Phase equilibria, structure and properties of intermediate phases in the Sm2O3 – Fe2O3 – CoO and Sm2O3 – CaO – CoO systems

Article abstract of DOI:10.1016/j.jallcom.2017.05.044

Phase equilibria in the Sm₂O₃ – Fe₂O₃ – CoO and Sm₂O₃ – CaO – CoO systems were systematically studied at 1100 °C in air. The following solid solutions: SmFe_1-xCo_xO

Full text of DOI:10.1016/j.jallcom.2017.05.044

Accepted Manuscript
Phase equilibria, structure and properties of intermediate phases in the Sm O –
2
3
Fe O – CoO and Sm O – CaO – CoO systems
2 3
2
3
A.P. Galayda, N.E. Volkova, L. Ya. Gavrilova, K.G. Balymov, V.A. Cherepanov
PII:
S0925-8388(17)31628-6
10.1016/j.jallcom.2017.05.044
JALCOM 41774
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 31 January 2017
Revised Date: 29 April 2017
Accepted Date: 5 May 2017
Please cite this article as: A.P. Galayda, N.E. Volkova, L.Y. Gavrilova, K.G. Balymov, V.A. Cherepanov,
Phase equilibria, structure and properties of intermediate phases in the Sm O – Fe O – CoO
2 3
2
3
and Sm O – CaO – CoO systems, Journal of Alloys and Compounds (2017), doi: 10.1016/
2
3
j.jallcom.2017.05.044.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Phase equilibria, structure and properties of intermediate phases in the
Sm O - Fe O - CoO and Sm O - CaO - CoO systems
2
3
2
3
2
3
A.P. Galayda, N.E. Volkova, L.Ya. Gavrilova, K.G. Balymov,
*
and V.A. Cherepanov
Institute of Natural Science and Mathematics, Ural Federal University,
Yekaterinburg, Russia
*
- corresponding author v.a.cherepanov@urfu.ru
Abstract
Phase equilibria in the Sm O – Fe O – CoO and Sm O – CaO – CoO systems
2
3
2
3
2
3
were systematically studied at 1100°C in air. The following solid solutions:
SmFe Co O (0≤x≤1), Ca Sm Co O (0.6≤y≤0.7) and Sm Ca CoO
1
-x
x
3
3-y
y
2
6-δ
2-r
r
4-δ
(0.8≤r≤1) have been detected in the studied systems. Structural parameters for the
single phase samples were refined using the Rietveld analysis. Thermal expansion,
electrical conductivity and Seebeck coefficient of SmFe Co O (x = 0.2, 0.5 and
1
-x
x
3-δ
0
.8), Ca Sm Co O and SmCaCoO were measured within the temperature
2
.3
0.7
2
6-δ
4-δ
range of 25–1100°C in air. The subsolidus phase diagrams for the ½ Sm O –
2
3
½
Fe O – CoO system and ½ Sm O – CaO – CoO system both at 1100°C in air
2
3
2
3
have been constructed.
Keywords: Phase equilibria; crystal structure; oxygen nonstoichiometry;
conductivity; thermal expansion
1
. Introduction
Materials with the perovskite structure and Ruddlesden-Popper phases based on
the rare earth, alkali earth and 3d-transition metals exhibit high mixed electronic-
ionic conductivity and oxygen mobility, good thermodynamic stability under
oxidizing atmosphere at high temperature and compatibility with electrolyte
materials [1-4]. In particular, samarium ferrites and cobaltites have received great
interest not only as electrode materials [1-4], but also as materials for the sensitive
gas sensors [5-7] and catalytic materials [8, 9]. They were also studied as
promising thermoelectric materials [10]. On the other hand, calcium cobaltite
Ca Co O is known as one of the best high temperature thermoelectric p-type oxide
3
4
9

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materials [11-13]. However, this compound is thermodynamically stable only
below 926°C, which limited its applicability.
A well-known approach that is often used in order to improve functional properties
of materials is doping. For example, Co-doped ferrites, such as SmFe Co O , are
1
-x
x
3
shown to be good sensor materials for ethanol, ozone and NO detection [14-16]. It
2
was found that cobalt substitution decreases the working temperature of the gas
sensor [14]. Yttrium and iron co-doping of Ca Co O modify its thermoelectric
3
4
9
properties [17]. Partially lanthanum-substituted Ca La Co O was considered as
3
-x
x
4
9
cathode for the proton conducting fuel cell [18].
Information on phase equilibria and thermodynamic stability of intermediate
phases formed in a system presented in a form of phase diagrams serves as a
physicochemical basis for the preparation and usage of any material. At the present
time available information on phase equilibria and thermodynamic characteristics
is devoted to the binary systems, such as Sm O -CoO [19-23], Sm O -Fe O [24-
2
3
2
3
2
3
2
8], CaO-CoO [29-32] and CaO-Fe O [33-35]. The short summary of available
2
3
information about the phases formed in the afore-mentioned binary systems is
presented below.
Sm-Co-O system. The only thermodynamically stable binary oxide SmCoO is
3
-δ
known in this system [19-23]. It can be heated in air up to incongruent melting at
344±4°C [19]. Samarium cobaltite possesses the perovskite structure with the
1
orthorhombic distortions; the unit cell parameters are a = 5.357 Å, b = 5.294 Å,
c = 7.513 Å (sp. gr. Pbnm) [36].
Sm-Fe-O system. Two complex oxides SmFeO and Sm Fe O were described in
3
-δ
3
5
12
the Sm–Fe–O system at 1100°C in air [24-28]. Samarium ferrite SmFeO can be
3
-δ
prepared by the solid state reaction [24, 26, 27], or by the citric acid route [25,37,
8] or by the decomposition of cyanide complexes [6, 8]. The crystal structure of
3
SmFeO3-δ was described as orthorhombic (sp.gr. Pbnm) with the unit cell
parameters a = 5.398 Å, b = 5.597 Å; c = 7.707 Å [38].

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Another binary oxide Sm Fe O that can be prepared in air by the solid state
3
5
12
reaction [24,26, 27] or by the citrate-nitrate technique [25] possesses the cubic
garnet structure (Ia3d sp. gr.) with the unit cell parameter a=12.530 Å [39].
Ca-Co-O system. Three complex oxides have been described in the Co-Co-O
system, namely, CaCo O , Ca Co O , and Ca Co O [29-32].
2
4
3
4
9+δ
3
2
6
The crystal structure of CaCo O depends on the synthesis conditions. CaCo O
2
4
2
4
obtained by the citrate-nitrate technology [32] or carbonates co-precipitation
method within the temperature range 500-650°C in oxygen or in air [40] possesses
the monoclinic cell with the unit cell parameters a = 11.490 Å, b = 5.772 Å,
c = 4.737 Å [32]. The high pressure synthesis at 1500°C and 6 GPa leads to the
formation of CaCo O with the CaFe O -type orthorhombic structure (sp.gr. Pnma,
2
4
2
4
the unit cell parameters a = 8.789 Å, b = 2.901 Å and c = 10.282Å) [41]. The
decomposition temperature for CaCo O in air was found to be located between
2
4
6
00°C and 700°C [32].
Widely studied thermoelectric oxide Ca Co O has a layered structure, formed by
3
4
9+δ
the alternating of Ca CoO layers with the rock-salt structure and CoO layers with
2
3
2
the CdI -type structure [17, 18]. Ca Co O reveals the monoclinic structure (sp.
2
3
4
9+δ
gr. P2 /m) [18] with the unit cell parameters a = 4.832Å, b = 36.467Å,
1
c = 10.819 Å, β = 98.05°. The upper temperature limit of Ca Co O in air was
3
4
9
found to be equal to 926°C [31], however, Tran et al. [32] reported that the
decomposition of this phase starts at 800°C.
Ca Co O was prepared by the citrate technique [42], or by the solid phase
3
2
6
synthesis [30, 43]. It has a pseudo-one-dimensional structure (sp. gr. R-3c) with the
unit cell parameters a = 9.079 Å, c = 10.381 Å [42]. The decomposition
temperature of Ca Co O in air was determined as 1026°C [31, 32].
3
2
6
The afore-mentioned data on thermodynamic stability of all known phases [29, 30]
and the phase equilibria study reported in refs. [31, 32] confirm the absence of the
intermediate complex oxides at 1100°C in air inside the Ca-Co-O system. The
solubility of cobalt oxide in calcium oxide at 1100°C in air was estimated as 7
mole %, while the solubility of CaO in CoO was evaluated as 14 mole % [31].

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Co-Fe-O system. The homogeneity ranges and crystal structure of the single phases
o
in the Co-Fe-O system existing at 1100 C in air (Co Fe O with 0≤u≤0.13;
1
-u
u
Co Fe O with -0.1≤v≤0.35 and Fe Co O with 0≤w≤0.03) had been described
1
+v
2-v
4
2-w
w
3
in the earlier paper [35].
It should be noted that phase equilibria in the Sm O – CaO – CoO system and in
2
3
the related La-, Nd- and Eu-containing systems in air have been studied by Wong-
o
Ng et al. [44-47], however, the phase diagrams were constructed at 885 C and as it
o
will be shown below the raise of temperature up to 1100 C significantly influences
the phase relations.
One more item of vital importance for oxide materials is the oxygen content and its
changes with temperature since it determines the defect structure and, as a result,
the main related functional properties, such as conductivity and ionic transport.
Since there is lack of information about phase equilibria, oxygen content and its
changes, electrical conductivity and thermal expansion in the present work we
have focused on the phase equilibria, oxygen content and some important
functional properties of intermediate phases, such as thermal expansion and
electrical conductivity in the Sm O - Fe O - CoO and Sm O – CaO - CoO
2
3
2
3
2
3
systems.
. Experimental
2
The complex oxides were prepared using a glycerin nitrate technique. Samarium
oxide Sm O (99.99% purity), calcium carbonate CaCO (“pure for analysis”
2
3
3
grade), metallic cobalt Co, iron oxalate FeC O ×2H O (“pure for analysis” grade),
2
4
2
nitric acid (“special purity” grade) and glycerin were used as the starting materials.
Metallic cobalt was obtained by reducing cobalt oxide Co O (“special purity”
3
4
grade) in the hydrogen flow at 400–600ºC during 6 h. Before weighting the
starting materials (Sm O and CaCO ) were annealed in order to remove adsorbed
2
3
3
gases and water. The appropriate amounts of starting materials were dissolved in
nitric acid, and then glycerin was added to the solution. The resulting gel was dried
in a porcelain cup, decomposed to the dark powder, then removed to an alumina
crucible and calcined in air at 700-1000ºC during 8-10 h. The final annealing was

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performed at 1100ºC in air during 120 h with intermediate grindings, followed by
quenching to room temperature with the cooling rate of about 500 º/min. The phase
composition of the annealed samples was determined by X-ray diffraction using
DRON-6 diffractometer (CuKα-radiation, angle range was 2Θ= 20°–120°, step
was 0.04°, 10 s/step), an Equinox-3000 instrument (CuKα-radiation, equipped by a
curved position sensitive detector CPS-590; the angle interval was 2Θ = 10°–90°,
acquisition time was 1–2 h, step was 0.012°) and Shimadzu XRD-7000 (CuKα-
radiation, angle range was 2Θ= 20°–90°, step was 0.03°, 5 s/step, temperature
range of 25–1100°С) in air. The structural parameters were refined by the Rietveld
profile method using the Fullprof-2008 package. Thermogravimetric analysis
(TGA) was carried out using a STA 409PC instrument (Netzsch) within the
temperature range of 25–1100°С in air in dynamic (heating/cooling rate 2K/min)
mode. Absolute values of oxygen content were determined by the reduction of the
samples in a hydrogen flux in TGA instrument at 1100ºC [48] and chromatometric
[49] titration. Thermal expansion measurements were carried out within the
temperature range of 25-1100ºC in air using the dilatometer DIL 402C (Netzsch) at
a heating/cooling rate of 5º/min. Total conductivity and Seebeck coefficient of
ceramic samples were measured in air by 4-probe method with platinum
electrodes. Magnetic properties were measured using vibrating sample
magnetometer LakeShore 7407 in the temperature range of 25–900°C in air.
Bar-shaped samples (3×4×25 mm) for TEC (thermal expansion coefficient) and
conductivity measurements were obtained by pressing the powders mixed with 2–3
drops of ethanol using a manually operated press followed by sintering under slow
heating, sintering at 1200°C for 14 h in air, and slow cooling (the rate of heating
and cooling was 50 º/h). Relative densities of the samples were evaluated by the
comparison of measured values to those calculated from XRD-data. The obtained
values of relative densities were higher than 90% for all samples. A possible
chemical interaction of SmFe Co O with the electrolyte materials Ce Sm O
1
-x
x
3-δ
0.8
0.2
2
and Zr Y O has been checked within the temperature range of 900–1100°C,
0
.85 0.15
2

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by annealing the corresponding mixtures with 1:1 weight ratio during 24 h in air
with the following XRD analysis.
3
. Results and discussion
3
.1. ½ Sm O – ½Fe O – CoO system
2
3
2
3
The phase equilibria in the ½ Sm O – ½ Fe O – CoO system were studied at
2
3
2
3
1
100°C in air using 37 samples with various compositions.
Solid solution SmFe Co O
1
-x
x
3-δ
In order to determine the homogeneity range of the SmFe Co O solid solution,
1
-x
x
3-δ
the samples with x= 0 - 1 (steps in x of 0.1) were prepared using the glycerin-
nitrate technique. The XRD results for quenched samples show that single-phase
solid solutions have formed at 1100°C in air within the entire composition range
(
Fig. 1 a). The XRD patterns of single phase SmFe Co O were indexed in the
1
-x
x
3-δ
orthorhombic structure (sp. gr. Pbnm). The results obtained were in good
agreement with those reported in ref. [12]. Fig. 1b illustrates X-ray diffraction
pattern for SmFe Co O refined by the Rietveld analysis, as an example. The
0
.7
0.3 3-δ
incorporation of cobalt into the iron sublattice results in a linear decrease of the
unit cell parameters and unit cell volume in accordance with the Vegard’s law
(Fig. 2 and Table 1), since the ionic radius of iron is larger than that of cobalt (for
CN = 6 rFe3+ = 0.785 Å; rCo3+ = 0.75 Å [50]).
The value of oxygen nonstoichiometry for SmFe Co O determined by the
0
.5
0.5 3-δ
reduction method inside TGA cell and recalculated to the room temperature was
equal to zero. TGA measurements reveal that the oxygen content in
SmFe Co O has not shown noticeable changes within the whole temperature
0
.5
0.5
3
o
range from room temperature up to 1100 C.
Fig. 3 illustrates typical temperature dependencies of the thermal expansion for
SmFe Co O with x = 0.2, 0.5 and 0.8. As can be seen, all samples exhibit
1
-x
x
3
nonlinear shape dependencies. Since no significant oxygen losses were detected at
heating such deviations from linearity could not be explained by the so-called
chemical expansion, the effect that occurs due to the change of oxidation state of
some cations [51-54]. Another reason that can induce such trend is possible

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redistribution of the electron density between Co and Fe due to their
electronegativity difference or changes in their spin states with the temperature.
The average TEC values within the temperature range of 800–1100°C are listed in
Table 2. The decrease of TEC values with the increase of iron content while the
oxygen content has not changed can be explained by higher bond energy for Fe –
O (409 kJ/mol) compared to Co – O (368 kJ/mol) [54].
The total conductivity of SmFe Co O increases with temperature (Fig. 4) in a
0
.5
0.5
3
typical semiconductor manner. The conductivity activation energy calculated from
the linear Arrhenius plot (insert in Fig. 4) is equal to 0.66 eV. The positive value of
Seebeck coefficient (Fig. 5) confirms that electron holes are predominant charge
carriers. The maximum value of Seebeck coefficient correspondent to the room
temperature is equal to 583 ꢀV/K.
The chemical compatibility of SmFe Co O (x = 0.2; 0.5; 0.8) with the solid
1
-x
x
3
electrolyte materials (Ce Sm O and Zr Y O ) has been checked in air
0
.8
0.2
2
0.85 0.15
2
within the temperature range of 800–1100°C. No chemical interaction between the
solid solutions and both electrolytes has been detected within the entire
temperature range (Fig. 6).
High conductivity values, the TEC values that are comparable to those for the
electrolyte materials, and the absence of interaction with the solid electrolytes
allow us to consider SmFe Co O (x = 0.2; 0.5; 0.8) as a potential candidate for a
1
-x
x
3
cathode material in SOFCs (solid oxide fuel cells).
Using XRD results for 37 quenched samples the projection of isobaric-isothermal
phase diagram (1100ºC in air) onto the compositional triangle ½ Sm O - ½ Fe O
2
3
2
3
–
CoO has been divided into 7 phase fields (Fig. 7). The overall compositions for
the examined samples are shown as points on the phase triangle. The homogeneity
ranges for the phases formed in the binary Fe O – CoO system at 1100ºC in air
2
3
have been taken from our previous work [35].
.2. ½ Sm O – CaO–CoO system
3
2
3
The phase equilibria in the ½ Sm O – CaO – CoO system were studied at 1100ºC
2
3
in air using 53 samples with various compositions.

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.2.1. ½ Sm O – CaO system
3
2
3
In order to check a possibility of partial dissolution of calcium in samarium oxide
Sm O a number of samples with overall formula Sm Ca O within the
2
3
2-z
z
3-δ
compositional range of 0.05≤z≤0.2 with the step in z of 0.05 have been prepared. It
was found that all samples of Sm Ca O within 0≤z≤0.1 range are single-phase.
2
-z
z
3-δ
All single phase solid solutions possess the monoclinic structure (sp. gr. C2/m).
The unit cell parameters refined by the Rietveld method are presented in Table 3.
3
.2.2. Sm Ca CoO cross section.
1
-x
x
3-δ
Contrary to the La O – CaO – CoO system where the solid solution
2
3
La Ca CoO with the distorted perovskite structure has formed at 1100ºC in air
1
-x
x
3-δ
within the range of 0≤x≤0.3 [55, 56] no signs of Ca substitution for Sm in SmCoO3
have been detected in the present work. All samples belonging to
“
Sm Ca CoO ” cross section had a non-single phase composition. The phase
1
-x
x
3-δ
composition of each sample depended on the particular Sm/Ca ratio. It should be
noted that partial substitution of Ca for Sm with the formation of Sm Ca CoO
1
-x
x
3-δ
solid solution (0≤x≤0.11) at 885ºC has been reported by Wong-Ng et al. [44]. It
means that the temperature influences the substitution limit in the Sm Ca CoO
1
-x
x
3-δ
solid solution significantly. The size difference between Sm and Ca ions compared
for example to La ions ( Sm
r
=1.24 Å,
r
La3+
=1.36 Å,
r
=1.34 Å [50]) should not
3
+
2+
Ca
be considered as the crucial reason since the single phase Sr Sm CoO is known
1
-x
x
3-δ
to be stable [36] although the size difference between Sr and Sm ions is even larger
2
+
r 2 =1.44 Å [50]). Due to the smaller size Ca ions are stronger bonded oxygen
+
Sr
2
+
atoms in the lattice compared to Sr and prevent oxygen release which can
compensate for the heterovalent substitution of Sm. At the same time, the rise of
2
+
temperature induces Co ions to decrease its oxidation state but Ca substitution for
3
+
Sm affects in the opposite way. This can explain the significant decrease of the
substitution limit for Ca ions with the temperature as well as perhaps with the
decrease of oxygen partial pressure. The annealing time at 85ºC in [44] was about
three weeks. So it is significant time to reach equilibrium state even for the

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relatively low temperature. The specific feature of the systems containing 3d-
transition metals is that the increase of temperature does not always lead to the
increase of the homogeneity range of complex oxides. Thermodynamically
preferable oxidation state of 3d-transition metal is more important. The increase of
temperature means the creation of more reducing conditions. There are two factors
which affect the average oxidation state of Co in lanthanum calcium cobaltite:
calcium content (x) and oxygen deficiency. Although the oxygen content is
decreasing with the temperature, it is not enough to compensate for the tendency of
Co ions to decrease its average oxidation state. The latter leads to the narrowing of
the homogeneity range, i.e. to the decrease of calcium content (x).
3
.2.3. Solid solution Ca Sm Co O
3
-y
y
2
6-δ
According to the phase diagram of the Ca – Co – O system [31, 32] the undoped
calcium cobaltite Ca Co O does not exist in air at temperatures higher than
3
2
6
1
026ºC. The homogeneity range for the Sm-doped Ca Sm Co O solid solution
3
-y
y
2
6-δ
at 1000ºC was estimated as y = 0-0.09 [42]. In this research we have studied the
influence of Sm-doping on the stability of Ca Sm Co O cobaltite at 1100ºC in
3
-y
y
2
6-δ
air, i.e. at the temperature that exceeded the decomposition value for the undoped
Ca Co O . The samples of Ca Sm Co O with y = 0 - 1 (∆y = 0.1) were prepared
3
2
6
3-y
y
2
6-δ
using the glycerin nitrate synthesis. According to the X-ray analysis the
homogeneity range of solid solution Ca Sm Co O appears within 0.6≤y≤0.7.
3-y
y
2
6-δ
Thus, the introduction of Sm enhances the thermal stability of the Ca Co O type
3
2
6
structure. One can see that the width of the homogeneity range of the
Ca Sm Co O solid solution at 1100ºC remains practically the same as it was at
3
-y
y
2
6-δ
1
000ºC, however, both limiting compositions are shifted towards Sm enrichment.
The XRD patterns of the single phase oxides were indexed in the rhombohedral
unit cell (sp. gr. R-3c). The unit cell parameters refined by the Rietveld method are
listed in Table 4.
On the other hand, the decrease of temperature down to 885ºC leads to the
complete disappearance of the homogeneity range in Ca Sm Co O according to
3
-y
y
2
6-δ
the results reported by Wong-Ng et al. [44]. The linear extrapolation of the Sm-

ACCEPTED MANUSCRIPT
rich boundary determined at 1000ºC in ref. [42] and at 1100ºC in the present work
to the temperature axis inside the “T – composition (y)” cross-section allows us to
suppose that noticeable substitution of Sm for Ca might be expected at a
temperature of about 980ºC and higher. However, it is very rough estimation
which needed further experimental confirmation.
The value of oxygen content for the slowly cooled Ca Sm Co O at room
2
.3
0.7
2
6-δ
temperature was estimated from the results of chromatometric titration as
6-δ) = 5.88±0.05. The mean oxidation state of cobalt ions is +2.53, which means
(
3
+
2+
that Co and Co ions coexisted in practically equal amounts and correspondently
distributed in Co1 and Co2 positions in the crystal lattice. TGA examination shows
that oxygen content remains practically unchanged from 25ºC up to 700ºC and
slight oxygen losses started at higher temperature (Fig. 8).
Fig. 9 shows the relative linear expansion of the complex oxide Ca Sm Co O
2.3
0.7
2
6-δ
versus temperature. The visible bent on the dilatometric plot can be explained by
the beginning of oxygen release at 700-750ºC. Thus the dependence ∆L/L=ƒ(T)
has been described by two linear equations in the temperature ranges of 25-750ºC
and 750-1000ºC. Low- and high-temperature TEC values for Ca Sm Co O are
2
.3
0.7
2
6-δ
listed in Table 2.
The total conductivity of Ca Sm Co O versus temperature is shown in Fig.10.
2
.3
0.7
2
6-δ
The increase of conductivity for Ca Sm Co O with temperature which is
2
.3
0.7
2
6-δ
similar to SmFe Co O can be caused by the rise of the charge carrier mobility.
0
.5
0.5
3
The activation energy for the conduction of Ca Sm Co O calculated from the
2.3
0.7
2
6-δ
linear plot in the Arrhenius coordinates (see insert in Fig. 10) is equal to 0.32 eV.
.2.4. «Ca CoO » – «Sm CoO » system.
3
2
4
2
4
Although both parent oxides have not existed at 1100ºC in air: «Ca CoO » was not
2
4
synthesized at all and Sm CoO was prepared in the conditions that suggest it is
2
4
probably a metastable phase [57], intermediate complex oxides Sm Ca CoO
2
-r
r
4-δ
with the K NiF -type structure were detected earlier for Sm [44, 58-60] and other
2
4
Ln (La [47, 55, 56], Nd [45, 61, 62]) systems. XRD of the samples with nominal
composition Sm Ca CoO quenched from 1100ºC in air proved a formation of
2
-r
r
4-δ

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single phase solid solutions within the range of 0.8≤r≤1. Equimolar SmCaCoO
4
-δ
possesses the ideal tetragonal K NiF -type structure (sp. gr. I4/mmm), while those
2
4
with smaller calcium content r= 0.8 and 0.9 revealed orthorhombic distortions (sp.
gr. Bmab), that is in good agreement with the results for the samples with r = 1 and
0
.9 reported by Thorogood et al. [60]. The XRD patterns for Sm Ca CoO with
2
-r
r
4-δ
the tetragonal (r=1) and orthorhombic (r=0.9) structures are shown in Fig. 11. The
structural parameters for all single phase Sm Ca CoO refined by the Rietveld
2
-r
r
4-δ
analysis are listed in Table 5.
The values of oxygen content for SmCaCoO4-δ at room temperature exhibited
slight deficit but were reproducible in both methods used in the present work:
3
.96±0.01 measured by the TGA reduction and 3.95±0.04 measured by the
chromatometric titration. The TGA measurements show minor oxygen losses
starting at about 600ºC that corresponded to 0.01 in δ values at 1100ºC. These
results differ from those obtained by Taguchi et al. [53] and by Thorogood et al.
[55] who reported the oxygen content in equimolar samarium calcium cobaltite
being equal to 4.04 and 4.15, correspondently. Such discrepancy can be explained
by the differences in the preparation conditions. In the present work SmCaCoO4-δ
was quenched from 1100ºC in air while Taguchi et al. [53] had synthesized it at
9
50ºC with the following annealing at 700ºC for 12 h in the flow of oxygen and the
samples obtained in [55] were slowly cooled in oxygen from 1100ºC to room
temperature with the rate of 2 º/min.
Thermal expansion properties of SmCaCoO were evaluated from the results of
4
-δ
dilatometry and XRD measurements. The average TEC values calculated from
both methods are listed in Table 5. Although nonlinear behavior of the thermal
expansion plot for SmCaCoO in vicinity of 600ºC (Fig. 12) can be associated
4
-δ
with the beginning of oxygen release it also might indicate a phase transition. High
temperature in situ XRD measurements have shown no signs of phase transition in
SmCaCoO (Fig. 13). Another guess was that such behavior might be related to
4
-δ
some magnetic transformations accompanied with the change of spin states of Co
ions. However, the temperature measurements of the magnetic moment show that

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SmCaCoO remains in the paramagnetic state in the temperature range of 25-
4
-δ
9
00°C (Figure 14a). The typical paramagnetic hysteresis loop measured at 900°C
is shown in Figure 14b. Therefore, the bent of dilatometric plot was likely
associated with the oxygen losses although they are small enough.
The total conductivity of SmCaCoO4-δ is practically independent on the
temperature up to 600°C and then rises significantly up to 150 S/cm at 1100°C
(Fig. 15a). On the contrary, Seebeck coefficient is increasing up to 600°C and
further raise in temperature leads to some decrease (Fig. 15b). The average
oxidation state of Co ions at room temperature in SmCaCoO calculated from the
4
-δ
value of oxygen content is + 2.92. It means that the majority of cobalt ions are in
3
+
2+
the form of Co state and the small part of them are in the form of Co state. The
positive value of Seebeck coefficient proves that most mobile charge carriers are
electron holes. The noticeable rise of conductivity that starts at about 600°C might
associate with the increase of electron hole’s mobility. Similar dependencies for
the conductivity and Seebeck coefficient versus temperature were measured for
LaSrCoO [63], although the value of conductivity was significantly lower and
4
maximum for Seebeck coefficient was located at room temperature.
3
.2.5. Phase diagram
Using the XRD data from all quenched samples, we have divided the projection of
isobaric–isothermal phase diagram of the Sm – Ca – Co – O system onto the
compositional triangle ½ Sm O – CaO – CoO into 11 phase fields (Fig. 16).
2
3
Comparing phase equilibria in the ½ Sm O – CaO – CoO system in air at 885°C
2
3
[44] and at 1100°C obtained in the present work one can see the temperature
significantly affects the stability and homogeneity ranges of all phases except
Sm Ca CoO that remains practically unchanged. The introduction of Sm has
2
-r
r
4-δ
stabilized Ca Co O type structure forming the Ca Sm Co O solid solutions. In
3
2
6
3-y
y
2
6-δ
contrast, neither undoped Ca Co O nor partially substituted Ca Sm Co O have
3
4
9
3-x
x
4
9-z
been found to exist at 1100°C. The Ca-substitution for Sm in the perovskite
structure Sm Ca CoO was also found to be negligibly small.
1
-x
x
3-δ

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4
. Conclusion
The phase equilibria in the Sm – Fe – Co – O and Sm – Ca – Co – O systems at
T=1100°C in air are presented in the form of the isothermal-isobaric projections to
the compositional triangles ½ Sm O – ½Fe O – CoO and ½ Sm O – CaO – CoO.
2
3
2
3
2
3
The only SmFe Co O (0≤x≤1) solid solution was found in the ½ Sm O –
1
-x
x
3-δ
2
3
½
Fe O – CoO system as intermediate phase. The average TECs values are
2
3
decreasing with an increase of cobalt content in SmFe Co O solid solutions. All
1
-x
x
3
samples are chemically inert with respect to the Ce Sm O and Zr Y O
0
.8
0.2
2
0.85 0.15
2
electrolytes up to 1100°C. SmFe Co O demonstrated typical semiconductor-like
1
-x
x
3
behavior within the temperature range of 25-1100°C in air.
Two types of solid solutions: Ca Sm Co O (0.6≤y≤0.7) and Sm Ca CoO
3
-y
y
2
6-δ
2-r
r
4-δ
(
0.8≤r≤1) exist in the ½ Sm O – CaO – CoO system. Ca Sm Co O possesses
2
3
3-y
y
2
6-δ
the rhombohedral structure (sp. gr. R-3c) and Sm Ca CoO crystallized in the
2
-r
r
4-δ
tetragonal (sp. gr. I4/mmm) (r=1) or in the orthorhombic structure (Bmab sp. gr.)
r=0.8-0.9). Seebeck coefficients for Ca Sm Co O and SmCaCoO4-δ reveal
(
2.3
0.7
2
6-δ
positive values within the entire temperature range that indicate predominant p-
type conductivity.
Since all studied solid solutions are good conducting materials, chemically inert to
the solid electrolytes whose TEC values are comparable and can be considered as
possible candidates for cathodes in SOFCs application.
Acknowledgements
This work was financially supported by the Ministry of Higher Education and
Science within the State Task № 4.2288.2017

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Table 1 – Unit cell parameters and atomic positions of SmFe Co O (sp. gr.
1
-y
y
3-δ
Pbnm: Sm – (x, y, 0.25); Fe/Co – (0.5, 0, 0); O1 – (x, y, 0.25); O2 - (x, y, z)) refined
by the Rietveld analysis
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
a, Å
5.400
5.593
7.708
232.83
0.0118
5.390
5.577
7.691
231.19
0.0138
5.378
5.554
7.668
229.01
0.0094
5.364
5.520
7.634
226.03
0.0127
5.363
5.488
7.620
224.26
5.340
5.453
7.584
220.83
5.324
5.422
7.554
218.09
5.316
5.412
7.548
217.17
5.308
5.394
7.536
215.76
5.297
5.371
7.518
213.91
5.286
5.353
7.499
212.18
b, Å
c, Å
3
V, Å
x
y
x
y
x
y
z
-0.0075 -0.0094 -0.0077 -0.0113 -0.0033 -0.0089 -0.0110
Sm
-0.0551 -0.0559 -0.0544 -0.0554 -0.0527 -0.0479 -0.0462 -0.0486 -0.0464 -0.0462 -0.0463
-0.1180 -0.1188 -0.0839 -0.0421 0.0737
0.5378 0.5393 0.5589 0.5319 0.5231
0.0800
0.5179
0.0777
0.5217
0.0991
0.5140
0.0990
0.5203
0.1095
0.5146
0.0914
0.5094
O1
O2
-0.1995 -0.1949 -0.1932 -0.2226 -0.2243 -0.2241 -0.2058 -0.2234 -0.2318 -0.2312 -0.2209
0.2585
0.0510
0.2623
0.0422
0.2395
0.0582
0.2202
0.0588
0.1962
0.0488
0.1864
0.0395
0.1831
0.0377
0.1961
0.0400
0.2081
0.0414
0.1975
0.0247
0.2065
0.0346
Table 2 – The average thermal expansion coefficients of SmFe Co O ,
1
-y
y
3-δ
Ca Sm Co O and SmCaCoO in air
2
.3
0.7
2
6-δ
4-δ
-
6
-1
Composition
SmFe Co O
T,°C
800–1100°C
TEC×10 , K
21.6
0
.2
0.8
3
3
3
SmFe Co O
20.3
16.2
0
.5
0.5
SmFe Co O
0
.8
0.2
2
7
2
5
5-750ºC
50-1000ºC
5-580ºC
80-1100ºC
13.9
17.9
17.7
20.2
Ca Sm Co O
2
.3
0.7
2
6-δ
SmCaCoO4-δ
Table 3 – The unit cell parameters of Sm Ca O , extracted from the Rietveld
2
-z
z
3-δ
refinement results, and relevant R-factors*
Composition
Sm O
a, Å
b, Å c, Å
°β
R , % R , %
Br
f
14.177 3.633 8.847 99.960
-
-
2
3
Sm Ca O 14.168 3.626 8.847 100.095 0.752 0.996
1
.95
0.05
3
Sm Ca O 14.170 3.624 8.847 100.111 0.842 0.813
1
.9
0.1
3
*
R is the Bragg factor, R is structural factor
Br
f

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Table 4 – The unit cell parameters of Ca Sm Co O , extracted from the Rietveld
3
-y
y
2
6-δ
refinement results, and relevant R-factors
3
Composition
a, Å
9.079
9.074
c, Å
10.587
10.606
V, Å
RBr, %
0.752
0.842
Rf, %
0.996
0.813
Ca Sm Co O
872.7
873.3
2
.3
0.7
2
6-δ
Ca Sm Co O
2
.4
0.6
2
6-δ
Table 5 – The unit cell parameters and atomic coordinates for Sm Ca CoO with
2
-r
r
4-δ
r = 1 (sp.gr. I4/mmm: Sm/Ca – (0, 0, z); Co – (0, 0, 0); O1 – (0.5, 0, 0); O2 – (0, 0,
z)) and with r = 0.9 and 0.8 (sp.gr. Bmab: Sm/Ca – (0, y, z); Co – (0, 0, 0); O1 –
(0.25, 0.25, z); O2 – (0,y, z))
SmCaCoO4-δ Sm Ca CoO
Sm Ca CoO
1.1
0.9
4-δ
1.2
0.8
4-δ
sp.gr.
a, Å
b, Å
c, Å
I4/mmm
3.719
3.719
11.873
z
Bmab
5.265
5.302
11.833
y
5.280
5.331
11.850
y
z
z
(
O1
O2
Sm,Ca) 0.3584
0.0049
0.2500
0.3609 0.0060 0.3613
0.0087 0.2500 0.0101
0
0.1919
-0.0381 0.1662 -0.0452 0.1680

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