Phase equilibria at 1100°C in air and crystal structure of solid solutions in the system LaCoO3-SrCoO2.5- SrNiO 3 - LaNiO3

Article abstract of DOI:10.1007/s10789-005-0021-4

Phase equilibria in the LaCoO₃-SrCoO_2.5- SrNiO₃ - LaNiO₃ system are studied at 1100°C in air using samples prepared by standard solid-state reactions and by the citrate route. The composition region and structure of La _{1 - x}Sr_xCo_{1 - y}NiyO_{3 - δ} solid solutions are determined by x-ray powder diffraction. The structure is refined by the Rietveld profile analysis method with Fullprof 2002. The solid solutions with 0 < x ≤ 0.5 have a rhombohedrally distorted perovskite structure (sp. gr. R3c). The rhombohedral distortion decreases with increasing Sr content, and the solid solutions with x > 0.5 have an ideal cubic structure (sp. gr. Pm3m). The composition dependences of lattice parameters for the La_{1 - x}Sr_xCo_{1 - y}Ni_yO _{3 - δ} solid solutions are presented.

Full text of DOI:10.1007/s10789-005-0021-4

Inorganic Materials, Vol. 40, No. 12, 2004, pp. 1336–1340. Translated from Neorganicheskie Materialy, Vol. 40, No. 12, 2004, pp. 1520–1525.
Original Russian Text Copyright © 2004 by Aksenova, Gavrilova, Cherepanov.
Phase Equilibria at 1100°C in Air
and Crystal Structure of Solid Solutions
in the System LaCoO –SrCoO –“SrNiO ”–“LaNiO ”
3
2.5
3
3
T. V. Aksenova, L. Ya. Gavrilova, and V. A. Cherepanov
Gorki State University, pr. Lenina 51, Yekaterinburg, 620083 Russia
e-mail: aksenovatanya@yandex.ru
Received June 10, 2004
Abstract—Phase equilibria in the LaCoO –SrCoO –“SrNiO ”–“LaNiO ” system are studied at 1100°C in air
3
2.5
3
3
using samples prepared by standard solid-state reactions and by the citrate route. The composition region and
structure of La1 – xSr Co Ni O solid solutions are determined by x-ray powder diffraction. The structure
x
1 – y
y
3 – δ
is refined by the Rietveld profile analysis method with Fullprof 2002. The solid solutions with 0 < x ≤ 0.5 have
a rhombohedrally distorted perovskite structure (sp. gr. R3c ). The rhombohedral distortion decreases with
increasing Sr content, and the solid solutions with x > 0.5 have an ideal cubic structure (sp. gr. Pm3m). The
composition dependences of lattice parameters for the La1 – xSr Co Ni O solid solutions are presented.
x
1 – y
y
3 – δ
INTRODUCTION
The Sr–Co–O system was reported to contain
phases with the compositions Sr Co O and
was prepared at 1000°C
3
2
7 – δ
Multicomponent perovskite solid solutions are of
practical importance in the search for new materials for
technological applications. Owing to their stability in
oxidizing environments and at high temperatures, their
high electrical conductivity, and high oxygen mobility
in their structure, such solid solutions find application
as electrode materials for various electrochemical
devices, catalysts for oxidation, oxygen membranes,
etc. Their engineering performance is highly dependent
on the optimization of preparation conditions and pre-
cise knowledge of the thermodynamic stability of the
phases involved.
SrCoO3 − δ [7, 8]. Sr Co O
7 – δ
3
2
in air by the ceramic route [7]. This compound has a
broad homogeneity range: 0.94 < δ < 1.22. According
to x-ray diffraction (XRD) data, it has a tetragonal
structure at small δ and an orthorhombic structure at
large δ. The composition SrCoO represents a number
x
of phases, differing in oxygen content. Takeda et al. [8]
investigated the stability of SrCoO in the range 2.29 <
x
x < 2.8. According to their results, the phase stable at
high temperatures and low oxygen pressures has the
perovskite structure and a relatively low oxygen con-
tent. In the range 2.3 < x < 2.42, they observed the coex-
La1 – xSr CoO
solid solutions (0 < x ≤ 0.5) were istence of perovskite and brownmillerite phases. At
x
3 – δ
first prepared by Jonker and Van Santen [1] in air by a lower temperatures and high oxygen pressures,
standard ceramic route. Using annealing in pure oxy- SrCoO3 – δ also has a perovskite-like structure.
gen at 1100°C, Taguchi et al. [2] obtained
Phase equilibria in the LaCoO –SrCoO –
3
2.5
La Sr CoO solid solutions with 0.5 ≤ x < 1. The
1
− x
x
3 – δ
“SrNiO ”–“LaNiO ” system are essentially unex-
3 3
composition range of La1 – xSr CoO
solid solutions
x
3 – δ
plored. Huang et al. [9] synthesized single-phase
La1 − xSr Co Ni O
in air was reported to extend to 0.7 [3] or 0.8 [4].
La1 Sr CoO solid solutions with x < 0.5 have a
rhombohedrally distorted perovskite structure. The
rhombohedral distortion decreases with increasing Sr
content, and the x = 0.5 solid solution has an ideal cubic
structure.
samples with 0 ≤ x ≤ 0.3 and
3 – δ
x
1 – y
y
− x
x
3 – δ
0 ≤ y ≤ 0.2. Beyond this composition region, the sam-
ples contained La NiO as a major impurity phase.
2
4
In this work, we report our findings on the phase
equilibria in the LaCoO –SrCoO –“SrNiO ”–
3
2.5
3
“
LaNiO ” system at 1100°C in air and the stability
3
Gopalakrishnan et al. [5] obtained a continuous regions and structures of solid solutions in this system.
series of LaCo1 – yNi O solid solutions (0 < y < 1) in
y
3 – δ
the range 1073–1223 K in an oxygen atmosphere.
EXPERIMENTAL
According to Gavrilova et al. [6], the homogeneity
range of LaCo1 – yNi O
at 1100°C in air is 0 ≤ y ≤
Samples for this investigation were synthesized by
y
3 – δ
0
.625. The samples in that study were prepared by standard solid-state reactions and by the citrate route.
solid-state reactions.
Before synthesis, the starting reagents (LaO-D La O ;
2 3
0
020-1685/04/4012-1336 © 2004 MAIK “Nauka/Interperiodica”

PHASE EQUILIBRIA AT 1100°C IN AIR AND CRYSTAL STRUCTURE OF SOLID SOLUTIONS
1337
extrapure-grade SrCO , Co O and NiO; and metallic
RESULTS AND DISCUSSION
The phase equilibria in the LaCoO –SrCoO –
3
3
4
cobalt) were calcined to remove adsorbed moisture and
gases: La O at 1100°C for 24 h, NiO and Co O at
3
2.5
2
3
3
4
“SrNiO ”–“LaNiO ” system were studied at 1100°C in
3
3
7
00°C for 2 h, and SrCO at 600°C. Cobalt metal was
3
air using 131 samples of various metal compositions.
Examination of annealed La1 – xSr CoO samples
prepared by reducing Co O at 600°C in flowing
hydrogen.
3
4
x
3 – δ
with x = 0.8, 0.85, and 0.9 showed that solid solutions
in this system exist in the range x = 0–0.85 and that the
maximum degree of Sr substitution for La is between
x = 0.85 and 0.9, which slightly exceeds the values
reported earlier [3, 4].
Solid-state reactions were conducted in air at tem-
peratures from 850 to 1100°C with several intermediate
grindings in ethanol and quenching every 24 h. The
total annealing time at 1100°C was 240–316 h.
According to XRD results for quenched samples
In the citrate process, La O , NiO, SrCO , and Co prepared by standard solid-state reactions and the cit-
2
3
3
rate route, the solid-solution range of SrCo1 – zNi O at
metal were dissolved in nitric acid, and crystalline citric
acid hydrate (C H O · 2H O) powder was added to the
z
2.5
1
100°C in air is 0 < z ≤ 0.15. The solid solutions have
6
8
7
2
the brownmillerite structure (sp. gr. Pnma). Beyond this
composition range, the major impurity phase is an
Sr Co O7 – δ-based solid solution. The samples with
resultant solution. Next, the solution was boiled down
in a porcelain dish, and the dry residue was heated from
3
2
3
00 to 800°C with several isothermal holds. In the final
0
.15 < z ≤ 0.4 consisted of three phases: the terminal
step, the sample was fired at 1100°C in air for 5–10 h
with several intermediate grindings.
solid solution SrCo Ni O was in equilibrium with
0.85 0.15 2.5
an Sr Co O7 – δ-based solid solution and NiO. The sam-
3
2
The phase composition of the samples was deter- ples with 0.4 ≤ z ≤ 0.7 consisted of two phases: an
Sr Co O -based solid solution and NiO. The XRD
mined by XRD on a DRON-UM1 diffractometer (CuKα
radiation, 10° ≤ 2θ ≤ 70°). In structure determination
and refinement of lattice parameters, we used the
Rietveld profile analysis method (Fullprof 2002 pro-
gram).
3
2
7 – δ
patterns of the samples with 0.7 ≤ z ≤ 1 showed reflec-
tions from Sr Co Ni O , NiO, and SrO.
3
0.9
1.1 7 – δ
Gavrilova et al. [6] obtained LaCo1 – yNi O
solid
y
3 – δ
solutions in the range 0 < y ≤ 0.625. Beyond this com-
4
3
3
2
2
1
1
00
50
00
50
00
50
00
5
0
0
2
0
30
40
50
60
70
2
θ, deg
Fig. 1. Rietveld refinement profile for single-phase La0.9Sr0.1Co0.9Ni0.1
upper continuous line is the calculated profile, and the lower continuous line is the difference plot.
O
: the open circles represent the raw XRD data, the
3 – δ
INORGANIC MATERIALS Vol. 40 No. 12 2004

1
338
AKSENOVA et al.
α, deg
0 < x < 0.03 consisted of two phases: La4 – xSr Ni O
x 3 10
9
9
9
9
9
9
9
8
0.6
0.5
0.4
0.3
0.2
0.1
0.0
9.9
and NiO, and the samples with 0.03 < x < 0.08 con-
sisted of La3.95Sr0.05Ni O , NiO, and La1.85Sr0.15NiO4.
3
10
At higher Sr contents, the XRD patterns showed no
reflections from La4 – xSr Ni O . The samples with
x
3
10
0.08 < x < 0.4 contained NiO in equilibrium with
La2 − xSr NiO , and the samples 0.4 < x < 1 consisted of
x
4
the terminal solid solution La Sr NiO , NiO, and
1
.2 0.8
4
SrO [10].
The
XRD
1 − y
patterns
3 – δ
of
single-phase
La1 − xSr Co Ni O
samples could be indexed in a
x
y
rhombohedral unit cell (hexagonal setting), sp. gr.
R3c . Figure 1 shows the Rietveld refinement profile
for single-phase La Sr Co Ni O3 – δ.
0
.9 0.1
0.9
0.1
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
As in the case of lanthanum cobaltite, the rhombo-
hedral distortion of the perovskite structure of
x
La1 − xSr Co Ni O decreases with increasing Sr
x
1 – y
y
3 – δ
Fig. 2. Composition dependence of the pseudocubic-cell
angle for La1 – xSr Co0.9Ni0.1 solid solutions.
content, and the solid solutions with x > 0.5 have a
nearly ideal cubic structure (sp. gr. Pm3m). Figure 2
shows the composition dependence of the pseudocubic-
O
x
3 – δ
position range, the major impurity phase was cell angle for the La1 – xSr
La (Ni Co ) O . The samples with 0.6 ≤ y ≤ 0.8 con- series.
Co0.9Ni0.1O3 – δ solid-solution
x
4
1 − x
x 3 10
sisted of three phases: LaCo Ni O
and
0
.4
0.6 3 – δ
The quinary system La–Sr–Co–Ni–O is difficult to
La Ni Co O solid solutions and NiO. The XRD represent even by a space figure. Considering binary
4
2.1
0.9 10
patterns of the samples with 0.8 ≤ y ≤ 1 showed reflec- oxides as the constituent components of the system
does not resolve the problem since, under the condi-
tions from NiO and La (Ni Co ) O .
4
1 – x
x 3 10
tions of this study (1100°C in air), the thermodynami-
In the “LaNiO ”–“SrNiO ” system, no solid solu- cally stable oxides are La O , SrO, CoO, and NiO,
3
3
2
3
tions were obtained at 1100°C in air. The samples with while the oxidation state of 3d transition metals in the
Table 1. Crystal data for La1 – xSr Co Ni O
solid solutions air-quenched from 1100°C (sp. gr. R3c: La(Sr) in (0 0
x
0.9 0.1 3 – δ
0
.25), Co(Ni) in (0 0 0), O in (x 0 0.25))
x
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
a, Å
5.444
5.444
13.135
0.457
1.929
2.953
2.491
2.705
3.283
3.328
3.40
5.438
5.438
13.164
0.453
1.932
2.974
2.464
2.710
3.291
2.326
3.46
5.434
5.434
13.186
0.469
1.922
2.880
2.554
2.705
3.296
3.324
2.66
5.427
5.427
13.228
0.480
1.918
2.822
2.606
2.707
3.307
3.322
2.58
5.423
5.423
13.235
0.501
1.915
2.704
2.719
2.705
3.308
3.320
2.96
5.418
5.418
13.275
0.480
1.918
2.817
2.602
2.711
3.317
3.318
4.53
5.419
5.419
13.288
0.495
1.924
2.538
2.880
2.717
3.322
3.319
3.12
5.426
5.426
13.317
0.508
1.920
2.665
2.761
2.717
3.329
3.323
7.87
b, Å
c, Å
xO
l(Co–O), Å
l(La–O), Å
l(La–O), Å
l(La–O), Å
l(La–Co), Å
l(La–Co), Å
RBr
Rf
3.20
2.62
2.10
2.62
2.42
3.52
2.65
6.51
INORGANIC MATERIALS Vol. 40 No. 12 2004

PHASE EQUILIBRIA AT 1100°C IN AIR AND CRYSTAL STRUCTURE OF SOLID SOLUTIONS
a, c, Å
1339
a, c, Å
1
3.5
3.4
3.3
1
1
1
1
3.144
3.140
3.136
3.132
c
1
1
c
1
5
3.2
.46
~
~
~
~
5
5
5
.454
5
5
5
.44
.42
.40
a
.448
.442
a
0.10
0.15
0.20
0.25
0.30
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
y
x
Fig. 3. Composition dependences of lattice parameters for
La1 Sr Co0.9Ni0.1 solid solutions.
Fig. 4. Composition dependences of lattice parameters for
La0.9Sr0.1Co Ni O solid solutions.
O
− x
x
3 – δ
1 − y
y
3 – δ
forming La1 – xSr Co1 – yNi O
y 3 – δ
greater than 2+.
solid solutions is 0.8) solid solutions. With increasing strontium content,
a decreases and passes through a minimum at x = 0.6,
x
while c rises over the entire solid-solution range. Ni
Phase equilibria in systems where the oxidation
state of one or several components is composition-
dependent can be represented using metal composi-
tions. In this approach, the oxygen content of con-
densed phases is not indicated in the phase diagram and
is taken to be equal to its equilibrium value under given
substitution for Co increases a, c, and the unit-cell vol-
3
+
ume, since the sixfold-coordinated ionic radius of Ni
(
3
+
0.56 Å) is greater than that of Co (0.52 Å). Figure 4
shows the composition dependences of lattice parame-
ters for La Sr Co Ni O solid solutions. The
0.9 0.1
1 – y
y
3 – δ
crystal data for La Sr Co Ni O (0.1 ≤ y ≤ 0.3)
0
.9 0.1
1 – y
y
3 – δ
conditions (T, p ). Thus, the system under consider-
O
2
solid solutions air-quenched from 1100°C are summa-
ation can be represented by a composition tetrahedron
whose vertices correspond to mole fractions of metallic
rized in Table 2.
n
Σni
i
components (ξ = ------- , where the sum is over all the
i
Table 2. Structural parameters of La Sr Co Ni O
0.9 0.1 1 – y y 3 – δ
solid solutions air-quenched from 1100°C (sp. gr. R3 c:
La(Sr) in (0 0 0.25), Co(Ni) in (0 0 0), O in (x 0 0.25))
constituent components except oxygen) equal to unity
1
(
-- La O , SrO, CoO, NiO). The midpoints of the edges
2 3
2
y
0.1
0.2
0.3
then represent 1 : 1 phases (or compositions): LaCoO3,
SrCoO , SrNiO , and LaNiO (the last two phases do
not exist under the conditions of this study).
a, Å
b, Å
5.444
5.444
13.135
0.457
1.929
2.953
2.491
2.705
3.283
3.328
3.40
5.449
5.449
13.139
0.461
1.928
2.934
2.515
2.704
3.285
3.331
4.64
5.450
5.450
13.143
0.390
2.043
3.398
2.174
2.794
3.302
3.400
3.0
2
.5
3
3
Using XRD data for quenched samples, we located c, Å
the composition region of La1 – xSr Co Ni O solid
x
1 – y
y
3 – δ
xO
solutions at 1100°C in air. With increasing Ni content (y),
the maximum Sr content (x) of La Sr Co Ni O
l(Co–O), Å
l(La–O), Å
1
– x
x
1 – y
y
3 – δ
decreases because, upon the introduction of an alka-
line-earth metal into LaCo1 – yNi O3 – δ, the charge com-
y
pensation is due to an increase in the average degree of l(La–O), Å
oxidation of the 3d transition metals, while Ni substitu-
l(La–O), Å
tion for Co in La1 – xSr CoO
reduces it.
x
3 – δ
l(La–Co), Å
Table
1
0.9
presents the crystal data for
0.1 3 – δ
La1 − xSr Co Ni O
solid solutions inferred from l(La–Co), Å
x
XRD data.
RBr
Figure 3 displays the composition dependences of
lattice parameters for La1 – xSr Co Ni O (0.1 ≤ x ≤
Rf
3.20
4.06
2.93
x
0.9
0.1 3 – δ
INORGANIC MATERIALS Vol. 40 No. 12 2004

1
340
AKSENOVA et al.
SrCoO2.5
“SrNiO3”
solid-solution series was shown to lie between x = 0.85
and 0.9. In the SrCoO3 – δ–“SrNiO ” system, we obtained
3
XI
II
X
SrCo1 – zNi O solid solutions with 0 < z ≤ 0.15.
z
2.5
VIII
XRD results were used to map out the 1100°C iso-
XII
VII
therm of the system LaCoO –SrCoO –“SrNiO ”–
0
.8
.6
.4
.2
3
2.5
3
“
LaNiO ” in air.
3
IX
0
0
0
ACKNOWLEDGMENTS
This work was supported in part by the US Civilian
Research & Development Foundation (grant no. REC
005) and the Russian Foundation for Basic Research
(grant nos. Ural-04-03-96136 and 04-03-32118).
I
III
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3 – δ
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2 – y
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1
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2
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“
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ovskitelike Compound SrCoO (2.29 ≤ x ≤ 2.80), Z.
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3
Ni-Doped LaCoO and LaFeO Perovskites, J. Electro-
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3
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0. Bobina, M.A., Yakovleva, N.A., Gavrilova, L.Ya., and
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System, Fiz. Khim. (Moscow), 2004, vol. 78, no. 8,
pp. 1527–1530.
solutions was located. The structural parameters of the
solid solutions were determined by the Rietveld profile
analysis method. The limit of the La1 – xSr CoO
x
3 – δ
INORGANIC MATERIALS Vol. 40 No. 12 2004