A Study of the Crystal Structure of SmCaCo1?xFexO4?δ and Sm0.9Ca1.1Fe1?yCoyO4?δ Solid Solutions

Article abstract of DOI:10.1134/S0022476619050111

Complex oxides with general compositions SmCaCo_1?xFe_xO_4?δ and Sm_0.9Ca_1.1Fe_1?yCo_yCO_4?δ are synthesized using the glycerol-nitrate technique at 1100 °C in air. By powder X-ray diffraction it is determined that SmCaCo_1?xFe_xO_4?δ solid solutions exist in a composition range 0 ≤ x ≤ 0.3 and Sm_0.9Ca_1.1Fe_1?yCo_yO_4?δ solid solutions exist in a composition range 0 ≤ y ≤ 0.7. The samples with high concentrations of cobalt ions are found to crystallize in the tetragonal unit cell (space group I4/mmm), whereas the solid solutions enriched with iron ions have the orthorhombic structure (space group Bmab). For all single phase samples the unit cell parameters and volume and the structural parameters (atomic coordinates and bond lengths) are calculated by the full-profile Rietveld method.

Full text of DOI:10.1134/S0022476619050111

ISSN 0022-4766, Journal of Structural Chemistry, 2019, Vol. 60, No. 5, pp. 789-795. © Pleiades Publishing, Inc., 2019.
Russian Text © The Author(s), 2019, published in Zhurnal Strukturnoi Khimii, 2019, Vol. 60, No. 5, pp. 823-829.
A STUDY OF THE CRYSTAL STRUCTURE OF
SmCaCo1–xFexO4–δ AND Sm0.9Ca1.1Fe1–yCoyO4–δ
SOLID SOLUTIONS
A. P. Galayda, N. E. Volkova, A. A. Startseva,
L. Ya. Gavrilova, and V. A. Cherepanov
Complex oxides with general compositions SmCaCo1–xFexO4–δ and Sm0.9Ca1.1Fe1–yCoyO4–δ are synthesized
using the glycerol-nitrate technique at 1100 °C in air. By powder X-ray diffraction it is determined that
SmCaCo1–xFexO4–δ solid solutions exist in a composition range 0 ≤ x ≤ 0.3 and Sm0.9Ca1.1Fe1–yCoyO4–δ solid
solutions exist in a composition range 0 ≤ y ≤ 0.7. The samples with high concentrations of cobalt ions are
found to crystallize in the tetragonal unit cell (space group I4/mmm), whereas the solid solutions enriched
with iron ions have the orthorhombic structure (space group Bmab). For all single phase samples the unit
cell parameters and volume and the structural parameters (atomic coordinates and bond lengths) are
calculated by the full-profile Rietveld method.
DOI: 10.1134/S0022476619050111
Keywords: complex oxides, Ruddlesden–Popper phases, ferrites, cobaltites, powder X-ray diffraction
analysis, crystal structure.
INTRODUCTION
Oxide materials based on rare earth elements and 3d transition metals are now broadly used in the design of many
electrochemical devices, such as high and intermediate temperature fuel cells, oxygen membranes, semiconductor gas sensors
[1-6]. The research interest in these compounds stems from a set of their physicochemical properties (high electron and ion
conductivity, stability in an oxidizing environment, thermal stability in a broad temperature range) largely determined by
their crystal structures [2, 3, 5, 7]. Oxides with the general composition Ln2TO4 (where Ln is the rare earth element (REE), T
is the 3d metal atom) with a layered structure formed of alternating blocks with perovskite and rock salt structures are
promising cathode materials for solid-oxide fuel cells (SOFC). However, unlike nickelates Ln2NiO4, REE ferrites and
cobaltites Ln2(Co,Fe)O4 do not form in air [8]. Partial substitution of alkaline-earth metals (AEM) for REE ions is
accompanied by an increase in the average oxidation state of the 3d metal ions, resulting in the stabilization of the above
structure [9-19]. Additional substitution in B-sublattice allows us to obtain materials with a high mixed electron-ion
conductivity and acceptable thermal expansion coefficients [20-23].
Calcium-substituted samarium cobaltites Sm2–xCaxCoO4–δ exist at 1100 °C in air in a narrow composition range
(0.8 ≤ x ≤ 1.0) [18]. The crystal structure of SmCaCoO4–δ oxide is described with the tetragonal unit cell of the space group
Institute of Natural Sciences and Mathematics, Ural Federal University, Ekaterinburg, Russia;
anastasia.galaida@urfu.ru. Original article submitted December 03, 2018; revised December 24, 2018; accepted December
25, 2018.
0022-4766/19/6005-0789 © 2019 by Pleiades Publishing, Ltd.
789

I4/mmm. A decrease in the calcium concentration in Sm2–xCaxCoO4–δ solid solutions (x = 0.8 and 0.9) is accompanied by
a change in the crystal structure to orthorhombic (space group Bmab). Such a symmetry reduction with a change in the ratio
between REEs and AEMs in A-sublattice has previously been noted for Ln2–xCaxCoO4 δ oxides with medium- and small-
radius lanthanides (Sm, Eu, Gd) [19]. There is practically no literature data on the existence of substituted ferrites with the
general composition Ln2–xCaxFeO4 δ. It is known that when the temperature decreases to 1100 °C, CaLaFeO4–δ complex oxide
synthesized at atmospheric pressure and 1500 °C decomposes into lanthanum ferrite LaFeO3–δ and calcium oxide CaO [24].
The work aims to determine the effect of cobalt and iron ion concentrations on the homogeneity ranges and the
crystal structures of SmCaCo1–xFexO4–δ and Sm0.9Ca1.1Fe1–yCoyO4–δ solid solutions.
EXPERIMENTAL
The samples for the study were synthesized using the glycerol-nitrate technique. The following precursors were used
to prepare the samples: samarium oxide Sm2O3 (SmO-L grade) and calcium carbonate CaCO3 (analytical grade), which were
preliminary annealed to remove adsorbed moisture and gases, metallic cobalt, iron oxalate FeC2O4·2H2O (analytical grade),
nitric acid HNO3 (analytical grade) to dissolve the precursors, and glycerol (analytical grade) as a chelating agent and organic
fuel for the reaction mixture pyrolysis. Metallic cobalt was obtained by reduction from Co3O4 oxide (high purity grade) at
500-600 °C in the hydrogen flow. The final annealing was carried out at 1100 °C in air for 120 h with intermediate grinding
in the ethanol medium and subsequent slow cooling to room temperature.
The phase composition of the annealed samples was determined by powder X-ray diffraction. The phase
composition of the samples was determined on an Equinox-3000 diffractometer (CuK radiation, in an angle range 2θ = 10-
α
90° with a step of 0.012°). The phases were identified using the ICDD database and the Fpeak software package (Institute of
Natural Sciences and Mathematics, Ural Federal University).
The structures of the samples under study were refined by the full-profile Rietveld method using the Fullprof 2008
software.
RESULTS AND DISCUSSION
SmCaCo Fe O solid solutions. In order to investigate whether it is possible to obtain complex oxides with the
1–x x 4–δ
general composition SmCaCo1–xFexO4–δ using the glycerol-nitrate technique the samples with 0 ≤ x ≤ 0.5 with a step ∆x = 0.1
were synthesized. From the powder X-ray diffraction (XRD) data it is found that the homogeneity range of
SmCaCo1–xFexO4–δ solid solutions lies within 0 ≤ x ≤ 0.3. The crystal structure of oxide with x = 0.1, which is similar to
unsubstituted samarium cobaltite SmCaCoO4–δ [18], is described with the tetragonal unit cell of the space group I4/mmm.
A subsequent increase in the iron content (x = 0.2 and 0.3) results in the symmetry reduction to orthorhombic (space group
Bmab). Fig. 1 depicts the X-ray diffraction patterns of SmCaCo0.9Fe0.1O4–δ (a) and SmCaCo0.7Fe0.3O4–δ (b) complex oxides
refined by the full-profile Rietveld method.
For all single-phase SmCaCo1–xFexO4–δ (0 ≤ x ≤ 0.3) samples the unit cell parameters and volumes, the atomic
coordinates (Table 1), the average bond lengths and the angles between the metal–oxygen bonds (Table 2) were calculated
from the X-ray diffraction data.
Both types of the structures are characterized by the statistical distribution of metal atoms in A and B sublattices.
The 3d metal ions (Con+/Fen+) occupy the crystallographic site 2a (0, 0, 0). In the samples with the tetragonal structure, the
samarium and calcium ions occupy the site 4e (0, 0, z), the oxygen ions (O1 and O2) occupy sites 4c (0, 1/2, 0) and
4e (0, 0, z). In the orthorhombic structure, cations (Sm3+/Ca2+) are displaced in the A sublattice along the crystallographic y
axis relative to the respective sites in the tetragonal structure. The tilt of the oxygen octahedra (CoO6) is accompanied by
a displacement of O1 and O2 oxygen ions along the z and y axes respectively. Then, in the description of the crystal structure
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Fig. 1. X-ray diffraction data for SmCaCo0.9Fe0.1O4–δ (a) and SmCaCo0.7Fe0.3O4–δ (b)
refined by the Rietveld method. Dots are the experimental data; the upper solid line is
the theoretical profile; the lower solid line is the difference between the experimental
data and the theoretical curve.
TABLE 1. Unit Cell Parameters and Atomic Coordinates for SmCaCo1–xFexO4–δ (0 ≤ x ≤ 0.3) and Sm0.9Ca1.1Fe1–yCoyO4–δ
(0 ≤ y ≤ 0.7) Solid Solutions
Sm / Ca
O1
O2
Composition Space group a, Ǻ
b, Ǻ
c, Ǻ
RBr,% Rf,%
y
z
z
y
z
x = 0.0 [15]
x = 0.1
x = 0.2
x = 0.3
y = 0.0
y = 0.1
y = 0.2
y = 0.3
y = 0.4
y = 0.5
y = 0.6
y = 0.7
I4/mmm*
3.719(1)
3.729(1)
11.873(1) 0.000(0) 0.358(1) 0.000(0) 0.000(0) 0.192(1)
–
–
11.854(1) 0.000(0) 0.360(1) 0.000(0) 0.000(0 0.170(0) 4.16 3.14
Bmab
5.286(1) 5.305(1) 11.861(1) –0.004(1) 0.361(1) –0.005(1) 0.049(1) 0.171(1) 5.90 7.14
5.297(1) 5.323(1) 11.875(1) –0.004(1) 0.361(1) –0.008(1) 0.056(1) 0.170(1) 4.34 5.84
5.390(1) 5.452(1) 12.039(1) –0.016(1) 0.356(1) –0.018(1) 0.058(1) 0.174(1) 6.97 6.97
5.378(1) 5.443(1) 11.982(1) –0.011(1) 0.358(1) –0.015(1) 0.062(1) 0.169(1) 4.75 5.79
5.359(1) 5.420(1) 11.963(1) –0.001(1) 0.358(1) –0.015(1) 0.058(1) 0.172(1) 5.41 6.57
5.338(1) 5.388(1) 11.948(1) 0.006(1) 0.358(1) –0.015(1) 0.009(1) 0.171(1) 4.09 8.42
5.344(1) 5.400(1) 11.942(1) –0.008(1) 0.358(1) –0.019(1) 0.054(1) 0.173(1) 7.41 7.41
5.331(1) 5.379(1) 11.927(1) –0.008(1) 0.359(1) –0.018(1) 0.050(1) 0.173(1) 7.12 7.43
5.302(1) 5.332(1) 11.901(1) –0.006(1) 0.359(1) –0.012(1) 0.037(1) 0.174(1) 6.81 6.81
I4/mmm
3.748(1)
11.889(1) 0.000(0) 0.359(1) 0.000(0) 0.000(0) 0.173(1) 4.51 3.21
* The atomic sites for the space group I4/mmm: Sm/Ca (0, 0, z); Co/Fe (0, 0, 0); O1 (0.5, 0, 0); O2 (0, 0, z); space
group Bmab: Sm/Ca (0, y, z); Co/Fe (0, 0, 0); O1 (0.25, 0.25, z); O2 (0, y, z).
of the samples the 2 ap× 2 ap×c unit cell should be used, where aр is the unit cell parameter of the ideal cubic perovskite;
c corresponds to the c unit cell parameter of the tetragonal phase of the K2NiF4 type. Fig. 2 shows the models of unit cells of
complex oxides with tetragonal (a) and orthorhombic (b) structures constructed using the Diamond software.
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TABLE 2. Average Bond Lengths and Angles Between the Metal–Oxygen Bondsin SmCaCo1–xFexO4–δ (0 ≤ x ≤ 0.3) and
Sm0.9Ca1.1Fe1–yCoyO4–δ (0 ≤ y ≤ 0.7)
Composition
A–O1, Ǻ
B–O1, Ǻ
A–O2, Ǻ
B–O2, Ǻ
O1–B–O1, deg
O1–B–O2, deg
x = 0.0
x = 0.1
x = 0.2
x = 0.3
y = 0.0
y = 0.1
y = 0.2
y = 0.3
y = 0.4
y = 0.5
y = 0.6
y = 0.7
2.484(2)
2.492(1)
2.502(1)
2.515(1)
2.586(2)
2.566(1)
2.555(1)
2.546(2)
2.550(1)
2.485(1)
2.520(2)
2.509(2)
1.857(1)
1.864(1)
1.881(1)
1.884(1)
1.929(2)
1.921(1)
1.910(1)
1.905(2)
1.909(1)
1.903(2)
1.886(1)
1.874(2)
2.284(2)
2.261(5)
2.593(2)
2.593(2)
2.634(1)
2.643(2)
2.702(1)
2.610(3)
2.614(1)
2.608(1)
2.596(1)
2.672(2)
1.999(2)
2.011(2)
2.017(1)
2.047(2)
2.103(1)
2.020(2)
2.052(2)
2.049(2)
2.074(1)
2.081(2)
2.083(2)
2.017(1)
90(0)
90(0)
90(0)
90(0)
89.7(1)
89.8(1)
88.6(1)
88.8(1)
89.1(1)
90.0(1)
90.0(1)
89.9(1)
89.9(1)
90(0)
93.3(1)
91.9(1)
91.8(1)
90.6(1)
90.7(1)
94.4(1)
93.5(1)
92.4(1)
91.4(1)
90(0)
Fig. 2. Model of the unit cell of complex oxides with tetragonal (a) and
orthorhombic (b) structures.
It is found that an increase in the concentration of iron ions in the SmCaCo1–xFexO4–δ solid solution results in
an increase in the unit cell parameters and volume due to a large iron ion radius as compared to the cobalt ion (for the
octahedral environment rHS/LS(Fe3+) = 0.645/0.55 Ǻ, rHS/LS(Co3+) = 0.61/0.545 Ǻ [25]).
In accordance with Vegard's rule, the concentration dependence of the unit cell volume for the SmCaCo1–xFexO4–δ
samples is linear (Fig. 3). For comparison, the unit cell volumes of the samples with the orthorhombic structure are reduced
to the tetragonal unit cell volume.
Sm Ca Fe Co O solid solutions. According to the powder XRD data, samarium-calcium ferrite
0.9 1.1 1–y y 4–δ
Sm0.9Ca1.1FeO4–δ crystallizes in the orthorhombic unit cell of the space group Bmab. Cobalt doping of Sm0.9Ca1.1FeO4–δ ferrite
in the B sublattice results in the formation of several Sm0.9Ca1.1Fe1–yCoyO4–δ solid solutions with 0 ≤ y ≤ 0.7. The X-ray
diffraction patterns of the samples with 0.1 ≤ y ≤ 0.6 are described with the orthorhombic perovskite-like unit cell of the
space group Bmab, while complex oxide with the cobalt content y = 0.7 has the tetragonal structure (space group I4/mmm).
792

Fig. 3. Dependence of the unit cell volume of the
SmCaCo1–xFexO4–δ solid solution on the concentration
of iron ions.
Fig. 4. X-ray diffraction data for Sm0.9Ca1.1Fe0.6Co0.4O4–δ refined by the
Rietveld method.
Fig. 4 depicts the X-ray diffraction pattern of Sm0.9Ca1.1Fe0.6Co0.4O4–δ complex oxide refined by the full-profile Rietveld
method.
For all single-phase Sm0.9Ca1.1Fe1–yCoyO4–δ oxides the structural parameters were calculated (Tables 1 and 2). With
an increase in the cobalt concentration in the samples the parameters monotonically decrease, which can be explained in
terms of size effects. The concentration dependences of the Sm0.9Ca1.1Fe1–yCoyO4–δ unit cell parameters are given in Fig. 5.
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Fig. 5. Dependence of the unit cell volume of the
Sm0.9Ca1.1Fe1–yCoyO4–δ solid solution on the
concentration of cobalt ions.
CONCLUSIONS
The SmCaCo1–xFexO4–δ (0 ≤ x ≤ 0.5) and Sm0.9Ca1.1Fe1–yCoyO4–δ (0.0 ≤ y ≤ 0.7) solid solutions were synthesized
using the glycerol-nitrate technique at 1100 °C in air. It is established that the REE/AEM ratio in the A sublattice and the
Fe/Co ratio in the B sublattice substantially affect the crystal structures of the samples: SmCaCo1–xFexO4–δ and
Sm0.9Ca1.1Fe1–yCoyO4–δ solid solutions enriched by cobalt crystallized in the tetragonal unit cell and the samples with a high
concentration of iron ions crystallized in the orthorhombic unit cell. An increase in the iron ion content in the samples results
in a monotonic increase in the unit cell parameters and volume of the samples.
CONFLICT OF INTERESTS
The authors declare that they have no conflict of interests.
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