Crystal structure and magnetic properties of high-oxygen pressure annealed Sr1-xLaxCo0.5Fe0.5O3-δ (0≤x≤0.5)

Article abstract of DOI:10.1016/j.jssc.2008.10.030

Structural and magnetic studies are presented for the perovskite type Sr_1-xLa_xCo_0.5Fe_0.5O_3-δ (0≤x≤0.5) materials annealed under moderately high-oxygen pressures of ～200 atm. A detailed analysis of the

Full text of DOI:10.1016/j.jssc.2008.10.030

ARTICLE IN PRESS
Journal of Solid State Chemistry 182 (2009) 280–288
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Crystal structure and magnetic properties of high-oxygen pressure annealed
Sr1ꢀxLaxCo0.5Fe0.5O3ꢀ (0pxp0.5)
d
Konrad Swierczek a,b,1, Bogdan Dabrowski a,b, , Leopoldo Suescun b,c, Stanislaw Kolesnik a
Ã
´
a Physics Department, Northern Illinois University, DeKalb, IL 60115, USA
b Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA
c Cryssmat-Lab/Detema, Facultad de Qu´ımica, Universidad de la Repu´blica, P.O. Box 1157, Montevideo, Uruguay
a r t i c l e i n f o
a b s t r a c t
Article history:
Structural and magnetic studies are presented for the perovskite type Sr1ꢀxLaxCo0.5Fe0.5O3ꢀ (0pxp0.5)
d
Received 28 July 2008
Received in revised form
10 October 2008
materials annealed under moderately high-oxygen pressures of ꢁ200 atm. A detailed analysis of the
room temperature neutron time-of-flight diffraction data reveals that the crystal structure of the
sample SrCo0.5Fe0.5O2.89(1), previously described as vacancy-disordered cubic, is similar to the formerly
reported, oxygen-vacancy ordered Sr8Fe8O23 compound, i.e. Sr8Co4Fe4O23 is tetragonal with the I4/mmm
symmetry. With an increase of the La content the studied materials become nearly oxygen
Accepted 18 October 2008
Available online 7 November 2008
Keywords:
¯
stoichiometric and a lowering of the crystal symmetry is observed from cubic Pm3m (x ¼ 0.1 and 0.2)
Sr1ꢀxLaxCo0.5Fe0.5O3
Phase transition
Neutron diffraction
Magnetic structure
Ferromagnetism
Sr8Co4Fe4O23
to tetragonal I4/mcm (x ¼ 0.3 and 0.4), and finally to monoclinic I12/c1 (x ¼ 0.5). Low-temperature
structural and magnetic measurements show a ferromagnetic ordering with the maximum Curie
temperature near 290 K at x ¼ 0.2.
& 2008 Published by Elsevier Inc.
1. Introduction
the fully oxygenated SrFeO3 requires anneal under 200 atm or
higher oxygen pressure [15,19], whereas similar annealing
produces oxygen deficient SrCoO2.88 [20]. The level of the oxygen
nonstoichiometry at high temperatures in the Sr1ꢀxLaxCo1ꢀy
Lanthanum strontium cobalt iron oxides with perovskite type
structure have been extensively studied due to high oxygen
permeability [1–7] and high mixed ionic–electronic conductivity
at elevated temperatures [5,8,9], that make them attractive for
applications as gas separation membranes and as the cathode
materials in solid oxide fuel cells [6,7,9–11]. From the structural
point of view two major phases have been obtained in the
FeyO3ꢀ system is determined by both, the Sr/La and Co/Fe ratios
d
and can exceed
cobalt content [5,21–24] as explained by the change of the
average oxidation state of cobalt and/or iron 4–x–2 with the
d ¼ 0.3. It increases with both strontium and
d
La3+/Sr2+ ratio and the difficulty of obtaining Co4+. This trend was
observed for the mixed compositions, SrCo1ꢀxFexO3 prepared by
high-temperature reaction [16,25] as well as by a standard
ceramic method using high oxygen pressure for oxidation. Even
after treating these materials under high pressure of 6 GPa in the
presence of oxidizer KClO4, certain oxygen deficiency was
observed [26]. It was reported, however, that electrochemical
oxidation allowed to obtain fully stoichiometric samples of
Sr1ꢀxLaxCo1ꢀyFeyO3ꢀ (SLCF) system, an oxygen vacancy ordered
d
brownmillerite type phase [12] and the cubic or pseudo-cubic
perovskite phase. The brownmillerite type phase is present for
materials with the oxygen vacancy concentration
d close to 0.5,
and is stable in air at temperatures below 800 1C [13–16]. For
samples with higher oxygen content the cubic related phases are
observed in which oxygen vacancies are distributed statistically
on the oxygen sublattice [15,17,18]. Also other vacancy ordered
phases (e.g. Sr8Fe8O23) besides the vacancy ordered brownmiller-
ite have been observed for the Fe and Sr rich end of the SLCF
series, as was reported by Hodges et al. [15]. While both oxygen
stoichiometric LaFeO3 and LaCoO3 can easily be obtained in air,
SrCoO3 [27,28] and SrCo0.5Fe0.5O3 [29].
A recent report on
stoichiometric SrCoO3 obtained by a high-pressure method [30]
remains to be confirmed, as the explanation for the observed
excess oxygen content (43) seems unsatisfactory. It is also worth
noting that, for distorted octahedral environment and high
iron content, the disproportionation reaction of Fe4+ to Fe3+ and
Fe5+ may take place, resulting in formation of valence state 5+ of
iron [31].
Previous structural reports have found a cubic perovskite
structure for the oxygen deficient SrCo0.8Fe0.2O2.78 [16] and for
SrCo0.5Fe0.5O2.88 compounds [32]. However, a number of small,
Ã
Corresponding author at: Physics Department, Northern Illinois University,
DeKalb, IL 60115, USA.
E-mail address: dabrowski@anl.gov (B. Dabrowski).
1
Permanent address: Faculty of Materials Science and Ceramics, AGH
University of Science and Technology, al. Mickiewicza 30, 30-059 Krakow, Poland.
0022-4596/$ - see front matter & 2008 Published by Elsevier Inc.
doi:10.1016/j.jssc.2008.10.030

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K. Swierczek et al. / Journal of Solid State Chemistry 182 (2009) 280–288
281
unindexed peaks visible in the published neutron powder
diffraction (NPD) patterns strongly suggest a deviation from the
simple cubic perovskite structure. Also in the case of high-
temperature NPD measurements performed for the SrCo0.8
content in the samples was determined by the weight difference
between the initial sample and total reduction products La2O3,
SrO, and metallic Co and Fe (confirmed by X-ray powder
diffraction) obtained by slow reduction in hydrogen atmosphere
(11/min) up to the point of no weight change. Fig. 1 shows
¯
Fe0.2O3ꢀ compound, which were fitted using the cubic Pm3m
d
symmetry in the 873–1173 K range at pO2 ¼ 1 atm, some small,
exemplary TGA measurements for the SrCo0.5Fe0.5O3ꢀ sample
d
unindexed peaks are present [24].
obtained from a high-pressure oxygen annealing under 200 atm at
430 1C. In the first step, the sample was heated and cooled in 0.5%
O2 in Ar, followed by a reduction in 50% H2 in Ar. The extended
equilibrium ranges of oxygen contents vs. temperature near 2.56
in 0.5% O2/Ar and 2.50 in 50% H2/Ar signal the presence of an
oxygen vacancy ordered brownmillerite phase. Stability of such
The Sr1ꢀxLaxCo1ꢀyFeyO3ꢀ system presents a vast variety of
d
magnetic and transport properties that are strongly correlated
with the chemical composition and oxygen content [33–37].
SrFeO3 is metallic down to 4 K, it is antiferromagnetic with
TNE130 K and shows a screw spin structure with /111S propaga-
tion vector [38,39]. It is well documented that Fe4+ ions present in
this structure are in the high-spin (HS) state [40–43]. There is a
strong hybridization present and the conduction band consists of
Fe 3d and O 2p orbitals with strong negative charge transfer
phases for various x in Sr1ꢀxLaxCo0.5Fe0.5O3ꢀ their structures
d
determined from NPD studies and properties will be described in
future publications. The TGA determined oxygen contents of the
high-oxygen-pressure annealed Sr1ꢀxLaxCo0.5Fe0.5O3ꢀ samples
d
between iron and oxygen [42,44]. Nearly stoichiometric SrCoO3ꢀ
were found at 2.89(1) and 2.98(1) for x ¼ 0 and 0.1, respectively.
The remaining samples x ¼ 0.2ꢀ0.5 were found fully oxygenated
to 3.00. Some of them, x ¼ 0.4 and 0.5, achieved that oxygen
content after slow cool in air.
Room temperature neutron time-of-flight powder diffraction
data were collected for all Sr1ꢀxLaxCo0.5Fe0.5O3 samples at the
Special Environment Powder Diffractometer (SEPD) at Argonne’s
Intense Pulsed Neutron Source (IPNS) [53]. The high-resolution
d
is also metallic and possesses ferromagnetic properties
below TC ¼ 222 K [45,46] or, in case of the electrochemically
oxidized sample, TC ¼ 280 K [28]. Co4+ cations are reported
to be either in the low [28,47] or intermediate-spin (IS) state
[48,49]. Negative charge transfer was also observed in this
material [50]. Ferromagnetic properties were observed for mixed
oxides SrCo1ꢀyFeyO3ꢀ in the 0.1pyp0.6 range with TC ¼ 340 K
d
for nearly stoichiometric SrCo0.6Fe0.4O3ꢀ [26]. An observation
backscattering data bank (2y ¼ 1441, Bank 1) was used for
d
of colossal magnetoresistance at low temperatures was also
reported [51,52].
structural refinement. Low-temperature data were also collected
to perform magnetic structure determination for the Sr0.9La0.1
Co0.5Fe0.5O2.98 and Sr0.6La0.4Co0.5Fe0.5O3 samples. Structural
refinements were performed by the Rietveld method with the
GSAS/EXPGUI suite of programs [54,55]. The background was
modeled using a shifted Chebyschev polynomial and the profile
function was modeled using the modified Thomson–Cox–Hasting
pseudo-Voigt function (TCHZ) with anisotropic broadening, con-
voluted with two back-to-back exponentials to model the intrinsic
TOF profile shape. Absorption and extinction correction para-
meters were also refined. Symmetry-allowed atomic coordinates
of all atoms in the unit cell were freely refined together with
isotropic thermal displacement parameters for Sr/La and Fe/Co
cations. In most of the cases, we were able to refine anisotropic
thermal parameters of oxygen ions and the total oxygen content
in the sample.
Results of the magnetic and transport properties measure-
ments for Sr0.5La0.5Co1ꢀyFeyO3 (0.0pyp0.6) indicate that the
partial replacement of Co by Fe ions favors insulating and glassy
magnetic behavior [37]. For the Sr0.3La0.7Co1ꢀyFeyO3 (0.0pyp0.6)
series a change from ferromagnetic behavior for cobalt rich
samples to antiferromagnetic G–type order for Sr0.3La0.7FeO3 was
observed. Interestingly, for the Sr0.3La0.7Co0.5Fe0.5O3 composition
the coexistence of ferromagnetic and antiferromagnetic domains
was shown [35]. LaCo0.5Fe0.5O3 was reported to have antiferro-
magnetic G–type structure below 300 K. A structural transition
¯
from R3c to low-temperature Pbnm symmetry in the 200–300 K
temperature range was also reported [36].
In this work we describe the structural and magnetic proper-
ties of the nearly stoichiometric system Sr1ꢀxLaxCo0.5Fe0.5O3
(0pxp0.5) with cubic or pseudo-cubic structures prepared by a
solid state reaction technique and oxygenated under moderately
high oxygen pressures.
2. Synthesis and experimental techniques
Polycrystalline Sr1ꢀxLaxCo0.5Fe0.5O3ꢀ samples in the 0.0px
d
p0.5 range were synthesized by the solid state reaction
of appropriate amounts of Fe2O3, Co3O4, La2O3 and SrCO3 (all
499.99% purity). Reactants were thoroughly mixed in an agate
mortar and fired in air several times in the 900–1150 1C range with
intermediate grindings. The reaction was monitored with X-ray
diffraction measurements, performed after each step of the
synthesis procedure using a Rigaku D/MAX powder diffractometer
in the 20–701 range with CuK
a radiation. The final step of the
synthesis procedure was an oxygenation performed under
180–200 bar of oxygen pressure at 410–500 1C followed by a very
slow (0.11/min) cooling to room temperature.
Thermogravimetric analysis (TGA) was performed on a Cahn
TG171 thermobalance in several flowing gas mixtures up to
1100 1C to determine the oxygen stoichiometry as a function of
oxygen pressure and temperature, and to determine the exact
oxygen content of the final samples. The weight of the approxi-
Fig. 1. Sequence of thermogravimetric measurements in 0.5% O2/Ar (heating and
cooling) and 50% H2/Ar (heating) for the SrCo0.5Fe0.5O3ꢀ sample obtained from
d
mately 1 g samples was measured with a 5
mg precision. Oxygen
200 atm O2 at 430 1C.

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282
The dc magnetization was measured using a magnetic property
Cobalt and iron cations occupy three different crystallographic
sites in Sr8Co4Fe4O23, as shown in Table 1. We checked for a
possibility of preferential occupation of these sites by taking
advantage of the very different neutron scattering powers of Fe
measurement system model MPMS-7 (Quantum Design). The
temperature dependence of magnetization was measured in the
temperature range 5–395 K on cooling in a low magnetic field
of 20 Oe. From that, the Curie temperature and the properties of
the paramagnetic state were determined. The isothermal magne-
tization hysteresis loops were measured at 5 K in magnetic fields
up to 70 kOe. The electrical resistivity was measured using a
physical property measurement system model 6000 (Quantum
Design). The temperature dependence of resistivity was measured
using a standard four-point method in the temperature range
5–395 K in a zero magnetic field and in a magnetic field of 70 kOe.
3. Results and discussion
3.1. Crystal structure of the vacancy ordered SrCo0.5Fe0.5O3ꢀ sample
d
An initial fit of the NPD data for the SrCo0.5Fe0.5O3ꢀ sample
d
¯
was performed using cubic Pm3m symmetry. We were able to fit
correctly high-intensity peaks; however number of low-
a
intensity peaks remained unfitted. Following the discussion
presented by Hodges et al. [15] we fitted the SrCo0.5Fe0.5O3ꢀ
d
data using a tetragonal I4/mmm model for the oxygen-vacancy-
ordered Sr8Fe8O23 compound (Fig. 2a). Apart from the improve-
ment of the residual values of the refinement (see: footnote in
Table 1), all of the previously unindexed peaks become fitted
correctly (Fig. 2a). The observed vacancy ordering (Fig. 2b) has an
important consequence for the interpretation of the physico-
chemical properties of this material, because lower oxygen
mobility can be expected as a result of such ordering; for example,
it would hinder an application of such material in intermediate
temperature solid oxide fuel cells (IT-SOFC), in which fast
oxygen transport in the cathode material plays an important
role in gaining high efficiency of a working cell. We believe that
the crystal structures of the samples with similar chemical
compositions showing the same low-intensity peaks and refined
as cubic in [16] and [32] are indeed tetragonal I4/mmm
manifesting oxygen-vacancy-ordering of the Sr8Fe8O23 type that
appears to be quite common for the SrCo1ꢀyFeyO3ꢀ compounds.
It is important to note that the X-ray diffraction data of
d
Fig. 2. (a) Room temperature NPD data and the Rietveld refinement profile using
I4/mmm space group for the Sr8Co4Fe4O23 sample. Top tick-marks denote allowed
reflections in I4/mmm. Bottom tick-marks are included to emphasize the
possibility of indexing the pattern using reflections in Pm–3m symmetry. The
SrCo0.5Fe0.5O3ꢀ show no deviation from cubic symmetry, as
d
expected from the structural distortion arising from ordering of
ꢁ20% of oxygen vacancies, making the results of neutron
diffraction of a critical importance to determine the correct
structure.
˚
inset shows a detail of the superstructure peaks in the 1–2 A d-spacing range to
emphasize that cubic model fails to describe the low-intensity peaks, which are
correctly fitted by the I4/mmm model. (b) Polyhedral network with ordered oxygen
vacancies in I4/mmm Sr8Co4Fe4O23 sample.
Table 1
Structural parameters for Sr8Co4Fe4O23 (RT).
3
˚
˚
˚
˚
Symmetry I4/mmm; a ¼ 10.88488(7) A, b ¼ 10.88488(7) A, c ¼ 7.68380(8) A, V ¼ 910.38(2) A
2
˚
Atom
Site
x
y
z
Uiso ꢂ 100 (A )
Sr(1)
8i
0.2577(2)
0.2491(4)
0
0
0
0.61(2)
0.88(2)
0.44(4)
0.62(2)
0.13(3)
1.67a
Sr(2)
8j
0
1/2
Co/Fe(1)
Co/Fe(2)
Co/Fe(3)
O(1)
4e
0
0.2510(11)
8f
1/4
1/4
1/4
4d
2b
16m
8h
16k
4c
1/2
0
1/4
0
0
1/2
O(2)
0.1229(3)
0.2460(3)
0.1264(2)
1/2
0.1229(3)
0.2460(3)
0.6264(2)
0
0.2355(3)
1/2
1.51a
O(3)
0.86a
O(4)
1/4
1.17a
O(5)
0
1.71a
w2 ¼ 1.439, Rwp ¼ 7.15%, and Rp ¼ 4.65%.
For comparison: w2 ¼ 2.126, Rwp ¼ 8.72%, and Rp ¼ 6.01% when refined in cubic symmetry.
a
Ueqv.

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283
and Co (9.45 and 2.49 fm, respectively). However, no improve-
ment of the refinement was achieved, suggesting that random
distribution of the Co and Fe ions is present. We also checked
for a possibility of additional nonstoichiometry in the oxygen
sublattice, however also in this case no improvement of the
refinement was obtained suggesting that the oxygen-vacancy
ordering present in this material is complete. Selected interatomic
distances and angles, shown in Table 2, are similar to the data
published for Sr8Fe8O23 [15]. The square–pyramidal coordinated
Co/Fe(1) site exhibits slightly smaller average bond length of
oxygen contents from NPD data were found consistent with the
TGA measurements as is shown in Tables 3–8.
Sr0.9La0.1Co0.5Fe0.5O2.98 sample is cubic at room temperature
and remains cubic down to 10 K (Table 3). The average Co/Fe–O
Table 3
Structural parameters for Sr0.9La0.1Co0.5Fe0.5O2.98(1) as a function of temperature
refined in Pm–3m space group.
10 K
100 K
200 K
RT
˚
1.899 A than the octahedral Co/Fe(2) and Co/Fe(3) sites with
˚
˚
a (A)
3.8262(1)
56.01(1)
0.33(1)
0.25(1)
0.64
3.8272(1)
56.06(1)
0.40(1)
0.28(1)
0.69
3.8313(1)
56.24(1)
0.57(2)
0.37(2)
0.87
3.8397(2)
56.61(1)
0.77(2)
0.49(2)
1.11
average bonds of 1.928 and 1.937 A, respectively. This small
3
˚
V (A )
difference, expected as a function of coordination, as well as a
small variance of the individual bond lengths does not support
charge or orbital ordering on a long-range scale. The oxygen
content of 2.875 (equal to 23/8) per 1 mol of the sample obtained
for an ideal vacancy ordered structure is consistent with the TGA
analysis which provided the oxygen content of 2.89(1).
2
˚
Sr/La (1a) Uiso ꢂ 100 (A )
2
˚
Co/Fe (1b) Uiso ꢂ 100 (A )
2
˚
O (3c) Ueqv ꢂ 100 (A )
˚
Sr/La–O (A) ꢂ 12
2.706(1)
1.913(1)
2.706(1)
1.914(1)
2.709(1)
1.916(1)
2.715(1)
1.920(1)
˚
Co/Fe–O (A) ꢂ 6
m
(
m
B)a
2.58(7)
2.42(7)
1.92(8)
–
3.2. Crystal structures of Sr1ꢀxLaxCo0.5Fe0.5O3 (0.1pxp0.5) samples
w2
Rwp (%)
1.388
7.77
1.376
7.69
1.321
7.54
1.445
5.63
3.83
Contrary to the x ¼ 0 sample, in case of the lanthanum
substituted materials, Sr1ꢀxLaxCo0.5Fe0.5O3 (0.1pxp0.5), we have
obtained nearly oxygen stoichiometric samples. The refined
Rp (%)
5.67
5.59
5.54
a
Refined with P1 magnetic symmetry for Co/Fe atoms with unit cell
constrained to the nuclear phase 1a (1/2,1/2,1/2); 1b (0,0,0); and 3c (1/2,0,0).
Table 2
˚
Interatomic distances (A) and bond angles (1) for the crystal structure of
Sr8Co4Fe4O23 (RT).
Table 4
Sr(1)–O(2)
Sr(1)–O(3)
Sr(1)–O(4)
Sr(1)–O(5)
Sr(2)–O(1)
Sr(2)–O(2)
Sr(2)–O(3)
Sr(2)–O(4)
Sr(2)–O(5)
2.687(2)
2.766(3)
2.678(1)
2.637(3)
2.712(4)
2.794(3)
2.678(3)
2.724(2)
2.731(4)
ꢂ 4
ꢂ 2
ꢂ 4
Structural parameters for Sr0.8La0.2Co0.5Fe0.5O2.99(1) refined in Pm-3m space
group (RT).
˚
A (A)
3.8410(2)
56.67(1)
0.76(2)
0.44(2)
1.09
3
˚
V (A )
ꢂ 4
ꢂ 2
ꢂ 4
2
˚
Sr/La (1a) Uiso ꢂ 100 (A )
2
˚
Co/Fe (1b) Uiso ꢂ 100 (A )
2
˚
O (3c) Ueqv ꢂ 100 (A )
˚
Sr/La–O (A) ꢂ 12
2.716(1)
1.920(1)
Square–pyramidal Co/Fe(1)
Co/Fe(1)–O(1)
˚
Co/Fe–O (A) ꢂ 6
1.913(9)
1.896(4)
93.6(3)
Co/Fe(1)–O(2)
ꢂ 4
ꢂ 4
ꢂ 4
ꢂ 2
ꢂ 4
ꢂ 4
w2 ¼ 1.371, Rwp ¼ 7.02%, and Rp ¼ 4.67%.
O(1)–Co/Fe(1)–O(2)
O(2)–Co/Fe(1)–O(2)
O(2)–Co/Fe(1)–O(2)
O(1)–O(2)
89.8(3)
172.8(5)
2.777(3)
2.676(4)
Table 5
O(2)–O(2)
Structural parameters for Sr0.7La0.3Co0.5Fe0.5O3.00(1) refined in I4/mcm space
group (RT).
Octahedral Co/Fe(2) site ꢂ 2
Co/Fe(2)–O(2)
˚
a (A)
5.4342(1)
7.6933(3)
227.19(1)
0.80(1)
0.38(1)
0.2558(3)
1.24
1.959(4)
1.922(1)
1.903(4)
91.4(2)
88.6(2)
90
ꢂ 2
ꢂ 2
ꢂ 2
ꢂ 2
ꢂ 2
ꢂ 4
ꢂ 4
˚
c (A)
Co/Fe(2)–O(3)
3
˚
V (A )
Co/Fe(2)–O(4)
2
˚
Sr/La (4b) Uiso ꢂ 100 (A )
O(2)–Co/Fe(2)–O(3)
O(2)–Co/Fe(2)–O(3)
O(2)–Co/Fe(2)–O(4)
O(3)–Co/Fe(2)–O(4)
O(2)–Co/Fe(2)–O(2)
O(3)–Co/Fe(2)–O(3)
O(4)–Co/Fe(2)–O(4)
O(2)–O(3)
2
˚
Co/Fe (4c) Uiso ꢂ 100 (A )
O(1) (8h) x
2
˚
O(1) Ueqv ꢂ 100 (A )
90
2
˚
O(2) (4a) Ueqv ꢂ 100 (A )
0.90
180
180
180
˚
Sr/La–O(1) (A) ꢂ 4
2.750(2)
2.687(2)
2.717(1)
1.922(1)
1.923(1)
177.3(2)
180
2.778(3)
2.711(4)
2.731(4)
2.705(3)
ꢂ 2
ꢂ 2
ꢂ 4
ꢂ 4
˚
Sr/La–O(1) (A) ꢂ 4
O(2)–O(3)
˚
Sr/La–O(2) (A) ꢂ 4
O(2)–O(4)
˚
Co/Fe–O(1) (A) ꢂ 4
O(3)–O(4)
˚
Co/Fe–O(2) (A) ꢂ 2
Co/Fe–O(1)–Co/Fe (1) ꢂ 2
Co/Fe–O(2)–Co/Fe (1)
Octahedral Co/Fe(3) site
Co/Fe(3)–O(4)
˚
/Sr/La–OSg (A)
/Co/Fe–OSg (A)
/Co/Fe–O–Co/FeSg (1)
2.718
1.945(4)
1.921(1)
90
ꢂ 4
ꢂ 2
ꢂ 4
ꢂ 8
ꢂ 4
ꢂ 8
˚
1.922
Co/Fe(3)–O(5)
178.2
O(4)–Co/Fe(3)–O(4)
O(4)–Co/Fe(3)–O(5)
O(4)–O(4)
tg
1.000
90
2.751(4)
2.734(2)
w2 ¼ 1.381, Rwp ¼ 5.69%, and Rp ¼ 3.80%.
O(4)–O(5)
4b (0,1/2,1.4), 4c (0,0,0), 8h (x,x+1/2,0),4a (0,0,1/4).

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K. Swierczek et al. / Journal of Solid State Chemistry 182 (2009) 280–288
Table 6
Table 8
Structural parameters for Sr0.6La0.4Co0.5Fe0.5O3.00(1) refined in I4/mcm space group
Structural parameters for Sr0.5La0.5Co0.5Fe0.5O3.02(1) refined in I12/c1 space
(RT) (oxygen content taken from TG data).
group (RT).
˚
˚
a (A)
5.4378(1)
7.6983(3)
227.64(1)
0.89(2)
0.35(2)
0.2583(3)
1.39
a (A)
5.4427(1)
5.4523(1)
7.7119(2)
90.256(1)
228.85(1)
0.5004(10)
0.80(1)
˚
˚
c (A)
b (A)
3
˚
˚
V (A )
c (A)
(1)
2
˚
Sr/La (4b) Uiso ꢂ 100 (A )
b
2
3
˚
˚
Co/Fe (4c) Uiso ꢂ 100 (A )
V (A )
O(1) (8h) x
Sr/La (4e) y
2
2
˚
˚
O(1) Ueqv ꢂ 100 (A )
Sr/La Uiso ꢂ 100 (A )
2
2
˚
˚
O(2) (4a) Ueqv ꢂ 100 (A )
1.08
Co/Fe (4a) Uiso ꢂ 100 (A )
O(1) (8f) x
0.49(1)
0.2659(5)
0.2382(7)
ꢀ0.0129(3)
0.0218(7)
1.29
O(1) y
˚
Sr/La–O(1) (A) ꢂ 4
2.766(2)
2.675(2)
2.719(1)
1.924(1)
1.925(1)
176.2(2)
180
O(1) z
˚
Sr/La–O(1) (A) ꢂ 4
O(2) (4e) y
˚
Sr/La–O(2) (A) ꢂ 4
2
˚
O(1) Ueqv ꢂ 100 (A )
˚
Co/Fe–O(1) (A) ꢂ 4
2
˚
O(2) Ueqv ꢂ 100 (A )
1.19
˚
Co/Fe–O(2) (A) ꢂ 2
Co/Fe–O(1)–Co/Fe (1) ꢂ 2
˚
Co/Fe–O(2)–Co/Fe (1)
Sr/La–O(1) (A) ꢂ 2
2.877(3)
2.729(5)
2.582(3)
2.721(4)
2.610(5)
2.843(5)
2.724(1)
1.947(4)
1.916(4)
1.932(1)
171.3(1)
173.0(2)
2.725
˚
˚
/Sr/La–OSg (A)
/Co/Fe–OSg (A)
/Co/Fe–O–Co/FeSg (1)
2.720
Sr/La–O(1) (A) ꢂ 2
˚
˚
1.924
Sr/La–O(1) (A) ꢂ 2
˚
177.5
Sr/La–O(1) (A) ꢂ 2
˚
tg
0.9997
Sr/La–O(2) (A)
˚
Sr/La–O(2) (A)
w2 ¼ 1.437, Rwp ¼ 6.73%, Rp ¼ 4.40%.
˚
Sr/La–O(2) (A) ꢂ 2
˚
Co/Fe–O(1) (A) ꢂ 2
˚
Co/Fe–O(1) (A) ꢂ 2
˚
Co/Fe–O(2) (A) ꢂ 2
Table 7
Co/Fe–O(1)–Co/Fe (1) ꢂ 2
Co/Fe–O(2)–Co/Fe (1)
/Sr/La–OSg
Structural parameters for Sr0.6La0.4Co0.5Fe0.5O3.00(1) as a function of temperature
refined in Ibam space group.
/Co/Fe–OSg
/Co/Fe–O–Co/FeSg (1)
tg
1.932
10 K
100 K
200 K
171.9
0.9973
˚
a (A)
5.4195(3)
5.4318(3)
7.6630(4)
225.58(3)
0.39(2)
5.4220(2)
5.4325(3)
7.6654(3)
225.79(1)
0.47(2)
5.4290(2)
5.4377(2)
7.6737(3)
226.54(1)
0.63(2)
˚
b (A)
w2 ¼ 1.636, Rwp ¼ 4.41%, Rp ¼ 2.95%.
4e (1/4,y,0), 4a (0,0,0), 8f (x,y,z).
˚
c (A)
3
˚
V (A )
2
˚
Sr/La (4b) Uiso ꢂ 100 (A )
2
˚
Co/Fe (4c) Uiso ꢂ 100 (A )
0.12(2)
0.15(2)
0.17(2)
O(1) (8j) x
O(1) y
0.2595(9)
0.2355(7)
0.69(4)
0.2593(9)
0.2361(7)
0.76(4)
0.2589(9)
0.2379(7)
0.76(3)
would expect an observation of the signs of symmetry lowering in
the diffraction data. However, within the resolution of our NPD
data we could not observe this effect. Therefore, to refine
magnetic moment we included a second phase for the NPD data
fitting, with the unit cell and Co/Fe positions constrained to the
nuclear cubic cell, and modeling only the magnetic contribution
to the data. The refined magnetic moment at 10 K was found to be
equal to 2.58(7) mB and agreed well with the magnetization
data presented in Section 3. Because we could not observe the
symmetry breaking, we assumed an arbitrary direction of the
magnetic moment. The observed increase of the magnetic
moment and decreases of the lattice parameter, bond lengths
and thermal parameters with decreasing temperature are typical
for the ferromagnetic oxide system (Table 3).
2
˚
O(1) Uiso ꢂ 100 (A )
2
˚
O(2) (4a) Uiso ꢂ 100 (A )
0.67(8)
0.62(7)
1.10(7)
˚
Sr/La–O(1) (A) ꢂ 4
2.777(2)
2.647(2)
2.716(1)
2.710(1)
1.901(5)
1.940(6)
1.916(1)
174.5(1)
180
2.776(2)
2.650(2)
2.716(1)
2.711(1)
1.903(6)
1.939(6)
1.916(1)
174.7(1)
180
2.773(2)
2.658(2)
2.719(1)
2.715(1)
1.910(6)
1.935(6)
1.918(1)
175.2(1)
180
˚
Sr/La–O(1) (A) ꢂ 4
˚
Sr/La–O(2) (A) ꢂ 2
˚
Sr/La–O(2) (A) ꢂ 2
˚
Co/Fe–O(1) (A) ꢂ 2
˚
Co/Fe–O(1) (A) ꢂ 2
˚
Co/Fe–O(2) (A) ꢂ 2
Co/Fe–O(1)–Co/Fe (1) ꢂ 2
Co/Fe–O(2)–Co/Fe (1)
˚
/Sr/La–OSg (A)
/Co/Fe–OSg (A)
/Co/Fe–O–Co/FeSg (1)
2.712
2.713
2.716
˚
1.919
1.919
1.921
176.3
176.4
176.8
Similarly to the above composition, the Sr0.8La0.2Co0.5Fe0.5O3
sample is cubic at RT and has minimally larger unit cell (Table 4)
as expected from the increase of the La3+ content. However, within
an error bar we did not observe the corresponding elongation of
the Co/Fe–O bond length that was expected for a slightly reduced
tg
0.9993
0.9997
0.9997
mz
w2
(
mB)
2.66(7)
2.32(7)
–
1.451
7.36
1.361
7.04
5.18
1.386
7.07
5.19
Rwp (%)
average oxidation state of 4–x–2d ¼ 3.8 as compared with 3.86 for
Rp (%)
5.43
the Sr0.9La0.1Co0.5Fe0.5O2.98 sample. The variable charge and spin
state of Co, which affects its ionic size may be a reason for that.
The room temperature NPD data for Sr0.7La0.3Co0.5Fe0.5O3
material could not be fitted with the cubic structure as additional
peaks were also observed, suggesting the presence of lower
symmetry phase. A relatively good fit was obtained using trigonal
4b (1/2,0,1/4), 4c (0,0,0), 8j (x,y,0), 4a (0,0,1/4).
˚
bond length of 1.920(1) A reflects probably combination of various
ionic sizes of Co and Fe at several valences and spin states. There is
no evidence for any kind of ordering on the Sr/La or Co/Fe
sublattices. In accordance with the magnetic data presented in the
following section, the Sr0.9La0.1Co0.5Fe0.5O2.98 compound is ferro-
magnetic below room temperature. As the onset of the ferromag-
netic ordering should break cubic symmetry of the system one
¯
R3c symmetry for the main phase; however, the best fit was
obtained using the tetragonal I4/mcm space group (see Table 5).
The secondary phase was identified as a perovskite type phase
¯
with R3c symmetry. The weight percentage of this phase was
refined at 4.0(5) %. In I4/mcm symmetry the dodecahedral
coordination sphere of Sr/La ions have three different sets of four

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K. Swierczek et al. / Journal of Solid State Chemistry 182 (2009) 280–288
285
identical Sr/La–O distances. Octahedral coordination of Co/Fe
cations is split into two sets of four and two identical Co/Fe–O
distances. The resulting average Co/Fe–O bond increases slightly
˚
to 1.922(1) A in line with reduced average valence of 3.7. Tolerance
factor t calculated using geometrical averages of bond distances
[56] is within experimental error equal to 1.0000. In I4/mcm
symmetry the Co/Fe–O octahedron is rotated along the c-axis,
with the resulting planar Co/Fe–O–Co/Fe bond angle of 177.31,
exhibiting just slight deviation from a straight bond. Both the
tolerance factor and the average Co/Fe–O–Co/Fe bond angle
(178.21) show that the tetragonal distortion is very small at room
temperature.
Refined structural parameters of the room temperature NPD
data for Sr0.6La0.4Co0.5Fe0.5O3 sample performed in tetragonal
symmetry are presented in Table 6. Apart from the main phase
¯
(I4/mcm), the secondary phase with R3c symmetry was also
observed (7.5(5) wt%). Probably due to the higher amount of the
secondary phase showing large degree of peak overlap with the
main one, we were not able to reliably refine the oxygen content
of the former, thus, the total oxygen content was fixed to 3.00 as
suggested by the TGA data (3.00(1)). Further increase of the
tetragonal distortion was observed as exhibited by an increase of
Fig. 3. Room temperature NPD data and the Rietveld refinement profile in I12/c1
space group for Sr0.5La0.5Co0.5Fe0.5O3 sample.
˚
the average Co/Fe–O bond length to 1.924(1) A and a deviation
from a straight planar Co/Fe–O–Co/Fe bond angle to 176.21 in line
with the reduced average valence to 3.6.
distances are present. The major difference with respect to the
tetragonal I4/mcm and orthorhombic Ibam symmetry observed for
the x ¼ 0.3 and 0.4 samples is the appearance of the Co/Fe–O
octahedral tilting along the a and b axes in addition to the c-axis
rotation, resulting in the apical and equatorial Co/Fe–O–Co/Fe
bond angles of 173.01 and 171.31, respectively. Comparing
the structural parameters for this composition with previous
samples one may observe further rapid distortion of the structure
(the lower tolerance factor of 0.9973 and the average bond angle
Co/Fe–O–Co/Fe of 171.91). The oxygen content of 3.02(1) refined
from neutron diffraction data is in agreement with TGA data
within experimental error.
The summary of observed average bond lengths Sr/La–O and
Co/Fe–O, normalized cell parameters, the tolerance factor, and the
normalized cubic volume are presented in Fig. 4 as a function of
the chemical composition x ¼ 0.1ꢀ0.5. The nonlinear dependence
of all the parameters is observed, with a larger distortion from the
ideal cubic structure developing for the higher lanthanum
content. This may be associated with the fact that substitution
of smaller strontium with larger lanthanum cations is compen-
sated by even faster increase of the size of Co/Fe caused by
nonlinear decrease of the oxidation state of Co and Fe as well as a
possible increase of the Co spin state. Furthermore, an increase of
the electrical resistivity (see: next section) that is also observed as
a result of the substitution suggests the localization of electrons,
which may also increase the average Co/Fe–O bond length
through the loss of the electron kinetic energy.
The observed lowering of the symmetry in the Sr1ꢀxLaxCo0.5
Fe0.5O3 series indicates that the size of the B-site ion increases
faster than that of the A-site ion, promoting BO6 octahedral tilting
that disturbs the perfect A-site coordination and decreases
calculated average A–O bond distances. Starting from the cubic
a0a0a0 tilt mode (using Glazer notation [57]) an initial antiphase
rotation along the c-axis is observed to yield a0a0cꢀ system
in the I4/mcm space group of Sr0.7La0.3Co0.5Fe0.5O3. The
orthorhombic Ibam structure found at low temperatures for the
Sr0.6La0.4Co0.5Fe0.5O3 sample shows the same kind of tilting as
the tetragonal I4/mcm structure. The symmetry loss appears from
the differentiation of Co/Fe–O equatorial bond distances that
breaks the four-fold symmetry of the Co/Fe site. An increasing
splitting with decreasing temperature is observed, indicating that
the transition from the tetragonal to orthorhombic phase on
cooling is not related to the change in the tilt mode. The degree of
For the Sr0.6La0.4Co0.5Fe0.5O3 sample low-temperature NPD
data were also collected. Results of refinements are shown in
Table 7. At 200 K tetragonal symmetry was no longer giving good
fit to the data. However, the use of the orthorhombic Ibam space
group resulted in a satisfactory fit. In this symmetry a further
separation of atomic distances is present: distances in dodecahe-
dral coordination of Sr/La ions are split into four groups, while in
case of the (Co/Fe)O6 octahedra three different interatomic
distances are present. While the azimuthal Co/Fe–O–Co/Fe bond
angle remains at 1801, two angles in the equatorial plane are equal
to 175.21. A refinement of similar quality was obtained for the
Pnma space group. However, a large number of anticipated
reflections, which were not present in the data, strongly support
the use of the higher symmetry Ibam space group. The
magnetization data presented in the following chapter revealed
ferromagnetic properties of this material below 185 K. The NPD
data refinement for 100 and 10 K is in a good agreement with the
magnetization data, the refined magnetic moments are 2.32(7)
and 2.66(7), respectively, and the moments are directed along the
crystallographic c-axis. It is worth pointing out that the structural
transition from the high-temperature tetragonal to the low-
temperature orthorhombic structure, which takes place between
300 and 200 K, is independent of the paramagnetic–ferromagnetic
transition. Typical temperature trends of decreasing cell and
thermal parameters (refined isotropically) and an increased
orthorhombic distortion as measured by the decreasing tolerance
factor and the average Co/Fe–O–Co/Fe bond angle were observed
(see Table 7).
In the case of the Sr0.5La0.5Co0.5Fe0.5O3 composition it was not
necessary to use elevated oxygen pressure to obtain oxygen
stoichiometric sample. The TGA measurements for the sample
heated and slowly cooled from 1150 1C in air revealed the perfect
3.00(1) stoichiometry. The room temperature NPD data for this
material could not be satisfactory fitted in either tetragonal or
orthorhombic symmetry. However, by using monoclinic I12/c1
space group we were able to obtain a very good fit (w2 ¼ 1.636,
Rwp ¼ 4.41%, Rp ¼ 2.95%) in comparison to the fit in which
orthorhombic space group was used (w2 ¼ 4.892, Rwp ¼ 7.63%,
Rp ¼ 5.15%). Fig. 3 shows the NPD data and Rietveld refinement
and the structural results are presented in Table 8. In the I12/c1
space group, multiple Sr/La–O and three different Co/Fe–O atomic

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K. Swierczek et al. / Journal of Solid State Chemistry 182 (2009) 280–288
286
tilting in the orthorhombic, low-temperature structures, however,
is still larger than the one observed in the room temperature
tetragonal one. The monoclinic I12/c1 structures show not only
antiphase octahedral tilting along the c-axis but also along the
other two cubic directions with similar magnitudes, defining the
aꢀaꢀcꢀ tilt mode (or aꢀbꢀbꢀ in the axes chosen by Glazer),
but also significant octahedral distortion indicating again the
existence of multiple structural distortions causing the symmetry
reduction. The three I4/mcm, Ibam and I12/c1 space groups do not
share group-subgroup relations, confirming that the symmetry
loss cannot be correlated with the continuous variation of one
structural feature in the system.
4. Magnetic and transport properties of Sr8Co4Fe4O23 and
Sr1ꢀxLaxCo0.5Fe0.5O3 samples
A typical magnetic behavior of Sr1ꢀxLaxCo0.5Fe0.5O3 is pre-
sented in Fig. 5. The measured materials x ¼ 0ꢀ0.3 demonstrate
ferromagnetic transitions at temperatures (TC) close to or below
room temperature. At T ¼ 5 K, magnetic hysteresis can be
observed with coercivity fields 200–2000 Oe, increasing with the
La substitution. The analysis of Msat requires to consider (without
an additional cation disproportionation) four magnetic ions Co4+
,
Co3+, Fe4+, and Fe3+ in different proportions depending on x.
Assuming uniform reduction of Fe and Co on the increase of x, the
respective cation concentrations are equal to 0.5(1–x–2
(Co,Fe)4+ and 0.5(x+2 ) for (Co,Fe)3+ (for a given oxygen deficiency
). Therefore, Msat can be expressed as a function of the spin
of each ion by the formula Msat ¼ (1–x–2 )SCo4++(1–x–2
d) for
d
d
d
d)
S
Fe4++(x+2d)SCo3++(x+2d)SFe3+. If we considered all magnetic ions
in the HS state, then SCo4+(HS:t2g3eg2) ¼ 5/2, SCo3+(HS:t2g4eg2) ¼ 2,
S
Fe4+(HS:t2g3eg1) ¼ 2, SFe3+(HS:t2g3eg2) ¼ 5/2. In this model, if all
the magnetic ions are involved in the ferromagnetic exchange,
Msat is constant and equal to 4.5, independent of x and d. This
model is not consistent with our results. Moreover, the HS state
for Co4+ is less likely than the IS state [SCo4+(IS:t2g4eg1) ¼ 3/2] in
perovskite cobaltites [30] and other spin states for Co4+ and Co3+
should be considered. To simplify the analysis, we take into
account the observation of the G-type antiferromagnetism with a
3+
small spontaneous magnetization ꢁ0.1 mB in LaCo Fe3+ O3 [36].
0.5 0.5
Therefore, we assume that (Co,Fe)3+ ions do not contribute to Msat
and only (Co,Fe)4+ participate in the ferromagnetic superex-
change. Together with the the IS state for Co4+ and the oxygen
deficiency from NPD (
d
¼ 0.125 for Sr8Co4Fe4O23, d ¼ 0.02 for
Fig. 4. Variation of: (a) average Sr/La–O and Co/Fe–O bond distances,
(b) lattice parameters compared to cubic lattice constant, (c) tolerance
factor tg (calculated using geometrical average, Ref. [56]), and (d) normalized
Fig. 5. Temperature dependence of magnetization for Sr0.9La0.1Co0.5Fe0.5O3ꢀ
d
cubic volume of the perovskite unit cell at RT as
a function of chemical
measured on ‘‘field cooling’’ (FC) in a magnetic field of 20 Oe. Inset: magnetic
composition x.
hysteresis loop at T ¼ 5 K.

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K. Swierczek et al. / Journal of Solid State Chemistry 182 (2009) 280–288
287
x ¼ 0.1, and
d
¼ 0.01 for x ¼ 0.2), the latter model gives us the
), which
dependence of Msat ¼ (1–x–2d)(SCo4++SFe4+) ¼ 3.5(1–x–2d
is presented in Fig. 6. This dependence can reproduce the
experimental data pretty well.
The molar magnetic susceptibility, wm ¼ M/H, in the paramag-
netic region above TC, was used to determine the effective
paramagnetic moment
meff, by fitting the Curie–Weiss formula
wm
¼
w0 þ ð1=8Þ ꢃ m2eff =ðT ꢀ
Y
Þ. In this formula w0 is a tempera-
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ture-independent susceptibility, meff ¼ g SðS þ 1Þ is the spin-only
effective paramagnetic moment (g ¼ 2 is the gyromagnetic factor
and S is the effective spin) and
Y is the paramagnetic Curie–Weiss
temperature. The values of
The resultant meff can be expressed by the spin of each
magnetic ion as m2eff ¼ 2[(1–x–2
)SCo4+(SCo4++1)+(1–xꢀ2 )SFe4+
(SFe4++1)+(x+2 )SCo3+(SCo3++1)+(x+2 )SFe3+(SFe3++1)]. The upper
limit for meff can be reached for all magnetic ions in the HS state.
[the line
meff and Y are also presented in Fig. 6.
d
d
d
d
That gives a constant value meff ¼ 5.43 for each x and
d
for Co4+:HS+Co3+:HS in Fig. 6(a)]. The more realistic choice of IS
states for both Co4+ and Co3+ [SCo4+(IS:t2g4eg1) ¼ 3/2, SCo3+
(IS:t2g5eg1) ¼ 1] gives lower meff [the line for Co4+:IS+Co3+:IS in
Fig. 6(a)]. The observed large values of meff may be related to the
presence of ferromagnetic clusters in the vicinity of TC. The
determined values of meff are only approximate because of using a
small fitting range 350–395 K that is not much higher than the
Curie temperatures. At these temperatures, the Curie–Weiss
formula may not be strictly valid. The paramagnetic Curie–Weiss
temperature
and follows closely TC for most compositions. The reason for the
larger differences between TC and for larger x is unknown. All
the parameters presented in Fig. 6 demonstrate a maximum
Y is positive as expected for ferromagnetic materials
Y
Fig. 7. Temperature dependence of electrical resistivity for Sr1ꢀxLaxCo0.5Fe0.5O3.
Solid lines represent the resistivity in a zero magnetic field. Dashed lines denote
the resistivity in a magnetic field of 70 kOe. Inset shows the zero-field resistivity at
T ¼ 300 K.
around x ¼ 0.1–0.2. The observed oxygen deficiency may result in
the lowering of TC for xo0.2.
Temperature dependence of electrical resistivity for Sr1ꢀxLax
Co0.5Fe0.5O3 is presented in Fig. 7, in a zero magnetic field an in a
magnetic field of 70 kOe. The undoped material shows metallic
resistivity with a slight upturn below ꢁ70 K. On La substitution,
the resistivity initially decreases and then increases with a
stronger tendency to become more insulating/semiconductive at
low temperatures. The inset to Fig. 7 demonstrates this tendency
for the resistivity at T ¼ 300 K. For all samples except the
nonmetallic x ¼ 0.5 a small decrease of resistivity below TC is
observed denoting decreased electron spin scattering. For a low-
La substitution, a slight magnetoresistance is observed just below
TC. This behavior is characteristic of those ferromagnetic oxides,
including manganites, ruthenates, and cobaltites, for which
resistivity is metallic above and below TC. The increase of La
substitution results in a moderately increased magnetoresistance
at lower temperatures, which may be a grain boundary effect.
5. Summary
We have obtained an oxygen-vacancy-ordered Sr8Co4Fe4O23
compound and nearly oxygen stoichiometric Sr1ꢀxLaxCo0.5Fe0.5O3
(0.1pxp0.5) samples by means of solid state synthesis and
annealing under moderately high oxygen pressures. Analysis of
the room temperature neutron diffraction data reveal that the
SrCo0.5Fe0.5O2.89 sample exhibits I4/mmm symmetry and is similar
to the formerly reported, oxygen-vacancy-ordered Sr8Fe8O23
compound [15]. Structural analysis of the NPD data clearly shows
that such an analysis must be performed carefully, as the ordering
of the oxygen sublattice may manifest itself in a form of very weak
Fig. 6. Dependence of Msat
dashed line shows the model for the dependence of Msat, described in the text. The
solid lines denote two models for
eff(x) with different spin states for Co4+/3+: high-
spin, HS and intermediate-spin, IS.
, meff (top) and Tc and Y (bottom) with La content. The
m

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K. Swierczek et al. / Journal of Solid State Chemistry 182 (2009) 280–288
288
peaks, not visible in the X-ray diffraction. At room temperature,
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¯
we have identified cubic symmetry Pm3m for samples with
x ¼ 0.1 and 0.2, tetragonal I4/mcm symmetry for samples
with x ¼ 0.3 and 0.4 and monoclinic I12/c1 symmetry for
Sr0.5La0.5Co0.5Fe0.5O3 compound. Rietveld analysis of NPD data
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accompanied by an even larger increase of the average B-site
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tolerance factor, and the lowering of the symmetry with
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a
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Work at NIU was supported by the NSF-DMR-0706610. Work at
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