X-ray diffraction investigation of phase stability in the Co-Cr-O and the Fe-Co-Cr-O systems in air at 1323 K

Article abstract of DOI:10.1016/j.jallcom.2005.04.187

Mixtures of Co₃O₄, Cr₂O₃, and Fe₂O₃ were annealed in air for times up to 147 h at 1323 K. Phase analysis was performed using X-ray diffraction. The mixtures of Cr ₂O₃ and Co₃O₄ powders reacted forming mixed oxides with the spinel structure. In addition to an oxide with the spinel structure an oxide with the rock salt structure was identified in the cobalt-rich mixtures, suggesting that these compositions belong to a two-phase field in the Cr-Co-O phase diagram. In the Cr-rich mixtures, both non-reacted Cr₂O₃ and CoCr₂O₄ were identified. The decomposition temperatures of Co₃O₄, Co ₂CrO₄, and Co_1.5Cr_1.5O₄ were estimated with thermogravimetry. On the basis of the results obtained a part of the Cr-Co-O phase diagram was constructed. Incorporation of Fe cations in the Co-Cr spinel was observed to expand the spinel lattice. Qualitative phase analyses of the Fe-Cr-Co oxide mixtures indicate that connecting the phase fields in the Fe-Co-O and Cr-Co-O system gives a fair prediction of the phase fields in the Fe-Cr-Co-O system at 1323 K (in air).

Full text of DOI:10.1016/j.jallcom.2005.04.187

Journal of Alloys and Compounds 402 (2005) 194–200
X-ray diffraction investigation of phase stability in the Co–Cr–O and the
Fe–Co–Cr–O systems in air at 1323 K
a,b,∗,1
b
b
a
Anette N. Hansson
, Søren Linderoth , Mogens Mogensen , Marcel A.J. Somers
a
Department of Manufacturing Engineering and Management, Technical University of Denmark, Kemitorvet b. 204, DK-2800 Kgs. Lyngby, Denmark
b
Materials Research Department, Risø National Laboratory, Frederiksborgvej 399 P.O. Box 49, DK-4000 Roskilde, Denmark
Received 7 April 2005; received in revised form 24 April 2005; accepted 27 April 2005
Available online 7 July 2005
Abstract
Mixtures of Co
diffraction.
The mixtures of Cr
structure an oxide with the rock salt structure was identified in the cobalt-rich mixtures, suggesting that these compositions belong to a
two-phase field in the Cr–Co–O phase diagram. In the Cr-rich mixtures, both non-reacted Cr and CoCr were identified.
The decomposition temperatures of Co , Co CrO were estimated with thermogravimetry. On the basis of the results
obtained a part of the Cr–Co–O phase diagram was constructed.
3
O
4
, Cr
2
O
3
, and Fe
2 3
O were annealed in air for times up to 147 h at 1323 K. Phase analysis was performed using X-ray
2
O
3
and Co O powders reacted forming mixed oxides with the spinel structure. In addition to an oxide with the spinel
3 4
2
O
3
2 4
O
3
O
4
2
4 4
, and Co1.5Cr1.5O
Incorporation of Fe cations in the Co–Cr spinel was observed to expand the spinel lattice. Qualitative phase analyses of the Fe–Cr–Co
oxide mixtures indicate that connecting the phase fields in the Fe–Co–O and Cr–Co–O system gives a fair prediction of the phase fields in
the Fe–Cr–Co–O system at 1323 K (in air).
©
2005 Elsevier B.V. All rights reserved.
Keywords: Phase diagram; Mixed Co–Cr oxides/spinels; Mixed Fe–Co–Cr oxides/spinels
1
. Introduction
773 and 1173 K [4–7]. Various Cr,Co-spinel compositions
were synthesised, mainly from the nitrates of Co and Cr by
prolonged annealing at a given temperature [4–6] and charac-
terised. Makkonen [4] obtained spinels after 240 h at 1173 K,
Bracconi et al. [5] after 500 h at 1123 K and O’Neill [6] after
40 h at 1073 K. Hanck and Laitinen [7] obtained Co2CrO4
by constant current electrolysis of K2CrO4 with CoCl2 at
773 K. The lattice parameters, a, of the mixed Cr–Co-spinels
as reported in the literature and those identified in the present
work are collected in Fig. 1. The solid line in Fig. 1 was cal-
culated using the equation O’Neill [6] obtained from a least
squares regression of his data (cf. Eq. (1)).
One of the candidate materials for the interconnector in
solid oxide fuel cell (SOFC) stacks is high chromium con-
taining metallic substrate coated with a Co3O4 [1] or a Co-
containing perovskite (e.g. LaCoO3 [1], (La,Sr)CoO3 [1–3])
layer. Oxidation of the interconnector material occurs dur-
ing operation of SOFCs at high temperature. In this respect
the coating/substrate interaction and the coating/oxidizing
atmosphere interaction become of importance. Crystallo-
graphic and thermodynamic data relating to the Co–Cr–O and
Fe–Co–Cr–O systems are scarce. Previously, the solid solu-
tion of Cr2O3 and Co3O4 with the spinel structure (hereafter
denoted spinel) has been studied at temperatures between
2
˚
a(± 0.0002)/A = 8.0832 + 0.2641 xCr − 0.0147(xCr) ,
3+
3+
3+
)
xCr = Cr /(Cr + Co
(1)
∗
Corresponding author. Tel.: +45 452 522 13; fax: +45 459 362 13.
The lattice parameter follows Vegard’s law [8], i.e. increases
linearly proportional to the fraction of the Cr-cations in
the spinel. Magnetic susceptibility and neutron diffraction
E-mail address: anh@ipl.dtu.dk (A.N. Hansson).
Now at Department of Manufacturing Engineering and Management,
1
Technical University of Denmark.
0
925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2005.04.187

A.N. Hansson et al. / Journal of Alloys and Compounds 402 (2005) 194–200
195
Fig. 3. The lattice parameter of Cr2−xFe2xO3. Data from the JC-PDS
database.
Fig. 1. The lattice parameter of the Co3−xCrxO4 spinel measured at room
temperature vs. the Cr content (×). The lattice parameters represented by
the solid line were calculated using Eq. (1) [6].
Thermodynamically, Fe2O3 and Cr2O3 are the most sta-
ble oxides of Fe and Cr, respectively, in air at 1323 K (and
at temperatures below). The oxides form a continuous series
of solid solutions according to the Fe–Cr–O phase diagram
data strongly suggest that Co3O4 [9], Co3−xCrxO4 [5] and
CoCr2O4 [10] are normal spinels at room temperature.
3+
Hence, the Cr -cations are expected to be located at the
[
13]. Lattice parameters of Cr2−2xFe2xO3 from the JC-PDS
3+
same positions as the Co -cations [4].
database show that substituting Cr-ions with Fe-ions in an
oxidewiththecorundumstructure(henceforthentitledcorun-
dum) results in larger a- and c-lattice parameters as shown in
Fig. 3.
The phase stability (in air) of the various types of oxides
in the Fe–Co–O system is shown in Fig. 2 as a function of the
fraction of Fe-cations [11]. It shows that spinel Co3O4 trans-
forms to CoO with the rock salt structure (hereafter named
rock salt) at about 1173 K [11]. At 1323 K, the CoxFe3−xO4
spinel is monophase for 0.67 < x < 1.46; monophase rock salt
develops for x > 2.45 [11].
Only few investigations were dedicated to the Fe–Co–Cr
oxide system. The investigations mainly focus on the cation
distribution in the spinel phase [12,13]. Rietveld refine-
ment of room temperature neutron diffraction data obtained
M o¨ ssbauer spectroscopy of CoxFe3−xO4 showed that for
3+
from CoFeCrO4 suggested that Cr -ions were located in
2
+
2+
x ≤ 1 the Co -cations replace the Fe -cations, whereas
2+
3+
the octahedral sites, whereas Co -ions and Fe -ions were
distributed over the remaining octahedral and tetrahedral
sites [14]. In situ M o¨ ssbauer spectroscopy of slowly cooled
3+
for x > 1 it is the Fe -cations, which are replaced by the
3
+
3+
Co -cations. The Co -ions were only found on octahe-
dral interstices with respect to the anion sublattice [11].
Rietveld refinement of X-ray diffraction patterns obtained
from CoFe2O4 and FeCo2O4 powders indicated that Fe -
cations and Co -cations are distributed over the tetrahedral
and octahedral positions (with respect to the anion sublattice)
(10 K/h) CoFe1.9Cr0.1O4 showed that cation site exchange
occurred during heating for temperatures above 295 K [15].
In the present work the oxide phases developing in the
Co–Cr–O and Fe–Co–Cr–O systems were investigated under
conditions of relevance for SOFC cathode conditions.
3+
2
+
[12].
2. Experimental
2.1. Synthesis of the mixed oxides
Mixtures of 1 g were prepared from Co3O4 (manufacturer:
Alfa Aesar, purity > 99.7%, grain size: dp,average = 7.5 ␮m),
Cr2O3 (manufacturer: Merck, purity > 99%, grain size:
dp,average = 2.3 ␮m) and Fe2O3 (manufacturer: Alfa Aesar,
purity > 99.945%, grain size: dp,average = 6.8 ␮m) in the cation
ratios given in Tables 1 and 2. The powders were crushed and
mixed in a mortar for approximately 5 min. The Co–Cr oxide
mixtures were placed in Al2O3 containers and annealed three
times at 1323 K in air. In the 1st and the 3rd run, the powders
were heated at 100 K/h and cooled at 300 K/h. In the 2nd run,
Fig. 2. [11] The Fe–Co oxide phase diagram in air at 1000–2100 K. x is x in
CoxFe3−xO4± δ.

1
96
A.N. Hansson et al. / Journal of Alloys and Compounds 402 (2005) 194–200
◦
◦
Table 1
or 10–100 . The 2θ-step size was either 0.05 (1st run, slits: 8
The investigated cobalt and chromium cation ratios
◦
and 0.3 mm) or 0.03 (2nd and 3rd run, slits: 8 and 0.15 mm).
Co:Cr 1:5
Co:Cr 1:2
Co:Cr 1:1
Co:Cr 2:1
Co:Cr 5:1
Peak positions were assessed by careful visual inspection
of each individual line profile. Relative intensities (IR) are
defined with respect to the strongest peak. STOE’s software
package was used for peak calibration and calculation of the
lattice parameters. The positions of the Si peaks (4–7 peaks)
were compared with the theoretical positions of Si; deviations
from theoretical positions were described as a function of
2θ with a straight line. The lattice parameters of the phases
developed were determined by minimisation of the absolute
difference of the 2θ values.
Co
Cr
1.00
5.08
1.00
1.98
1.00
0.97
1.97
1.00
4.83
1.00
the heating rate was 200 K/h and the powders were quenched
in air after the annealing. The annealing time after reaching
1
3
323 K was 56 h in the 1st run, 85 h in the 2nd and 6 h in the
rd.
The Fe–Cr–Co oxide mixtures were annealed twice. The
mixtures were held at temperature for 56 h in the 1st run and
2 h in the 2nd run. The heating and cooling rates were 100
7
2.4. Thermal analysis
and 300 K/h in the 1st run and 200 K/h and air quenching in
the 2nd run.
The decomposition temperatures of Co O , “Cr:Co 1:2”
3
4
The 2nd anneal was carried out to ensure that the mix-
tures had reached equilibrium, whereas the purpose of the
(Section 2.2) and “Co:Cr 1:1” were determined with ther-
mogravimetry (TGA), using a Netzsch STA 409C/D. The
heating rate was 300 K/h up to 773 K; thereafter the heating
rate was 60 K/h. The temperature, where the sample weight
started to fall, was taken as an indication of the decomposition
temperature.
3rd anneal was to investigate whether the cooling rate had
an influence on the phase constitution of the powders. In-
between two successive anneals, the mixtures were re-mixed
in a mortar for approximately 5 min.
2
.2. Synthesis from the nitrates of Co and Cr
3. Results
The powder was produced according to the glycine–nitrate
combustion method [16]. An aqueous solution of
3.1. The Cr–Co oxide mixtures
Cr(NO3)3·9H2O (manufacturer: Alfa Aesar, purity:
The lattice parameters and the relative intensities of the
9
9.999%) and Co(NO3)2·6H2O (manufacturer: Merck,
oxide phases identified after each annealing treatment of the
Cr–Co oxide mixtures are given in Table 3. The “Cr:Co 2:1”
and “Cr:Co 1:1” mixtures are homogeneous spinel phases
after the 2nd anneal. In the “Cr:Co 5:1” mixture coexistence
of the same spinel phase as for the “Cr:Co 2:1” mixture
and non-reacted Cr2O3 were observed. The lattice param-
eter of the spinel phase identified in the “Cr:Co 5:1” and the
purity: 99%), respectively, were prepared. The metal con-
centration was determined by transforming a known amount
of each solution into the pure oxide by annealing the solution
for 5 h at 1073 K in air. A Cr–Co solution with the metal
ratio Cr:Co equal to 1:2.09 was mixed with glycine. Water
was evaporated until combustion occurred, during which the
powder was formed. The powder was annealed for 65 h at
“
Cr:Co 2:1” mixtures is comparable to the value reported
1
123 or 1323 K and quenched in air.
˚
for CoCr2O4 (JC-PDS 22-1084): a = 8.3299 A. The lattice
parameter found for the homogeneous spinel phase identi-
fied in the “Cr:Co 1:1” mixture could not be compared to any
of the Cr–Co-spinel phases reported in the JC-PDS database.
In the “Cr:Co 1:2” and “Cr:Co 1:5” mixtures two spinel
phases and a rock salt phase were identified. The relative
intensities of the identified phases did not change from the
2
.3. X-ray diffraction analysis
The annealed mixtures were characterised with X-ray
diffraction (XRD) at room temperature. Silicon powder was
used as a reference in the XRD measurements following the
1
st and 2nd run. The XRD measurements were performed
1
st to the 3rd anneal of “Cr:Co 1:2” mixture, which strongly
on an STOE (θ–θ) diffractometer with Cu K␣ radiation. The
suggeststhatthereactionhascompletedduringthe1stanneal.
The identified spinel phases have lattice parameters
smaller than those observed for the spinel phase in the “Cr:Co
diffractometer was equipped with an energy disperse detec-
◦
tor. The diffractograms were obtained in the 2θ range 15–90
1:1” mixture and for CoCr2O4. The smallest lattice param-
Table 2
eters observed for a spinel phase, are slightly larger than
reported for Co3O4 (a = 8.084 A; JC-PDS 43-1003). It is
The investigated iron, chromium and cobalt cation ratios
˚
Fe:Cr:Co
:6:3
Fe:Cr:Co
1:4:2
Fe:Cr:Co
1:1:2
Fe:Cr:Co
1:2:4
Fe:Cr:Co
1:3:6
noted that the lattice parameter of the starting powder Co O
is in accordance with the reported JC-PDS value. The largest
of the lattice parameters found for the spinel phases in the
3
4
1
Co 3.00
2.00
4.00
1.00
1.97
1.00
1.00
3.95
1.95
1.00
6.10
3.04
1.00
Cr
Fe
6.00
1.00
“
Cr:Co 1:2” and “Cr:Co 1:5” mixtures is larger than the value
˚
reported for Co2CrO4 (a = 8.17 A; JC-PDS 24-0326). The lat-

A.N. Hansson et al. / Journal of Alloys and Compounds 402 (2005) 194–200
197
Table 3
The lattice parameter (a, c) (± 0.001 A˚ ) and the relative intensity (IR) of the starting materials and of the oxides formed during annealing of the chromium and
cobalt oxide mixtures in air
Corundum (R − 3¯ c)
Spinel (Fd3m)
¯
¯
Rock Salt (Fm3m)
Cr:Co
Anneal
a ( A˚ )
c ( A˚ )
IR
a ( A˚ )
IR
a ( A˚ )
IR
Cr2O3
4.959
4.959
4.959
13.594
13.591
13.594
1
0.73
0.69
5
2
1
1
1
:1
:1
:1
:2
:5
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
8.333
8.333
1
1
4.961
13.591
0.01
8.333
1
8.333
1
8.331
1
8.312 & 8.263
8.267
0.2 & 1
1
8.267
1
8.249 & 8.090
1 & 0.24
4.268
0.05
8.249 & 8.089
8.227 & 8.087
8.241 & 8.094
8.234 & 8.088
8.084
1 & 0.25
0.47 & 1
0.58 & 0.12
0.62 & 1
1
4.265
4.261
4.262
4.267
0.05
0.12
1
0.13
Co3O4
Anneal 1: 100 K/h—56 h at 1323 K—300 K/h. Anneal 2: 200 K/h—85 h at 1323 K—air quenching. Anneal 3: 100 K/h—6 h at 1323 K—300 K/h.
tice parameter of the rock salt phase is slightly larger than
that of CoO (a = 4.26 A; JC-PDS 43-1004). In the diffrac-
the selected samples a transformation from Co-rich spinel to
Co-rich rock salt phase is held responsible for the observed
weight loss. This interpretation is consistent with the low
temperature XRD results for low and high cooling rates (cf.
Table 3).
˚
tograms obtained after the second run, i.e. after quenching
fromtheannealingtemperature, thefractionofrocksaltphase
is considerably higher than in the diffractograms of the slowly
cooled samples. The reverse applied for the fraction of spinel
˚
phase with lattice parameter of approximately 8.09 A: a larger
3.4. The Fe–Cr–Co oxide mixtures
abundance of spinel phase in the diffractograms of the slowly
cooled samples. This observation indicates that the spinel
The lattice parameters and the relative intensities of the
oxidephasesidentifiedafterannealingoftheFe–Cr–Cooxide
mixtures are given in Table 4.
˚
with a = 8.09 A developed during cooling, as a result of a
conversion of rock salt CoO to spinel Co3O4.
A single oxide phase was obtained after annealing of the
Fe:Cr:Co 1:1:2” mixture at 1323 K. CoFeCrO4 is the only
3
.2. “Cr:Co 1:2” oxide powder obtained from the
“
nitrates of Co and Cr
ternary Fe–Cr–Co oxide included in the JC-PDS database
˚
(
a = 8.34 A, 28-1234). Both the composition and the lattice
The obtained powder was very inhomogeneous and con-
sisted of green and black particles. After annealing at
parameter of this spinel differ from those identified in the
“
“
Fe:Cr:Co 1:1:2” mixture. The X-ray diffractogram of the
Fe:Cr:Co 1:1:2” spinel phase is shown in Fig. 4.
1
323 K the powder consisted of two spinel phases (a = 8.248,
˚
˚
8
.092 A) and a rock salt phase (a = 4.265 A), which is equiv-
Annealing of the “Fe:Cr:Co 1:6:3” and “Fe:Cr:Co 1:4:2”
alent to the result obtained by mixing of the Cr2O3 and
Co3O4 oxide powder. Only one spinel phase was identified
mixtures resulted in the development of both corundum and
spinel phases. The spinel peaks in the diffractograms showed
a low intensity shoulder at higher 2θ values, especially after
quenching. This is attributed to in-homogeneities in the spinel
phase (perhaps caused by cation exchange during cooling).
The lattice parameters of the identified phases in the two
mixtures are similar and the phases may very likely be the
same. As compared to the spinel phase in the “Fe:Cr:Co
1:1:2” mixture, the lattice parameter of the spinel phases in
the “Fe:Cr:Co 1:6:3” and “Fe:Cr:Co 1:4:2” mixtures is much
larger; the lattice parameter of the corundum phase is slightly
larger than that of the Cr2O3 starting powder.
˚
after annealing at 1123 K. The lattice parameter was 8.207 A,
˚
which is larger than that reported for Co2CrO4 (a = 8.17 A,
JC-PDS 24-0326).
3
.3. Thermal analysis of selected powders
Co3O4 transformed to CoO at 1163 K. A weight loss is
observed due to oxygen release.
The weight started to decrease at 1270 K for the “Cr:Co
1
:2” oxide mixture and at 1426 K for the “Co:Cr 1:1” oxide
mixture. The weight continued to decrease until the heat-
ing was stopped, indicating that the phase transformation
was not completed when thermal analysis was terminated. In
Two spinel phases and a rock salt phase formed during
annealing of the “Fe:Cr:Co 1:2:4” and “Fe:Cr:Co 1:3:6” mix-
tures. Both spinel phases have lattice parameters smaller than

1
98
A.N. Hansson et al. / Journal of Alloys and Compounds 402 (2005) 194–200
Table 4
The lattice parameter (a, c) (± 0.001 A˚ ) and the relative intensity (IR) of the oxides formed during annealing of the “Fe:Cr:Co” oxide mixtures in air
Fe:Cr:Co
¯
¯
¯
Rock Salt (Fm3m)
Anneal
Corundum (R − 3c)
Spinel (Fd3m)
a ( A˚ )
c ( A˚ )
IR
a ( A˚ )
IR
a ( A˚ )
IR
1:6:3
1:4:2
1:3:6
1:2:4
1:1:2
1
2
1
2
1
2
1
2
1
2
4.971
5.000
4.894
4.988
13.673
13.614
13.934
13.673
0.02
0.02
0.03
0.06
8.350
8.347
8.353
8.357
8.269 & 8.098
8.265
8.274 & 8.096
8.273
8.299
8.297
1
1
1
1
1 & 0.08
1
1 & 0.05
1
1
1
4.259
4.259
4.270
4.261
0.03
0.13
0.01
0.07
Anneal 1: 100 K/h—56 h at 1323 K—300 K/h. Anneal 2: 200 K/h—72 h at 1323 K—air quenching.
the “Fe:Cr:Co 1:1:2” and larger than Co3O4. The rock salt
phase has a lattice parameter somewhat larger than that of
CoO. The spinel phase with a lattice parameter of approxi-
to 1 at.% using Eq. (1). Since the spinel phase results from a
rock salt to spinel transition during cooling, it is expected that
the rock salt phase contained a similar amount of Cr-cations
at high temperature. At 1273 K CoO in air can maximally
dissolve 1 at.% Cr [17].
˚
mately 8.10 A is present in larger amounts after slow cooling
than after quenching. The reverse is observed for the rock salt
phase. This suggests that the spinel phase forms as a result
of a rock salt to spinel transition during cooling.
The lattice parameter of the “Cr:Co 1:1” spinel and
CoCr2O4 agrees excellently with those reported in the lit-
erature on spinels with similar composition (cf. Fig. 1). The
data in Fig. 1 suggest that annealing of the “Cr:Co 1:2” and
“Cr:Co 1:5” mixtures results in a single spinel phase. In the
corresponding literatures [2–5], the spinels were mainly syn-
thesised from metal ion nitrates, which resulted in mixing of
the metal ions on an atomic scale. It could be argued that the
coexistence of three oxide phases instead of a single spinel
phase after 147 h of annealing of the “Cr:Co 1:2” and “Cr:Co
1:5” mixtures at 1323 K is due to insufficient homogenisation
time for the present oxide mixtures. In this respect it should
be mentioned that annealing of the “Cr:Co 1:2” oxide pow-
der, obtained from the nitrates of Co2+ and Cr , for 65 h at
1323 K also showed the occurrence of two spinel phases and
a rock salt phase. It is noted that the temperature was higher
and that the reaction time was longer than in the investigation
of O’Neill [6] for the synthesis of Co2CrO4. Furthermore,
using the same annealing time (65 h) for a powder at 1123 K,
a single spinel phase formed. Clearly, insufficient reaction
time at 1323 K cannot explain the occurrence of more than
one oxide phase. Instead the results suggest that a two-phase
field, containing a spinel phase and a rock salt phase occurs in
Cr–Co oxide system at 1323 K in air, rather than a continuous
series of solid solutions ranging from Co3O4 to CoCr2O4.
The observation of a continuous weight decrease for tem-
peratures above 1270 K during the TGA measurements of
4
. Discussion
4
.1. Phase stability in Cr–Co oxide mixtures
˚
The lattice parameters of the spinel phase with a = 8.09 A
and the rock salt phase, identified in the “Cr:Co 1:2” and
Cr:Co 1:5” mixtures, are slightly larger than those of the
“
Co3O4 starting powder and CoO, respectively. Since the lat-
tice parameter of the Co–Cr spinel phases increases with
increasing Cr-content (Fig. 1), it is expected that this spinel
phase contains some Cr-cations. The Cr-content is estimated
3+
2
+
“
Cr:Co 1:2” oxide powder, obtained from the nitrates of Co
3+
and Cr (65 h at 1123 K), corroborate this interpretation and
indicate that the fraction of rock salt phase increases with
temperature.
UsingEq. (1), theminimumCr-contentofthemixedspinel
phase is estimated at approximately 19 at.% (at 1323 K), at
lower Cr-contents a two-phase region of this spinel phase and
Fig. 4. The X-ray diffractogram of the “Fe:Cr:Co 1:1:2” spinel. The Si-peaks
are obtained from Si-reference powder.

A.N. Hansson et al. / Journal of Alloys and Compounds 402 (2005) 194–200
199
Fig. 5. An illustration of the Cr–Co–O phase diagram in air (the dashed
line represents an approximate boundary for the two-phase field between
the rock salt and the spinel).
Fig. 6. The lattice parameters of the spinel phases obtained after annealing
the “Cr:Co 1:2”, “Fe:Cr:Co 1:3:6”, “Fe:Cr:Co 1:2:4” and “Fe:Cr:Co 1:1:2”
mixtures at 1323 K in air vs. the Fe-content.
a rock salt phase, containing appr. 1 at.% Cr, forms. Since
the lattice parameter of the corundum phase obtained in the
in this work is plotted into the diagram; they are labelled with
their cation ratio numbers only.
“
Cr:Co 5:1” mixture is similar to that of the Cr2O3 starting
It is evident that the “Fe:Cr:Co 1:1:2” (“112”) mixture lies
within the homogeneous spinel phase field. It is therefore
not surprising that only one phase is identified. Likewise,
the isothermal section of the quasi-ternary system predicts
that the “Fe:Cr:Co 1:2:4” (“124”) and the “Fe:Cr:Co 1:3:6”
powder, it is suggested that Co-cations is not dissolved in
Cr2O3 to any appreciable extent.
Data relating to the phase stability of oxides in the
Co–Cr–O system in air are summarized in Fig. 5. The squares
represent the temperatures at which Co3O4, Co2CrO4 and
Co1.5Cr1.5O4 start to decompose into spinel and rock salt
phases. A tentative shape of the spinel + rock salt two-phase
region is indicated as well.
(“136”) mixtures are within a two-phase field where a rock
salt phase and a spinel phase coexist in equilibrium, which
was indeed found.
Corundum and spinel phases developed during the anneal-
ing of the “Fe:Cr:Co 1:4:2” (“142”) and the “Fe:Cr:Co 1:6:3”
4
.2. Phase stability in Fe–Cr–Co oxide mixtures
(“163”) mixtures, which also is in accordance with their loca-
tion in the quasi-ternary diagram. The results indicate that
connecting the phase field of the Cr–Co–O, the Fe–Co–O,
and the Fe–Cr–O systems provides a useful prediction of the
The lattice parameters of the spinel and corundum phases
developed in the “Fe:Cr:Co 1:6:3” and “Fe:Cr:Co 1:4:2” mix-
tures are larger than those in the CoCr2O4 and the Cr2O3
starting powders. Since the CoCr2O4 is the mixed Cr–Co-
spinel with the largest lattice parameter (cf. Fig. 1) this shows
that the spinel lattice has expanded upon incorporating iron
cations. Analogously, lattice parameters for the corundum
phase larger than for the Cr2O3 starting powder, strongly
indicate that iron is incorporated in the corundum phase (cf.
Fig. 3: iron expands the lattice).
The lattice parameters of the main spinel phase identified
in the “Fe:Cr:Co 1:3:6”, “Fe:Cr:Co 1:2:4” and “Fe:Cr:Co
1
:1:2” mixtures (Table 4) are larger than those obtained for
the “Cr:Co 1:2” mixture, which has the same Cr: Co ratio
Table 3). Fig. 6 shows that the lattice parameter of these
(
spinel phases increases linearly with the Fe-content.
A Gibbs triangle in the Fe–Cr–Co–O system at 1323 K is
given in Fig. 7. The diagram has been constructed by con-
necting the phase field boundaries of the Cr–Co–O system
suggested in Fig. 5 (the black circles in Fig. 7) with the
Fe–Co–O [11] (the black squares in Fig. 7) and the Fe–Cr–O
Fig. 7. Comparison of the Cr–Co–O, Fe–Co–O [11], and Fe–Cr–O [13]
systems at 1323 K in air and the phases identified in the “Fe:Cr:Co” mixtures
annealed at 1323 K in air. The numbers refer to the cation ratios of the
mixtures.
[13] systems at 1323 K in air. The phase constitution in rela-
tion to the composition of the “Fe:Cr:Co” mixtures studied

2
00
A.N. Hansson et al. / Journal of Alloys and Compounds 402 (2005) 194–200
phase boundaries in the Fe–Cr–Co–O system at 1323 K in
air.
with increasing Cr-content in accordance with existing
literature.
Substitution of Fe-cations into a Cr–Co-spinel results in an
expansion of the spinel lattice. Phase analyses of Fe–Cr–Co
oxide mixture showed a new Fe:Cr:Co (1:1:2) spinel with a
4
.3. The use of Co, Cr, (Fe)-containing components in
SOFCs cathode environment
˚
lattice parameter: a = 8.298 A. A pragmatic prediction of the
On start-up and operation of SOFCs, an interaction is
expected to occur between the interconnect material and its
neighbouring component (i.e. coating, contact layer, or cath-
ode). Co–Cr spinel layers have been observed to develop
as a result of reaction diffusion between the Cr2O3 oxide
layer formed on the steel interconnector and Co-containing
coatings [1–3]. Such layers are also likely to develop in
the contact areas between a Co-containing cathode, i.e.
La0.6Sr0.4Fe0.8Co0.2O3, and a non-coated steel interconnec-
tor. It is common to heat up to a temperature higher than the
operation temperature at the start-up of a SOFC stack with
glass sealings, which have to be softened in order to deform.
The layers shouldstay compact and well adheringto the metal
as well as to the cell. Thus, it is important to avoid phase
changes (or at least any big changes in molar volume of the
involved phases) during start-up as well as during operation
isothermal section at 1323 K of the Fe–Cr–Co oxide phase
diagram in air is obtained by connecting the phase boundaries
in the Cr–Co oxide, the Fe–Co oxide and the Fe–Cr oxide
phase diagrams. This is in a favourable correspondence with
the experimental phase constitution.
These data contributes to a basis for designing the com-
position and the processes of protective contact coatings on
FeCr stainless steels for interconnectors in SOFC stacks.
References
[
[
1] Y. Larring, T. Norby, J. Electrochem. Soc. 147 (9) (2000) 3251–3256.
2] W.J. Quadakkers, H. Greiner, M. H a¨ nsel, A. Pattanaik, A.S. Khanna,
W. Mall e´ ner, Solid State Ionics 91 (1996) 55–67.
[3] T. Kadowaki, T. Shiomitsu, E. Matsuda, H. Nakagawa, H.
Tsuneizumi, Solid State Ionics 67 (1993) 65–69.
[
[
4] R.J. Makkonen, Suom. Kemistil. B 35 (1962) 230–234.
5] P. Bracconi, L. Berthod, L.-C. Dufour, Ann. Chim. France 4 (1979)
(i.e. thermal cycling) as this may introduce additional stress,
which in worst case can result in loss of electric contract in
the SOFC.
The work presented here is believed to be a useful con-
tribution to the necessary background knowledge for SOFC
stack developers.
3
31–338.
[6] H.S.T.C. O’Neill, Mineral. Mag. 67 (3) (2003) 547–554.
[
[
7] K.W. Hanck, H.A. Laitinen, J. Inorg. Nucl. Chem. 33 (1971) 63–73.
8] W.D. Kingery, H.K. Bowen, D.R. Uhlmann, Introduction to Ceram-
ics, 2nd ed., Wiley, USA, 1976, pp. 131–139.
[
9] W.L. Roth, J. Phys. Chem. Solids 25 (5) (1964) 1–10.
[
[
10] N. Menyuk, K. Dwight, A. Wold, J. Phys. 25 (5) (1964) 528–536.
11] P.J. Murray, J.W. Linnett, J. Phys. Chem. Solids 37 (1976) 619–
624.
5
. Conclusion
[
12] T.A.S. Ferreira, J.C. Waerenborgh, M.H.R.M. Mendon c¸ a, M.R.
Nunes, F.M. Costa, Solid State Sci. 5 (2003) 383–392.
13] J.R. Taylor, A.T. Dinsdale, Z. Metallkd. 84 (5) (1993) 335–345.
14] A.K. Azad, S.-G. Eriksson, S.M. Yunus, J. Eriksen, H. Rundl o¨ f,
Physica B 327 (2003) 1–8.
Literature data strongly suggest that the mixed Co–Cr
oxides form a continuous series of solid solutions with the
spinel structure ranging from Co3O4 to CoCr2O4 in air at
[
[
[
[
[
15] S.W. Lee, S.Y. An, S.B. Kim, G.Y. Ahn, C.S. Kim, J. Korean Phys.
1
023 and 1173 K. The present work shows that the Cr–Co–O
Soc. 37 (4) (2000) 443–446.
16] L.A. Chick, L.R. Pederson, G.D. Maupin, J.L. Bates, L.E. Thomas,
G.J. Exarhos, Mater. Lett. 10 (1/2) (1990) 6–12.
17] M. Ijjaali, K. Kowalski, T. Bak, B. Dupre, C. Gleitzer, J. Nowotny,
M. Rekas, C.C. Sorrell, Ionics 7 (2001) 351–359.
phase diagram at 1323 K contains a two-phase field where
a rock salt phase, containing approximately 1 at.% Cr, and
a spinel phase with a Cr content of approximately 19 at.%
coexist. The lattice parameter of the Cr–Co-spinel increases