Phase equilibria and structure of solid solutions in the La-Co-Fe-O system at 1100°C

Article abstract of DOI:10.1023/B:INMA.0000041328.85504.3d

Phase equilibria in the La-Co-Fe-O system are studied at 1100°C in air using samples prepared by the citrate, nitrate, and conventional ceramic routes. The stability regions and structures of solid solutions in the La-Co-Fe-O system are determined by x-ra

Full text of DOI:10.1023/B:INMA.0000041328.85504.3d

Inorganic Materials, Vol. 40, No. 9, 2004, pp. 955–959. Translated from Neorganicheskie Materialy, Vol. 40, No. 9, 2004, pp. 1093–1097.
Original Russian Text Copyright © 2004 by Proskurina, Cherepanov, Golynets, Voronin.
Phase Equilibria and Structure of Solid Solutions
in the La–Co–Fe–O System at 1100°C
N. V. Proskurina*, V. A. Cherepanov*, O. S. Golynets*, and V. I. Voronin**
* Ural State University, pr. Lenina 51, Yekaterinburg, 620083 Russia
** Institute of Metal Physics, Ural Division, Russian Academy of Sciences,
ul. S. Kovalevskoi 14, Yekaterinburg, 620219 Russia
e-mail: natalya.proskurina@usu.ru
Received March 4, 2004
Abstract—Phase equilibria in the La–Co–Fe–O system are studied at 1100°C in air using samples prepared by
the citrate, nitrate, and conventional ceramic routes. The stability regions and structures of solid solutions in the
La–Co–Fe–O system are determined by x-ray powder diffraction: LaCo1 – yFeyO3 – δ (0 < y ≤ 0.25, sp. gr. R3c;
0.775 ≤ y < 1, sp. gr. Pbnm), Co1 – yFeyO (0 < y ≤ 0.13, NaCl-type structure, sp. gr. Fm3m), and Fe3 – xCoxO4
(0.84 ≤ x ≤ 1.38, sp. gr. Fd3m). The structural parameters of phase-pure solid solutions are determined by the
Rietveld method. The composition dependences of lattice parameters are presented for LaCo1 – yFeyO3 – δ (0 <
y ≤ 0.25) and Fe3 – xCoxO4 (0.84 ≤ x ≤ 1.38). The 1100°C isotherm of the pseudoternary system La2O3–CoO–
Fe2O3 in air is constructed.
INTRODUCTION
state reactions were conducted in air at temperatures
from 850 to 1100°C with several intermediate grindings.
The annealing time in the last step was 80–400 h.
ABO3 perovskite oxides with A = rare-earth and/or
alkaline-earth metals and B = Mn, Cr, Co, Ni, and Fe
have been recently the subject of intense attention
owing to their high catalytic activity and good electrical
and magnetic properties [1–4]. Of particular interest is
the use of Sr-, Ca-, and Ni-substituted cobaltites and
ferrites in fabricating gas-tight oxygen-ion-conducting
ceramic membranes for methane conversion applica-
tions [4, 5]. The broad application field of the materials
in question is due to the high stability of the perovskite
structure, which offers the possibility of widely varying
their oxygen stoichiometry and performing extensive
A- and/or B-site doping with no significant structural
changes. However, little or no data have been reported
on the phase equilibria in oxide systems containing
rare-earth and 3d transition metals.
In the nitrate and citrate processes, the starting
reagents were dissolved in a small excess of dilute
nitric acid. Next, two procedures were used. In one of
them, to the resultant solution was added crystalline cit-
ric acid hydrate powder, and the solution was boiled
down. The dry residue was slowly heated from 300 to
800°C with several isothermal holds. In the final step,
the sample was fired at 1100°C in air for 80–120 h. In
the other procedure, the nitrate mixture was melted and
decomposed at 200–300°C with vigorous stirring until
no gaseous reaction products were released. The result-
ant mixture was ground in an agate mortar and decom-
posed by firing at 800°C for 10 h. The powder thus
obtained was pressed at 5–10 MPa into disk-shaped
samples 8 mm in diameter, which were then heat-
treated at 1100°C for 24 h.
In this paper, we report our findings on the stability
regions and structures of solid solutions in the La–Co–
Fe–O system at 1100°C in air.
In the three procedures, the samples were cooled to
room temperature at a rate of 300°C/min. X-ray diffrac-
tion (XRD) studies were carried out on DRF-4.0 (phase
analysis) and DRON-UM1 (structural analysis) diffrac-
tometers (CuKα radiation, 2θ = 10°–75°). In structure
determination and refinement of lattice parameters, we
used the Rietveld profile analysis method.
EXPERIMENTAL
Samples for this investigation were synthesized by
standard solid-state reactions and by the nitrate and cit-
rate routes. Before synthesis, the starting reagents
(99.99+%-pure La2O3, extrapure-grade Co3O4 and
Fe2O3, and metallic cobalt and iron) were calcined in air
for 3–4 h to remove adsorbed gases and moisture: La2O3
at 1200°C, Co3O4 at 750°C, and Fe2O3 at 500°C. Cobalt
RESULTS AND DISCUSSION
The phase equilibria and crystal structure of solid
and iron metals were obtained by reducing the corre- solutions in the La–Co–Fe–O system were studied at
sponding oxides at 600°C in flowing hydrogen. Solid- 1100°C, using 90 samples prepared as described above.
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956
PROSKURINA et al.
a, Å
a, b, c, Å
4.260
7.875
7.830
7.785
c
(a)
4.258
4.256
4.254
4.252
4.250
(a)
5.580
5.535
5.490
b
a
4.248
V, A3
77.3
V, A3
243
242
241
240
239
238
(b)
77.2
77.1
77.0
76.9
76.8
(b)
76.7
0.75
0.80
0.85
0.90
0.95
1.00 y
0
0.02 0.04 0.06 0.08 0.10 0.120.14 y
Fig. 1. (a) Unit-cell parameters and (b) volume as functions
of Fe content for LaCo
tions.
Fe O (0.775 ≤ y < 1) solid solu-
Fig. 2. (a) Unit-cell parameter and (b) volume as functions
of Fe content for Co Fe O (0 < y ≤ 0.13) solid solutions.
1 – y
y 3
1 – y
y
To ascertain whether LaCo1 – yFeyO3 – δ solid solu-
tions exist at 1100°C in air, we prepared samples with
0 < y ≤ 1.0 at 0.05 intervals. As shown by XRD exami-
nation of quenched samples, LaCo1 – yFeyO3 – δ solid
solutions exist in the composition ranges 0 < y ≤ 0.25
LaCo1 – yFeyO3 – d
solid
solutions.
Earlier,
LaCo1 − yFeyO3 – δ solid solutions were prepared by Rao
et al. [2] and Sagdahl et al. [3], but detailed data on the
synthesis and structure of these solid solutions are not
available in the literature.
Table 1. Structural parameters of LaCo1 – yFeyO3 (0.0 ≤ y ≤ 0.25) samples air-quenched from 1000°C (sp. gr. R3 c: La in
(0 0 0.25), Co(Fe) in (0 0 0), O in (X 0 0.25))
y
0
0.05
0.1
0.15
0.2
0.25
X
0.4765(4)
0.4379(3)
0.4434(3)
0.4525(3)
0.4490(4)
0.4508(2)
×3
×3
×6
2.8454
2.5900
2.6888
3.0581
2.3820
2.7098
3.0312
2.4145
2.7088
2.9823
2.4644
2.7051
3.0029
2.4468
2.7078
2.9964
2.4592
2.7108
l (La–O), Å
×2
×6
3.2697
3.3220
3.2734
3.3250
3.2765
3.3284
3.2788
3.3292
3.2795
3.3309
3.2849
3.3347
l (La–Co), Å
l (Co–O), Å
V, Å3
×6
1.9147
1.9419
1.9389
1.9323
1.9358
1.9369
335.4(0)
335.7(1)
335.9(2)
336.9(2)
337.4(1)
338.7(1)
a, Å
5.435(1)
5.440(0)
5.446(0)
5.447(0)
5.450(1)
5.456(0)
c, Å
13.079(3)
13.094(1)
13.106(1)
13.115(1)
13.118(1)
13.140(1)
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PHASE EQUILIBRIA AND STRUCTURE OF SOLID SOLUTIONS
957
1/2(La2O3)
0
1.0
0.2
0.8
Co
II
I
III
0.4
0.6
LaCoO3
0.6
LaFeO3
0.4
V
IX
VII
IV
0.8
0.2
VIII
VI
1.0
0
0.6
0.8
1.0
0
0.2
0.4
CoO
1/2(Fe2O3)
CoFe2O4
Fe
5
Fig. 3. 1100°C isotherm of the pseudoternary system La O –CoO–Fe O at p = 0.21 × 10 Pa; (I–IX) see Table 3.
2
3
2 3
O2
radius of Fe3+ (r = 0.645 Å for CN = 6) compared to that
of Co3+(r = 0.61 Å for CN = 6) [6].
Solid solutions with 0.775 ≤ y < 1 crystallize in the
LaFeO3 structure (orthorhombic symmetry, sp. gr.
Pbnm). Their unit-cell parameters as functions of Fe
content are shown in Fig. 1.
and 0.775 ≤ y < 1. The use of starting mixtures prepared
by the nitrate or citrate route reduced the synthesis time
from 240 to 70 h.
The XRD patterns of phase-pure solid solutions
with 0 < y ≤ 0.25 could be indexed in a rhombohedral
The samples with 0.25 < y < 0.775 consisted of a
unit cell (sp. gr. R3c). The unit-cell parameters of these
solid solutions and the positional parameters and bond
distances in their structure are listed in Table 1. With
increasing Fe content, the unit-cell parameters and vol-
mixture of the R3 c and Pbnm terminal solid solutions.
Phase equilibria in the Co–Fe–O system. Under
the conditions typically used to study phase equilibria
ume increase steadily, in line with the larger ionic in 3d transition metal–oxygen systems, the equilibrium
Table 2. Structural parameters of Fe3 – xCoxO4 solid solutions (sp. gr. Fd3m: Fe in (0.125 0.125 0.125), Co(1) in (0.5 0.5
0.5), Co(2) in (0 0 0), O(1) in (X Y Z))
x
0.9
1.0
1.05
1.2
X = Y = Z
a, Å
0.254(1)
8.378(1)
0.261 (2)
8.386 (1)
0.259 (4)
8.385 (1)
0.259 (3)
8.366 (1)
V, Å3
587.9(1)
589.8 (1)
589.5 (1)
585.5 (1)
RB
4.29
3.44
4.28
3.48
5.29
4.79
3.25
3.74
Rf
INORGANIC MATERIALS Vol. 40 No. 9 2004

958
PROSKURINA et al.
samples annealed at 1100°C for 120–200 h (except for
y = 0.15 and 0.2) were single-phase. The XRD patterns
of the samples with y > 0.13 showed, along with reflec-
tions from the major phase CoO, peaks attributable to a
spinel structure. The CoO-based solid solutions belong
to the Fm3m space group. It is well known [8] that
cobalt oxide in air is a nonstoichiometric, cation-defi-
cient compound, Co1– zO, with a disordered arrange-
ment of the Co vacancies. Fe substitution for Co must
increase the deviation from stoichiometry, reducing the
lattice parameters of the material, as observed in our
data (Fig. 2).
Table 3. Phase fields in the composition triangle of the La–
Co–Fe–O system
Phase field in Fig. 3
Phases in equilibrium
I
LaCo1 – yFeyO3 (0.0 ≤ y ≤ 0.25)
La2O3
II
LaCo0.75Fe0.25O3
LaCo0.225Fe0.775O3
La2O3
III
IV
V
LaCo1 – yFeyO3 (0.775 ≤ y ≤ 1.0)
La2O3
La–Co–Fe–O phase diagram. Phase relations in a
quaternary system can be represented using a tetrahe-
dron. A more convenient, planar representation can be
obtained using the method of sections. This method,
however, is inapplicable to the system under consider-
ation because the starting 3d metal oxides are in differ-
ent oxidation states, and their compositions and the
compositions of the reaction products do not all lie in
the same plane. For this reason, we used projections
onto the plane of metallic components, an approach
often used to represent such systems. The oxygen con-
tent of condensed phases in each point of a projection
is assumed to be equal to the thermodynamically equi-
librium value and is not indicated in the composition
triangle.
LaCo1 – yFeyO3 (0.0 ≤ y ≤ 0.25)
Co1 – yFeyO (0.0 ≤ y ≤ 0.13)
LaCo0.75Fe0.25O3
LaCo0.225Fe0.775O3
Co0.87Fe0.13
LaCo0.225Fe0.775O3
Co0.87Fe0.13
O
VI
O
Co1.38Fe1.62O4
VII
LaCo1 – yFeyO3 (0.775 ≤ y ≤ 0.97)
CoxFe3 – xO4 (0.84 ≤ x ≤ 1.38)
LaCo0.03Fe0.97O3
Based on XRD data, the composition triangle of the
VIII
La–Co–Fe–O system at 1100°C and pO = 0.21 ×
2
Co0.84Fe2.16O4
105 Pa can be divided into nine phase fields (Fig. 3).
The triangles in the phase diagram represent the com-
positions studied. The designations of the phase fields
are explained in Table 3.
Co0.06Fe1.94O3
IX
LaCo1 – yFeyO3 (0.97 ≤ y ≤ 1.0)
Fe2 – xCoxO3 (0.0 ≤ x ≤ 0.03)
CONCLUSIONS
phases in the Co–Fe–O system are CoO (NaCl struc-
ture) and hematite, Fe2O3. Earlier, this system was
reported to contain solid solutions based on these
oxides [7–9] and also Fe3 – xCoxO4 spinel solid solu-
tions [1], but these solid solutions have not been studied
in sufficient detail, and available data are, to some
extent, contradictory.
The stability regions and structures of solid solu-
tions in the La–Co–Fe–O system at 1100°C in air are
determined.
ACKNOWLEDGMENTS
This work was supported by the US Civilian R & D
Foundation (project REC-005), the RF Ministry of Edu-
cation (Basic Research in the Natural Sciences, grant no.
E02-5.0-221), and grant no. 03-03-20006 BNTS_a.
As shown by XRD examination of quenched sam-
ples, hematite-based Fe2 – xCoxO3 solid solutions exist
in the composition range 0 < x ≤ 0.03. Thus, under the
conditions of this study (1100°C, air) the Co2O3 solu-
bility in Fe2O3 (about 3 mol %) is notably lower than
that reported earlier [8, 9].
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INORGANIC MATERIALS Vol. 40 No. 9 2004

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