve at ca. 600 1C. This agrees well
100
96
92
88 (a)
100
with the formation of
a compound of formula
99
La0.5Sr0.5FeO2.75 which can be calculated on the
assumption that all the Fe4+ in La0.5Sr0.5FeO3 is
reduced to Fe3+. The identification of two sextet
patterns is indicative of two Fe3+ sites. The sextet with
smaller chemical isomer shift (d=0.27 mm sÀ1) is char-
acteristic of Fe3+ in coordination lower than octahe-
dral, as has been observed previously in other oxygen-
deficient perovskite-related materials [8], whilst the
other sextet is characteristic of Fe3+ in octahedral sites.
It is interesting to note that while the pure material is
paramagnetic at room temperature, the material ob-
tained after treatment in the flowing hydrogen and
nitrogen at 650 1C is magnetically ordered at 298 K. This
can be associated with the different strengths of
magnetic interactions operating in these compounds.
It is known [9] that the Fe3+–Fe3+ interaction is
stronger than Fe4+–Fe4+ and Fe4+–Fe3+ interactions.
98
100
(b)
99
97
98
97
(c)
-12
-9
-6
-3
0
3
6
9
12
Velocity (mms-1)
Fig. 3. (a) 57Fe Mossbauer spectra recorded at 298 K from (a)
La0.5Sr0.5FeO3 and following treatment in 90% hydrogen/10%
nitrogen at (b) 650 1C and (c) 1150 1C.
Table 1
57Fe Mossbauer parameters recorded from La0.5Sr0.5FeO3 following treatment in 90% hydrogen/10% nitrogen
a
Assignment
d Æ 0:01
D
or 2ꢀ b70.02
G Æ 0:02
H Æ 0:5
Spectral
area73%
(mm sÀ1
)
(mm sÀ1
)
(mm sÀ1
)
(T)
La0.5Sr0.5FeO3
Fe4+
Fe3+
0.03
0.25
0
0
0.39
0.39
0
0
50
50
Following treatment in 90%
hydrogen/10% nitrogen at 650 1C
Fe3+
Fe3+
0.27
0.37
–0.40
0.19
0.61
0.61
52.1
52.9
47
53
1150 1C
Fe3+
Fe0
Fe0
0.31
0.00
0.58
0
0.60
0.43
0.42
32.4
33.2
0
42
51
7
À0.08
0
aQuadrupole split absorption.
bMagnetically split absorption.

ARTICLE IN PRESS
F.J. Berry et al. / Journal of Solid State Chemistry 178 (2005) 961–969
964
Therefore, the presence of only Fe3+ ions in the material
heated at 650 1C in hydrogen and nitrogen results in
stronger superexchange interactions than in the original
material and, therefore, in magnetic order at room
temperature and in the existence of a hyperfine magnetic
field.
20
15
10
5
101
100.5
100
99.5
99
98.5
98
The spectrum recorded from the material heated at
ca. 1250 1C, that is, after the second peak in the TPR
profile showed the superposition of two sextets and a
singlet (Fig. 3c). One of the sextets, characteristic of
97.5
97
0
Fe3+ (d=ca. 0.31 mm sÀ1), demonstrated
a small
-5
96.5
hyperfine magnetic field (H=ca. 32.4 T). The para-
meters are similar to those reported [6] for LaSrFeO4
which adopts the K2NiF4-type structure. The other
sextet (H=ca. 33.2 T) and the singlet (d=ca.
0.08 mm sÀ1) are characteristic of large- and small-
particle metallic a-iron, respectively [5]. Hence, treat-
ment of La0.5Sr0.5FeO3 at elevated temperature in a
mixture of flowing hydrogen and nitrogen induces both
structural rearrangement of the perovskite-related lat-
tice to the K2NiF4-type structure and reduction to
metallic iron. It is worth noting that, even after
treatment at the high temperature in the reducing
atmosphere, a noticeable amount of iron remains in
the trivalent state.
0
100 200 300 400 500 600 700 800
Temperature / °C
Fig. 4. TGA (heavy line) and DSC (light line) curves recorded from
La0.5Sr0.5FeO3 in flowing 10% hydrogen/90% nitrogen.
(a)
(a)
(b)
(c)
(d)
(b)
The Fe K-edge XANES (and their first derivative)
recorded from pure La0.5Sr0.5FeO3 and the materials
obtained after treatment at 650 and 1150 1C are shown
in Fig. 5 together with the Fe K-edge XANES of
metallic iron used as a reference. The position of the
absorption edge in the Fe K-edge XANES of La0.5Sr0.5
FeO3 appears at ca. 7128.5 eV (Table 2). This is ca.
1.9 eV higher than the position of the Fe K-edge
absorption edge of LaFeO3 that only contains trivalent
iron. It has been recently reported [10] that the energy
difference between the point at half absorption in the
(c)
(d)
7100 7120 7140 7160 7180 7100 7120 7140 7160 7180
Energy / eV Energy / eV
Fig. 5. (Left) Iron K-edge XANES recorded from (a) La0.5Sr0.5FeO3
and following treatment in 90% hydrogen/10% nitrogen at (b) 650 1C
and (c) 1150 1C. (d) Iron K-edge XANES recorded from metallic iron.
(Right) Corresponding first derivatives of the data.
Table 2
X-ray absorption edge positions recorded from the X-ray absorption near edge structure
Edge positiona (eV)70.3
Edge positionb (eV)70.3
Fe
Co
La
Sr
Fe
Co
La
Sr
Fe (metal foil)
Co (metal foil)
LaFeO3
7119.7
—
—
7716.0
—
—
—
—
—
—
—
7112.3
—
—
7709.0
—
—
—
—
—
—
—
—
5482.3
—
—
5484.3
—
7125.5
—
—
7126.6
—
—
LaCoO3
La2O3
SrCO3
7720.6
—
7723.6
—
5480.6
5484.3
16,110.2
16,110.2
16,110.2
16,110.2
16,110.0
16,110.2
16,110.2
La0.5Sr0.5FeO3
(TPR treated) 650 1C
(TPR treated) 1250 1C
La0.5Sr0.5CoO3
(TPR treated) 520 1C
(TPR treated) 1150 1C
7126.5
7124.9
7121.9
—
—
—
5480.2
5480.2
5480.2
5480.5
5480.8
5481.0
16,102.5
16,104.3
16,103.8
16,105.1
16,105.0
16,104.9
7128.5
7127.4
7112.3
—
—
—
5483.5
5483.4
5483.2
5483.9
5483.9
5484.2
—
—
7721.8
7716.6
7716.2
7724.2
7709.0
7709.0
—
—
—
—
aCorresponds to the absorption at half-height of the edge step.
bCorresponds to the first most intense maximum in the derivative of the XANES.

ARTICLE IN PRESS
F.J. Berry et al. / Journal of Solid State Chemistry 178 (2005) 961–969
965
edge stepof SrFeO
(a material in which iron is
3Àd
8
4
mainly present as Fe4+) and the first inflexion point in
the Fe K-edge XANES of metallic iron is ca. 14 eV.
Inspection of Table 2 shows a similar energy difference
between La0.5Sr0.5FeO3 and metallic iron. The results
are consistent with the presence of Fe4+ in this material
as indicated by Mossbauer spectroscopy. The position
of the Fe K-absorption edge recorded from the material
following treatment at 650 1C shifted to 7127.4 eV, a
value that is closer to that of LaFeO3. It is also
interesting to note that the 1s-3d pre-edge feature at
7115.9 eV in the Fe K-edge XANES of pure La0.5Sr0.5
FeO3 shifts to 7114.6 eV in the Fe K-edge XANES of the
material treated at 650 1C. A shift of ca. 1.5 eV is also
observed at the edge crest (Fig. 5). All these changes are
indicative of the reduction of Fe4+–Fe3+ as shown by
Mossbauer spectroscopy. Furthermore, inspection of
Fig. 5 suggests that the pre-edge feature in the Fe K-
edge XANES of La0.5Sr0.5FeO3 (characteristic of
distorted octahedral geometry) increases in intensity in
the Fe K-edge XANES of the material treated under the
reducing atmosphere at 650 1C. This is compatible with
the existence of a fraction of Fe3+ in coordination lower
than octahedral [11] as also shown by Mossbauer
spectroscopy. Finally, the first most intense maximum
in the derivative of the Fe K-edge XANES of the
material treated at 1150 1C appears at 7112.3 eV, a value
which coincides with the position of the Fe K-edge of
metallic iron. Inspection of Fig. 5 also shows that the Fe
K-edge XANES of this material contains some features
compatible with the presence of Fe3+ (e.g., peak in the
first derivative at 7127.4 eV) which corresponds with the
presence of the Fe3+-containing phase SrLaFeO4 as
identified also by both XRD and 57Fe Mossbauer
spectroscopy. Summarizing, the movement to lower
energy of the X-ray absorption Fe K-edge XANES
(clearly observed in Fig. 5 and Table 2 if the criterion of
defining the position of the edge as the energy at half-
height of the edge stepis taken) with increasing
treatment in the hydrogen/nitrogen gas mixture is
consistent with reduction of iron. In contrast to the
variation of the Fe K-edge X-ray absorption edge
positions, the La LIII-edge- and Sr K-edge-X-ray
absorption edge positions did not change significantly
with treatment in the flowing mixture of hydrogen and
0
-4
-8
9
6
3
0
(a)
(b)
-3
-6
-9
12
8
4
0
-4
-8
-12
(c)
2
4
6
8
10
12
k / Å-1
1.5
1.0
0.5
(a)
(b)
(c)
0.0
1.0
0.5
0.0
1.5
1.0
0.5
0.0
0
2
4
6
8
10
r / Å
Fig. 6. (Top) Fe K-edge EXAFS recorded from (a) La0.5Sr0.5FeO3 and
following treatment in 90% hydrogen/10% nitrogen at (b) 650 1C and
(c) 1150 1C. (Bottom) The corresponding Fourier transforms of the
EXAFS. (The experimental data are indicated by the solid line and the
fits to the data are indicated by the broken line.)
which coordinate iron in LaFeO3 [5,12]. This is
consistent with the presence of Fe4+ in La0.5Sr0.5FeO3
as shown by 57Fe Mossbauer spectroscopy. A better fit
was obtained by refining the first shell oxygen coordina-
tion to three oxygen atoms at 1.87 A, a distance which is
characteristic of Fe4+ [13], and three at 1.98 A which
reflects the equal amount of Fe4+ and Fe3+ in this
material.
Following treatment in hydrogen and nitrogen at
650 1C the Fe K-edge EXAFS showed the first shell of
six oxygen atoms to be located at ca. 1.94 A and to be
best separated into three oxygen atoms at 1.89 A and
three at 2.00 A (Table 3). The results suggest that, in the
material resulting from treatment in hydrogen and
nitrogen at 650 1C, half of the Fe3+ are in lower
coordination than octahedral. In this respect, we would
nitrogen being characteristic of La3+ and Sr2+
respectively (Table 2).
,
The Fe K-edge EXAFS is shown in Fig. 6, top. The
fitting of the data recorded from the reduced materials
was complicated by the multiphasic nature of the
products; hence, Table 3 contains only the best fit
parameters in the region 0–4 A. The first peak corre-
sponding to six oxygen atoms in the Fourier transform
of the data recorded from La0.5Sr0.5FeO3 (Fig. 6a,
bottom) was located at ca. 1.92 A which is shorter than
the distances (1.98–2.00 A) for the six oxygen atoms

ARTICLE IN PRESS
F.J. Berry et al. / Journal of Solid State Chemistry 178 (2005) 961–969
966
Table 3
Best fit parameters to the EXAFS data recorded from La0.5Sr0.5FeO3
La0.5Sr0.5FeO3
La0.5Sr0.5FeO3 treated at 650 1C
La0.5Sr0.5FeO3 treated at 1250 1C
2s2
2s2
2s2
Coordination
number and
[atom type]
d (A)
(70.02)
Coordination
number and
[atom type]
d (A)
(70.02)
Coordination
number and
[atom type]
d (A)
(70.02)
(A2)
(A2)
(A2)
Fe K-edge
3 [O]
3 [O]
8 [La]
6 [Fe]
1.87
1.98
3.21
3.94
0.006
0.004
0.033
0.009
3 [O]
3 [O]
8 [La]
6 [Fe]
1.89
2.00
3.19
3.95
0.011
0.015
0.034
0.023
4 [O]
2 [O]
4.5 [Fe]
1.96
2.20
2.48
0.019
0.012
0.013
La LIII-edge
Sr K-edge
3 [O]
6 [O]
3 [O]
2.59
2.81
3.49
0.007
0.031
0.004
3 [O]
6 [O]
3 [O]
2.53
2.74
3.47
0.015
0.039
0.049
1 [O]
4 [O]
4 [O]
2.44
2.63
2.85
0.003
0.001
0.002
3 [O]
6 [O]
3 [O]
2.56
2.75
3.32
0.011
0.024
0.001
3 [O]
6 [O]
3 [O]
2.54
2.76
3.30
0.020
0.056
0.001
1 [O]
4 [O]
4 [O]
2.39
2.59
2.79
0.001
0.004
0.010
x
Co
mention that a tentative structure has been proposed for
the perovskite-related oxide of composition SrFeO2.75
[8] in which every other oxygen atom is lost from certain
strings such that half of the iron ions remains
octahedrally coordinated and half become five coordi-
nated. Also, in the reduced phase of formula
La1À2xCa2xFeO3Àx which contains only Fe3+, the
existence of tetrahedral sites has been confirmed for x
values as small as 0.10 [8]. In the absence of more
structural data, we cannot decide whether the sextet
with the lower isomer shift arises from five coordinated
or tetrahedrally coordinated Fe3+. However, if this
material formed by partial reduction at 650 1C con-
tained equal quantities of octahedrally and tetrahedrally
coordinated Fe3+ then an oxygen content of 2.5, as in
the ordered brownmillerite structure in which each
tetrahedrally coordinated iron is surrounded by two
oxygen vacancies, would be required which is not the
case.
La2O3
o
SrO
SrCoO3-type
.
.
/o
*
x
.
.
o
.
o
o
.
o
x
.
(c)
.
/o
(b)
(a)
*
*
*
*
*
*
10
20
30
40
50
60
70
80
2θ / degrees
Fig. 7. XRD patterns recorded from (a) La0.5Sr0.5CoO3 and following
sequential treatment in 90% hydrogen/10% nitrogen at (b) 520 1C and
(c) 1150 1C.
The first shell coordination of iron in the Fe K-edge
EXAFS recorded from La0.5Sr0.5FeO3 following treat-
ment in flowing hydrogen and nitrogen at 1150 1C was
fitted according to the evidence from XRD and
Mossbauer spectroscopy for reduction to LaSrFeO4
and metallic iron. Hence, the coordination of iron by
four oxygen atoms at 1.96 A and two at 2.20 A agrees
well with the neutron diffraction data recorded
form LaSrFeO4 [6] whilst the iron atoms at ca.
2.48 A represent the metallic iron (Fig. 6c, bottom and
Table 3).
lanthanum and strontium (Table 3). Treatment in the
90% hydrogen/10% nitrogen gas mixture at 1150 1C
induced a change in the first shell coordination
of lanthanum and strontium to nine oxygen atoms
(Table 3). This is in good agreement with the XRD
pattern (Fig. 1d) that showed the formation of
LaSrFeO4, which contains lanthanum and strontium
in nine-fold oxygen coordination [6].
The La LIII-edge and Sr K-edge EXAFS (Table 3)
recorded from La0.5Sr0.5FeO3 showed lanthanum and
strontium to be coordinated by 12 oxygen atoms as
would be expected given the evidence from XRD that
the material adopts the cubic SrFeO3-type structure.
The material treated in hydrogen at ca. 650 1C showed
little change in the first shell oxygen coordination of
3.2. La0.5Sr0.5CoO3
The XRD pattern recorded from the compound
La0.5Sr0.5CoO3 is shown in Fig. 7a. The pattern is
characteristic of the SrCoO3-type structure. The TPR
profile recorded from La0.5Sr0.5CoO3 (Fig. 2) showed
the second peak to move to much lower temperature as

ARTICLE IN PRESS
F.J. Berry et al. / Journal of Solid State Chemistry 178 (2005) 961–969
967
10
5
(a)
0
(a)
-5
-10
20
(b)
(c)
(d)
(a)
(b)
(c)
(d)
10
0
(b)
(c)
-10
-20
20
10
0
-10
-20
7700 7720 7740 7760 7780
Energy / eV
7700 7720 7740 7760 7780
Energy / eV
Fig. 8. (Left) Co K-edge XANES recorded from (a) La0.5Sr0.5CoO3
and following treatment in flowing 90% hydrogen/10% nitrogen at (b)
520 1C and (c) 1150 1C. (d) Co K-edge XANES recorded from metallic
cobalt. (Right) Corresponding first derivatives of the data.
2
4
6
8
-1
10
12
k /Å
1.5
1.0
0.5
compared to that recorded from La0.5Sr0.5FeO3. The
XRD pattern recorded from the material treated at
520 1C (Fig. 7b), i.e., after the first reduction peak in the
TPR profile (Fig. 2), showed the material to be poorly
crystalline but to contain broad peaks which may be
associated with the most intense peaks of strontium- and
lanthanum-oxides. After TPR treatment at 1150 1C, the
material had decomposed to strontium- and lanthanum-
oxides and metallic cobalt (Fig. 7c).
The X-ray absorption edge position at 7724.2 eV of
the Co K-edge XANES recorded from La0.5Sr0.5CoO3
(Fig. 8a, Table 2) is ca. 0.6 eV larger than the edge
position of the Co K-edge XANES recorded from
LaCoO3 [5], a material which contains only trivalent
cobalt. The energy at half-height of the edge step
(Table 2) correlates well with recently published
XANES data for La0.5Sr0.5CoO3 [14,15] and reflects
the presence in this compound of Co4+ and Co3+. The
Co K-edge XANES recorded from the material follow-
ing treatment in flowing hydrogen and nitrogen at
520 1C was almost identical to that recorded from
metallic cobalt (Fig. 8b and 7d). The result demonstrates
that, under these mild conditions, the formation of
metallic cobalt is readily achieved (see also Table 2). The
Co K-edge XANES recorded from the material heated
in the reducing atmosphere at the higher temperature of
1150 1C also showed the presence of metallic cobalt
(Fig. 8c).
(a)
0.0
3
2
1
(b)
(c)
0
3
2
1
0
0
2
4
6
8
10
r / Å
Fig. 9. (Top) Co K-edge EXAFS recorded from (a) La0.5Sr0.5CoO3
and following treatment in flowing 90% hydrogen/10% nitrogen at (b)
520 1C and (c) 1150 1C. (Bottom) The corresponding Fourier trans-
forms of the above EXAFS. (The experimental data are indicated by
the solid line and the fits to the data are indicated by the broken line.)
of three oxygen atoms at a distance of 1.85 A
(corresponding to the presence of Co4+ in this material)
and three oxygen atoms at a distance of 1.96 A
(accounting for Co3+). In contrast, the EXAFS
recorded from the reduced phases showed only the
coordination of cobalt by cobalt (Fig. 9b and c, bottom
and Table 4).
The X-ray absorption edge position of the LaIII- and
Sr K-edges obtained from the XANES data (Table 2)
did not change significantly with treatment in the
reducing environment. The La LIII-edge EXAFS re-
corded from La0.5Sr0.5CoO3 (Fig. 10a, Table 4) showed
the lanthanum to be coordinated to 12 oxygen atoms in
the perovskite-related structure [16]. Treatment at 520
and 1150 1C in flowing hydrogen and nitrogen gave
seven-fold oxygen coordination characteristic of La2O3
[17] (Fig. 10b and c, Table 4). The Sr K-edge EXAFS
recorded from La0.5Sr0.5CoO3 (Fig. 11a, Table 4)
The Co K-edge EXAFS (Fig. 9, Table 4) endorsed the
XANES data. The EXAFS data recorded from
La0.5Sr0.5CoO3 were initially fitted to six oxygen atoms
(Fig. 9a, bottom) at 1.90 A, a value close to that
reported previously for cobalt–oxygen distances in
La0.5Sr0.5CoO3 [14,15]. A better fit was obtained by
adopting the approach used to fit the data from
La0.5Sr0.5FeO3, and involving fitting the data to a model
in which the first oxygen shell was split (Table 4) to a set

ARTICLE IN PRESS
F.J. Berry et al. / Journal of Solid State Chemistry 178 (2005) 961–969
968
Table 4
Best fit parameters to the EXAFS data recorded from La0.5Sr0.5CoO3
La0.5Sr0.5CoO3
La0.5Sr0.5CoO3 treated at 520 1C
La0.5Sr0.5CoO3 treated at 1150 1C
Coordination number d (A)
2s2
2s2
2s2
Coordination number d (A)
and [atom type]
Coordination number d (A)
and [atom type]
(A2)
(A2)
(A2)
(70.02)
(70.02)
and [atom type]
(70.02)
Co K-edge
3 [O]
3 [O]
1.85
1.96
3.17
3.89
0.005
0.004
0.031
0.009
8 [Co]
6 [Co]
12 [Co]
2.48
3.49
4.31
0.014
0.028
0.018
8 [Co]
6 [Co]
12 [Co]
2.48
2.45
4.30
0.012
0.035
0.015
8 [La]
6 [Co]
La LIII-edge
Sr K-edge
3 [O]
6 [O]
3 [O]
2.55
2.74
3.41
0.002
0.020
0.008
4 [O]
3 [O]
2.52
2.70
0.004
0.006
4 [O]
3 [O]
2.52
2.67
0.003
0.007
3 [O]
6 [O]
3 [O]
2.58
2.77
3.28
0.003
0.014
0.003
6 [O]
4 [Sr]
2.55
3.54
0.024
0.041
6 [O]
12 [Sr]
2.57
3.61
0.021
0.045
8
4
0
8
4
(a)
(b)
-4
0
-8
4
-4
(a)
(b)
-8
4
2
0
-2
-4
2
0
-2
-4
4
6
4
2
0
-2
-4
(c)
2
0
-2
-4
-6
(c)
2
4
6
8
10
k / Å-1
2
4
6
8
10
k / Å-1
0.9
0.6
0.3
0.0
0.9
0.6
0.3
(a)
(a)
(b)
0.0
0.6
0.4
0.2
0.4
0.2
(b)
(c)
0.0
0.6
0.0
0.8
0.4
0.2
0.0
0.6
0.4
0.2
0.0
0
(c)
0
2
4
6
8
10
2
4
6
8
10
r /Å
r /Å
Fig. 10. (Top) La LIII-edge EXAFS recorded from (a) La0.5Sr0.5CoO3
and following treatment in flowing 90% hydrogen/10% nitrogen at (b)
520 1C and (c) 1150 1C. (Bottom) The corresponding Fourier trans-
forms of the above EXAFS. (The experimental data are indicated by
the solid line and the fits to the data are indicated by the broken line.)
Fig. 11. (Top) Sr K-edge EXAFS recorded from (a) La0.5Sr0.5CoO3
and following treatment in flowing 90% hydrogen/10% nitrogen at (b)
520 1C and (c) 1150 1C. (Bottom) The corresponding Fourier trans-
forms of the above EXAFS. (The experimental data are indicated by
the solid line and the fits to the data are indicated by the broken line.)

ARTICLE IN PRESS
F.J. Berry et al. / Journal of Solid State Chemistry 178 (2005) 961–969
969
showed the strontium to be coordinated to 12 oxygen
References
atoms in the perovskite-related structure. The data
recorded following treatment in hydrogen and nitrogen
at 520 and 1150 1C (Fig. 11b and c, Table 3) showed the
strontium atom to be coordinated to six oxygen atoms
at a distance of ca. 2.55–2.57 A characteristic of the rock
salt-related strontium oxide structure.
[1] Y. Nishihata, J. Mizuki, T. Akao, H. Tanaka, M. Uenishi,
M. Kimura, T. Okamoto, H. Hamada, Nature 418 (2002) 164.
[2] T. Nakamoro, M. Misona, Y. Yoneda, J. Catal. 83 (1983) 151.
[3] J. Ohno, S. Nagata, H. Sato, Solid State Ionics 3–4 (1981) 439.
[4] G. Briceno, X.-D. Xiang, H. Chang, X. Sun, P.G. Schulz, Science
270 (1995) 273.
[5] F.J. Berry, X. Ren, J. Gancedo, J.F. Marco, J. Solid State Chem.
17 (2004) 2101.
[6] J.L. Soubeyroux, P. Courbin, L. Fournes, D. Fruchart, G. Le
Flem, J. Solid State Chem. 31 (1980) 313–320.
4. Conclusion
[7] L. Fournes, A. Wattiaux, A. Demourgues, P. Bezdicka, J.C.
Grenier, M. Pouchard, J. Etourneau, J. Phys. IV, Colloque C1 7
(1997) C1-353.
The results show that treatment of the material of
composition La0.5Sr0.5FeO3 (which contains 50% Fe4+
and 50% Fe3+) at 600 1C in a flowing 90% hydrogen/
10% nitrogen atmosphere results in the formation of an
oxygen-deficient perovskite-related phase containing
only trivalent iron. Further heating at 1150 1C results
in the formation of the Fe3+-containing phase SrLa
FeO4, which has a K2NiF4-type structure, and metallic
iron. The material of composition La0.5Sr0.5CoO3 is
more susceptible to reduction than the compound
La0.5Sr0.5FeO3 since after heating at 520 1C in the
reducing hydrogen/nitrogen atmosphere all the Co4+
and Co3+ present in the original material is reduced to
metallic cobalt with the concomitant formation of
strontium- and lanthanum-oxides. The results show
that in perovskite-related phases of the type La0.5Sr0.5
MO3 (M=Fe, Co) the cobalt-containing variant is more
amenable to reduction in reducing gaseous environ-
ments that its iron-containing counterpart.
[8] T.C. Gibb, J. Chem. Soc. Dalton Trans. (1985) 1455.
[9] J.B. Yang, W.B. Yelon, W.J. James, Z. Chu, M. Kornecki, Y.X.
Xie, X.D. Zhou, H.U. Anderson, A.G. Joshi, S.K. Malik, Phys.
Rev. B 66 (2002) 184415.
[10] G.M. Veith, I.D. Fawcett, M. Greenblatt, M. Croft, I. Nowik,
Int. J. Inorg. Mater. 2 (2000) 513.
[11] G.A. Waychunas, M.J. Apted, G.E. Brown, J. Phys. Chem.
Miner. 10 (1983) 1.
[12] H. Falcon, A.E. Goeta, G. Punte, R.E. Carbonio, J. Solid State
Chem. 133 (1997) 379.
[13] M. Holzapfel, O. Proux, P. Strobel, C. Darie, M. Borowski,
M. Morcrette, J. Mater. Chem. 14 (2004) 102.
[14] A.C. Masset, O. Toulemonde, D. Pelloquin, E. Suard,
A. Maignan, F. Studer, M. Hervieu, C. Michel, Int. J. Inorg.
Mater. 2 (2000) 687.
[15] J.E. Sunstrom IV, K.V. Ramanujachary, M. Greenblatt, J. Solid
State Chem. 139 (1998) 388.
[16] P. Porta, S. de Rossi, M. Faticanti, G. Minelli, I. Pettiti, L. Lisi,
M. Turco, J. Solid State Chem. 146 (1999) 291.
[17] F. Ali, A.V. Chadwick, M.E. Smith, J. Mater. Chem. 7 (1997)
285.