Structure and properties of layered perovskites Ba1-xLnxFe1-yCoyO3-δ (Ln = Pr, Sm, Gd)

Article abstract of DOI:10.1016/j.jallcom.2018.12.391

A series of samples of overall composition Ba_1-xLn_xFe_1-yCo_yO_3-δ (Ln = Pr, Sm, Gd) with x = 0.3–0.4; y = 0–0.5 were prepared by glycerin nitrate technique in air. The crystal structure of single-phase Ba_1-xLn_xFe_1-yCo_yO_3-δ (Ln = Pr, Sm, Gd) determined by XRD was described as cubic (sp. gr. Pm3ˉm). However, transmission electronic microscopy revealed that both Sm- and Gd-doped oxides possess tetragonal structure with 5-fold and 3-fold increased c parameter respectively. Oxygen content in the complex oxides has been determined in air over wide temperature range by means of thermogravimetry and iodometric titration. The change in oxygen content with temperature for the phases with five-layered ordering was significantly smaller than for the disordered phases.

Full text of DOI:10.1016/j.jallcom.2018.12.391

Accepted Manuscript
Structure and properties of layered perovskites Ba Ln Fe Co O
1-x 1-y y 3-δ
(Ln = Pr, Sm,
x
Gd)
N.E. Volkova, M. Yu. Mychinko, I.B. Golovachev, A.E. Makarova, M.V. Bazueva, E.I.
Zyaikin, L. Ya. Gavrilova, V.A. Cherepanov
PII:
S0925-8388(18)34962-4
DOI:
https://doi.org/10.1016/j.jallcom.2018.12.391
JALCOM 49034
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 18 October 2018
Revised Date: 25 December 2018
Accepted Date: 31 December 2018
Please cite this article as: N.E. Volkova, M.Y. Mychinko, I.B. Golovachev, A.E. Makarova, M.V.
Bazueva, E.I. Zyaikin, L.Y. Gavrilova, V.A. Cherepanov, Structure and properties of layered perovskites
Ba Ln Fe Co O (Ln = Pr, Sm, Gd), Journal of Alloys and Compounds (2019), doi: https://
1-y y 3-δ
1-x
x
doi.org/10.1016/j.jallcom.2018.12.391.
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ACCEPTED MANUSCRIPT
STRUCTURE AND PROPERTIES OF LAYERED PEROVSKITES
Ba1-xLnxFe1-yCoyO3-δ (Ln = Pr, Sm, Gd)
N.E. Volkova*, M.Yu. Mychinko, I.B. Golovachev, A.E. Makarova, M.V. Bazueva,
E.I. Zyaikin, L.Ya. Gavrilova, V.A. Cherepanov
Institute of Natural Science and Mathematics, Ural Federal University,
Yekaterinburg, Russia
* - corresponding author nadezhda.volkova@urfu.ru
Abstract: A series of samples of overall composition Ba1-xLnxFe1-yCoyO3-δ (Ln =
Pr, Sm, Gd) with x = 0.3-0.4; y = 0-0.5 were prepared by glycerin nitrate technique
in air. The crystal structure of single-phase Ba1-xLnxFe1-yCoyO3-δ (Ln = Pr, Sm, Gd)
Ŭ
determined by XRD was described as cubic (sp. gr. Pm3m). However, transmission
electronic microscopy revealed that both Sm- and Gd-doped oxides possess
tetragonal structure with 5-fold and 3-fold increased c parameter respectively.
Oxygen content in the complex oxides has been determined in air over wide
temperature range by means of thermogravimetry and iodometric titration. The
change in oxygen content with temperature for the phases with five-layered
ordering was significantly smaller than for the disordered phases.
Keywords: oxide materials, crystal structure, X-ray diffraction, transmission
electron microscopy, oxygen nonstoichiometry
1. Introduction
Perovskite oxide materials with the general formula of ABO3, where A is
alkali earth or rare earth metals and B is transition metal, have attracted much
attention as cathodes for solid oxide fuel cells (SOFC) because of their high
electronic conductivity and fast mobility of oxygen ions [1-4]. The introduction of
Ba2+ and Ln3+ ions with significantly different radii into the A-sites leads to
formation of the layered perovskite-type structures which have formed due to the
cations ordering in the alternating layers [5-13]. Depending on the nature of rare
earth and 3d metal it was possible to obtain double [5-7, 13], triple [9, 10, 12], or
quintuple [8, 11] perovskites. Formation of the five-fold layered Ln2–εBa3+εFe5O15–δ
phases is possible in the Fe-based systems in air [8, 11, 14]. Such 5-fold layered

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structure is formed by alternation of the layers containing exclusively samarium or
barium together with the mixed (Sm,Ba) layers along the c-axis: Sm–Ba–(Sm,Ba)–
(Sm,Ba)–Ba–Sm [8]. Recent studies revealed that partial substitution of cobalt for
iron stabilized quintuple layered structure [14, 15]. In the present work we have
shown the effect of rare earth element nature and role of Ln/Ba and Fe/Co ratio for
the crystal structure and oxygen content for the Ba1-xLnxFe1-yCoyO3-δ (Ln = Pr, Sm,
Gd; x = 0.3-0.4; y = 0-0.5) phases.
2. Experimental
The samples of overall composition Ba1-xLnxFe1-yCoyO3-δ (Ln = Pr, Sm, Gd; x
= 0.3-0.4; y = 0.1-0.5) were prepared using a glycerin nitrate technique. Rare earth
oxides (Pr6O11, Sm2O3 and Gd2O3) with the purity not less than 99.99%, barium
carbonate BaCO3 (99.9%), metallic cobalt Co, iron oxalate FeC2O4×2H2O
(99.0%), nitric acid and glycerin were used as the starting materials. Metallic
cobalt was obtained by reducing cobalt oxide Co3O4 (99.9%) in the hydrogen flow
at 400–600ºC during 6 h. The appropriate amounts of starting materials were
dissolved in 4.5 M nitric acid, and then glycerin was added to the solution.
Following heating first led to the formation of viscous gel that subsequently
transformed to brown powder. The final annealing was performed at 1100ºC in air
during 120 h with intermediate grindings, followed by a slow cooling down to
room temperature at the rate of about 100°/h. The phase composition of the
annealed samples was determined by X-ray diffraction (XRD) using a Shimadzu
XRD-7000 instrument (CuKα-radiation, angle range 2Θ= 20°–90°, step 0.03°, 5
s/step) in air. The structural parameters were refined by the Rietveld profile
method using the Fullprof-2008 package.
SAED and HRTEM images of the single phase samples that were deposited in
a form of suspension in ethanol on lacy carbon grids have been collected using a
JEOL JEM 2100 and FEI Technai G2 30 UT microscopes operated at 200 and 300
kV respectively.
Thermogravimetric analysis (TGA) was carried out using a STA 409PC
instrument (Netzsch) within the temperature range 25–1100 °С. Two measurement

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modes were employed, namely: static (10 h dwells at a fixed temperature, with the
temperature steps of 25–50°C) and dynamic (continuous heating/cooling rate of
1°C/min). The absolute values of oxygen content were determined by the
iodometric titration method described elsewhere [14].
3. Results and discussion
The compositions of single phase oxides which were confirmed by the XRD
examination of the studied Ba1-xLnxFe1-yCoyO3-δ (Ln = Pr, Sm, Gd; x = 0.3-0.4; y =
0-0.5) samples slowly cooled from 1100°С to room temperature are listed in Table
1.
The XRD patterns of all single phase Ba1-xLnxFe1-yCoyO3-δ samples were
refined either by the Rietveld method or by the Le Bail refinement within the cubic
Ŭ
structure (space group Pm3m). Typical XRD patterns for the selected single phase
samples are shown in Figure 1. The values of unit cell parameters, unit cell
volumes, and R-factors refined form the XRD results for the single phase oxides
are listed in Table 2.
It is obvious that concentration of 3d metals in the Ba1-xLnxFe1-yCoyO3-δ
phases has no significant effect to the unit cell parameters, which values are
consistent with the previously obtained results [14]. In contrast, the values of unit
cell parameter noticeably decreased with the decrease of lanthanide ion radii and
3+
its content (rBa2+=1.61 Å; rPr =1.32 Å; rSm3+=1.24 Å; rGd3+=1.11 Å [16]).
It was shown earlier that due to the chemical twinning along the orthogonal
directions XRD analysis cannot detect the formation of layered ordered supercell,
but electron microscopy required to prove it [8]. Therefore the next step of
structural examination was a transmission electron microscopy study.
The detailed TEM study of praseodymium-containing oxides Ba1-xPrxFe1-
yCoyO3-δ confirmed that regardless of Pr/Ba and Fe/Co ratio all oxides possess the
ideal cubic perovskite structure with random distribution of Pr and Ba in the A-site
positions as well as Fe and Co in the B-site sublattice (Fig. 2).

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This result differs the phase relations for rare earth barium ferrites compared
to that observed in the related cobaltite systems in air, where formation of layered
double perovskites have started from PrBaCo2O6-δ [5].
A decrease of Ln size drastically changes phase formation. Similarly to the
earlier reported Sm2-εBa3+εFe5O14.22 [8] and Sm2Ba3Fe3Co2O14.07 [14] (the latter
formula recalculated to the form of “simple perovskite” by dividing on 5 can be
written as Sm0.4Ba0.6Fe0.6Co0.4O3-δ) all samples with the intermediate Co-content
studied in this work also exhibited a quintuple supercell. The SAED and HRTEM
images for Sm2Ba3Fe3.5Co1.5O15-5δ (Sm0.4Ba0.6Fe0.7Co0.3O3-δ) shown in Fig. 3
illustrate 5-fold ordering and chemical twinning along three cubic directions of the
perovskite basic structure. The different brightness of the dots originate because of
difference in the scattering factors of different atoms forming correspondent
atomic columns reflecting 5-fold alternating of A-site atoms: “Sm – Ba – (Sm,Ba)
– (Sm,Ba) – Ba – Sm” [8, 14]. Partial substitution of Co for Fe stabilizes 5-fold
superstructure, which can be obtained as for the ideal (2/3) Sm/Ba ratio so for the
“nonstoichiometric” ((2-ε)/(3+ε)) Sm/Ba ratio.
It worth to note that although partial substitution of Co for Fe stabilizes
quintuple perovskite structure on the other hand the double perovskite
SmBaCo2O6-δ was found to be stable at 1100°C in air in the Sm2O3 – BaO – CoO
system [7]. Moreover, partial substitution of Fe for Co is possible within the
double perovskite structure SmBaCo2-yFeyO6-δ up to y = 1.1 [17].
Here we can also mentioned that according to our previous works Nd-
containing Co-doped ferrites demonstrate intermediate behavior between Pr- and
Sm-containing systems in respect to phase formation [11, 15, 18]: the domains of
quintuple perovskite were incorporated into the disordered perovskite matrix,
Nd/Ba ratio was shown to be a critical factor for the formation of 5-fold structure,
Co substitution stabilized the ordered quintuple structure and finally, neodymium
barium cobaltite with the double perovskite structure can be partially doped by iron
NdBaCo2-yFeyO6-δ.

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More complicate situation occurs in the Gd-containing system. Earlier it was
shown that stabilization of quintuple perovskite structure in the Eu-containing
system can be achieved only by Co for Fe partial substitution yielding the formula
Eu2Ba3Fe3Co2O13.72 [14]. Similar behavior was detected in the Gd-containing
system. Co-rich sample of the Gd0.365Ba0.635Fe0.6Co0.4O3-δ composition crystallized
in the form of quintuple layered perovskite, so its formula should be written as
Gd1.825Ba3.175Fe3Co2O15-5δ. The ordering of atomic columns with low, medium and
high brightness along z axis that obviously observed in the HRTEM image (Fig. 4)
confirms electron diffraction data and can be described as “Gd–Ba–Ba,Gd–Ba,Gd–
Ba–Gd” alternation of A-site cations in corresponding layers yielding to the
formula Gd2-εBa3+εFe3Co2O15-5δ. However, the SAED and HRTEM images for the
sample Gd0.365Ba0.625Fe0.9Co0.1O3-δ with poorest Co content differ significantly.
Additional to the simple perovskite structure spots and their relative brightness
indicate the formation of so-called triple perovskite (or 123-phase) with the “Gd-
Ba-Ba-Gd” alternation of A-site cations (Fig. 5). Taking into account this type of
ordering the formula of the Gd0.365Ba0.625Fe0.9Co0.1O3-δ oxide should be written as
follows: Gd1+εBa2-εFe2.7Co0.3O8+w (ε≈0.1).
Since Ba0.365Gd0.635Fe0.9Co0.1O3-δ and Ba0.365Gd0.635Fe0.6Co0.4O3-δ samples
should be considered as triple Gd1+εBa2-εFe2.7Co0.3O8+w (ε≈0.1) and quintuple
Gd2-εBa3+εFe3Co2O15-5δ (ε≈0.175) perovskites respectively, the question arise what
is the structure of oxide with a nominal composition in between of them, for
example Ba0.365Gd0.635Fe0.8Co0.2O3-δ? TEM image for this sample is presented in
Fig 6. Electron diffraction pattern taken along [110] zone axis supposed to be the
sum of two different diffractions since all additional reflexes have different
brightness and positions. Some of these reflexes are more intensive and located in
between the positions that are specific for triple and quintuple ordered phases
mentioned above, so we assume them as a sum of two closely located reflexes.
HRTEM images taken along area selected for electron diffraction shows randomly
distributed bright and dark spots along z axis of unit cell which can be considered

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as random alternating of the unit cells with triple and quintuple orderings.
Therefore the sample with intermediate composition can be treated as intergrowth
of triple and quintuple perovskites.
Similarly to a samarium barium double cobaltite corresponding gadolinium-
based oxide can also formed Fe-substituted solid solutions GdBaCo2-yFeyO6-δ up to
y=1.0 [19, 20]
The oxygen content and average oxidation state of the iron and cobalt ions in
the Ba1-xLnxFe1-yCoyO3-δ phases, either slowly cooled to room temperature
(calculated from the results of iodometric titration) or those related to 1100°C
(obtained from the TGA measurements) are listed in Table 3.
A decrease in the Ln3+ size and concentration leads to the decrease of oxygen
content and mean oxidation state of 3d transition metals. These trends can be
explained in terms of the bond energy values: the value of standard enthalpies of
formation increasing in the row: GdO (–78.7 kJ/mol); SmO (–135.6 kJ/mol); PrO
(–147.7 kJ/mol) [21]. The oxygen content slightly decreases with the raise of
cobalt concentration in Ba1-xLnxFe1-yCoyO3-δ. This can be understood considering
the fact that cobalt is more electronegative compared to iron (χFe=1.64; χCo =1.7
according to the Allred and Rochow scale [22]). Therefore cobalt acts as acceptor
V ••
′
Co
of electrons ( Fe ), enhancing formation of oxygen vacancies ( ). Concentration
O
dependencies of mean oxidation state for 3d-transition metals (iron and cobalt) in
Ba1-xLnxFe1-yCoyO3-δ are presented in Fig. 7.
Raw experimental results of TGA measurements for Ba1-xLnxFe1-yCoyO3-δ, vs.
temperature in air obtained in the dynamic mode with the cooling/heating rate
1°/min (shown by the lines) and obtained in the static mode (isothermal dwells at
each temperature shown by points), nicely coincided with each other, which
confirms fast oxygen release/uptake kinetics (see Fig. S1 in the Supplementary
file). Taking into account the absolute values of oxygen content extracted from the
results of iodometric titration the values of (3 – δ) were calculated from the TG
result within the entire temperature range (Fig. 8).

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The largest change in oxygen content has been found for the disordered
perovskites. The triple and quintuple perovskites exhibited much smaller variation
in oxygen content. This is consistent with the observation for the quintuple
perovskites about predominant location of oxygen vacancies between the mixed
(Sm,Ba) layers [8].
4. Conclusion
The present study have shown that crystal chemistry of oxide systems
containing rare earth elements (Pr, Sm, Gd), Ba and 3d-transition metals (Fe, Co)
at fixed conditions (1100oC, air) is complicate and strongly depended on cation’s
nature and their ratio in A-site (Ln/Ba) and B-site (Fe/Co) positions. Pr-containing
system reveals the formation of disordered perovskite structure, while samarium
barium ferrites exhibited a formation of quintuple layered structure. Depending on
Ln/Ba ratio, Gd-containing system reveals a formation of triple or quintuple
perovskite structure.
Acknowledgements
This work was supported in part by the Russian Science Foundation (Grant №
18-73-00159) and the Russian Foundation for Basic Research (Grant № 18-33-
00822 mol_a).
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Table 1 – The compositions of the single phase Ba1-xLnxFe1-yCoyO3-δ (Ln = Pr, Sm,
Gd) samples
Ln
Pr
x
0.3
y
0.1-0.5
0.1-0.3
0.1-0.3
0.1-0.4
0.4
Sm
Gd
0.4
0.365
Table 2 – Unit cell parameters of Ba1-xLnxFe1-yCoyO3-δ (Ln = Pr, Sm, Gd) and
relevant agreement factors extracted from the Le Bail refinement (*) or the
Rietveld refinement results (**)
Ln
x
y
a, Å
V, Å3
RBr, %
0.706*
0.642*
5.96**
1.67*
0.566*
3.81**
1.49*
3.83**
0.621*
0.801*
0.529*
1.19*
0.765*
0.989*
0.518*
Rf, %
0.734*
0.638*
3.94**
1.15*
0.446*
2.34**
1.01*
2.67**
0.795*
0.765*
0.449*
1.16*
0.712*
0.940*
0.502*
0.1
0.2
0.3
0.4
0.5
0.1
0.2
0.3
0.1
0.2
0.3
0.1
0.2
0.3
0.4
3.961(1)
3.952(1)
3.952(1)
3.952(1)
3.954(1)
3.934(1)
3.929(1)
3.928(1)
3.928(1)
3.926(1)
3.924(1)
3.927(1)
3.927(1)
3.925(1)
3.924(1)
62.14(2)
61.72(2)
61.72(1)
61.72(2)
61.82(2)
60.88(2)
60.65(2)
60.60(2)
60.61(2)
60.51(2)
60.42(2)
60.55(2)
60.55(2)
60.46(2)
60.42(2)
0.3
Pr
0.4
0.4
Sm
Gd
0.365

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Table 3 – Oxygen content (3-δ), mean oxidation state of iron and cobalt (nMe) ions
in Ba1-xLnxFe1-yCoyO3-δ at 25 and 1100°C.
3-δ
(25°C)
3-δ
nMe
nMe (25°C)
Ln
x
y
(1100°C) (1100°C)
2.74±0.04
3.17
0.1
2.89±0.04
3.48
0.3
0.2
0.3
0.1
0.2
0.3
0.1
0.2
0.3
0.1
2.85±0.04
2.83±0.04
2.90±0.04
2.88±0.04
2.86±0.04
2.76±0.04
2.75±0.04
2.74±0.04
2.71±0.04
3.39
3.36
3.40
3.35
3.31
3.12
3.10
3.08
3.06
2.70±0.04
3.09
3.03
–
2.66±0.04
Pr
–
0.4
0.4
–
–
–
–
–
–
Sm
2.65±0.04
–
2.90
–
Gd 0.365
2.70±0.04
3.02

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Figure 1 – X-ray diffraction patterns of Ba1-xLnxFe1-yCoyO3-δ
Fig. 2 – SAED image taken along [001] [111] and [110] zones axis for
Ba0.7Pr0.3Fe0.7Co0.3O3-δ
Fig. 3 – HRTEM image for Sm2Ba3Fe3.5Co1.5O15-5δ taken along [100] axis
(corresponding SAED pattern as inset)
Fig. 4 – SAED images of Gd0.365Ba0.635Fe0.6Co0.4O3-δ taken along (a) [100], (b)
[001], (c) [110] and (d, c) HRTEM images taken along [100] zone axis (intensity
profile line along z axis as inset)
Fig. 5 – HRTEM image of Gd0.365Ba0.635Fe0.9Co0.1O3-δ taken along [100] zone axis
(corresponding SAED pattern as inset)
Fig. 6 – SAED image of Gd0.365Ba0.635Fe0.8Co0.2O3-δ taken along (a) [100], (b)
comparison of obtained reflexes with different type of orderings, (c) HRTEM
image taken along [110] zone axis
Fig. 7 – The mean oxidation state of 3d-transition metals (iron and cobalt) vs.
lanthanide concentration (x) in Ba1-xLnxFe1-yCoyO3-δ (Ln = Pr, Sm, Gd) at 25 °C in
air.
Fig. 8 – Temperature dependencies of oxygen content in Ba1-xLnxFe1-yCoyO3-δ (Ln
= Pr, Sm, Gd) at atmospheric oxygen pressure.

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The Ba1-xLnxFe1-yCoyO3-δ oxides have been prepared by glycerol-nitrate technique;
The homogeneity ranges for the Ba1-xLnxFe1-yCoyO3-δ solid solutions were determined;
Crystal structure for the Ba1-xLnxFe1-yCoyO3-δ δ solid solutions was revealed;
The temperature dependencies of oxygen content in Ba1-xLnxFe1-yCoyO3-δ were
determined.