The isothermal section of the Ce-Co-Sb ternary system at 400 °C

Article abstract of DOI:10.1016/j.jallcom.2008.03.061

Phase equilibria were established in the Ce-Co-Sb ternary system at 400 °C based on X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and energy dispersion spectroscopy (EDS) techniques. Fourteen binary compounds Ce₂₄Co_11<

Full text of DOI:10.1016/j.jallcom.2008.03.061

Journal of Alloys and Compounds 471 (2009) 60–63
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
◦
The isothermal section of the Ce–Co–Sb ternary system at 400 C
∗
R.M. Luo, F.S. Liu, J.Q. Li , X.W. Feng
College of Materials Science and Engineering, Shenzhen University and Shenzhen Key Laboratory of Special Functional Materials,
Shenzhen 518060, PR China
a r t i c l e i n f o
a b s t r a c t
◦
Article history:
Phase equilibria were established in the Ce–Co–Sb ternary system at 400 C based on X-ray powder
diffraction (XRD), scanning electron microscopy (SEM) and energy dispersion spectroscopy (EDS) tech-
niques. Fourteen binary compounds Ce24Co11 , CeCo2, CeCo3, Ce2Co7, Ce5Co19 , Ce2Co17 , CoSb, CoSb2, CoSb3,
Ce2Sb, Ce5Sb3, Ce4Sb3, CeSb and CeSb2 have been confirmed, and three ternary compounds CeCoSb3,
Received 7 December 2007
Received in revised form 12 March 2008
Accepted 12 March 2008
Available online 9 May 2008
◦
CeCo1−xSb2 and CeCo1.33 Sb2 are found in this system at 400 C. The isothermal section of this sys-
tem at 400 C consists of 20 single-phase regions, 40 two-phase regions and 21 three-phase regions.
The compound CeCo1−xSb2 has ZrCuSi2-type structure with space group P4/nmm, a = 0.3878 nm and
c = 0.9849 nm. The crystal structure for the compound CeCoSb3 is determined to be CeNiSb3-type (Pbcm)
with a = 1.27697 nm, b = 0.60601 nm and c = 1.8435 nm. The compound CeCo1.33 Sb2 is of orthorhombic sys-
tem with a = 0.6083 nm, b = 0.60118 nm and c = 1.04596 nm. The homogeneity range of CoSb phase is about
◦
Keywords:
Rare earth alloys and compounds
Phase diagram
X-ray powder diffraction
2
at.%. The solubilities for the other single-phase regions were not observed.
©
2008 Elsevier B.V. All rights reserved.
1
. Introduction
alloys were re-melted two or three times in order to achieve complete fusion and
homogeneous composition. All samples were subjected to annealing for homoge-
nization in evacuated quartz ampoules in a resistance furnace. The homogenization
temperatures of the alloys were chosen on the basis of the binary phase diagrams of
The Co–Sb system is drawing more and more attention due to
its special feature found in recent years. The compound CoSb3 in
this system has the typical skutterudite structure exhibiting excel-
lent transport properties, such as extremely high carrier mobility,
that are desirable for good thermoelectric performance [1,2]. Rare
earth-filled skutterudites have been identified as potential thermo-
electric materials due to their reduced thermal conductivity [3,4].
Understanding the phase relationships of Re–Co–Sb system is help-
ful for the development of relative materials. The investigations of
phase diagrams of the Eu–Co–Sb [5], Gd–Co–Sb [6], Nd–Co–Sb [7]
and Pr–Co–Sb [8] have been reported, while no reports have been
found on the interaction in the Ce–Co–Sb ternary systems. In this
work, we investigated the phase equilibria in the Ce–Co–Sb ternary
◦
the Ce–Sb, Ce–Co and Sb–Co systems. The homogenization was performed at 600 C
for 20 days for the samples containing equal to or more than 40 at.% Sb, while at
◦
900 C for 15 days for the samples containing less than 40 at.% Sb. They are all then
◦
◦
◦
cooled to 400 C at the rate of 10 C/min, kept at 400 C for 3 days and subsequently
quenched into liquid nitrogen. 118 alloy samples in total were prepared for this
investigation.
The treated alloys were powdered for X-ray powder diffraction (XRD) analy-
sis. The phases in each alloy were identified by XRD with Cu K␣ radiation (Bruker,
D8 Advance SS 18 kW) and JADE 5.0 software. The software Topas 3.0 was used for
Rietveld refinement [9]. The morphology and composition of the phase were inves-
tigated using a cold field emission scanning electron microscope (SEM) with energy
dispersive analysis (EDS) (HITACHI S-4700).
3. Results and discussion
◦
system at 400 C.
3.1. Phase analysis
2.
Experimental detail
Cerium (purity of 99.99%), antimony (purity of 99.95%) and cobalt (purity of
9.99%) were used as raw materials. The alloys with 40 at.% Sb or more were pre-
We have studied the binary systems Co–Sb, Ce–Co and Ce–Sb
at 400 C to identify the binary compounds. The X-ray diffraction
9
◦
pared by high frequency melting under a high purity argon in quartz tube which
was first evacuated to high vacuum to avoid the weight lost of the samples owing to
the high volatility of antimony. The alloys with the Sb content less than 40 at.% were
prepared in an electric arc furnace using a non-consumable tungsten electrode and
a water-cooled copper tray under the protection of a pure argon atmosphere. The
analysis confirmed the existence of the 14 binary compounds in
◦
the Ce–Co–Sb ternary system at 400 C: Ce24Co , CeCo , CeCo ,
11
2
3
Ce2Co7, Ce5Co19 and Ce2Co17 in the Ce–Co system; CoSb, CoSb2
and CoSb3 in the Co–Sb system; Ce Sb, Ce Sb , Ce Sb , CeSb and
2
5
3
4
3
CeSb2 in the Ce–Sb system. The binary compounds found in this
work are in well agreement with those compounds reported in
the literatures for Ce–Co [10], Co–Sb [11] and Ce–Sb [12] binary
systems. The structure types and lattice parameters of these com-
∗ Corresponding author. Tel.: +86 755 26538528; fax: +86 755 26536239.
E-mail address: junqinli@szu.edu.cn (J.Q. Li).
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2008.03.061

R.M. Luo et al. / Journal of Alloys and Compounds 471 (2009) 60–63
61
Table 1
Structure data and temperature of the phase transition of compounds in the Ce–Co–Sb ternary system
Ta ( C)
◦
Reference
Compounds
Space group
Structure type
Lattice parameters (nm)
a
b
c
Ce24Co11
CeCo2
Ce2Co7
CeCo3
Ce5Co19
Ce2Co17
CoSb
CoSb2
CoSb3
Ce2Sb
P63mc
FD 3¯ m
P63/mmc
R 3¯ m
R 3¯ m
P63/mmc
P63/mmc
Pnnm
Ce24Co11
Cu2Mg
Ce2Ni7
Be3Nb
Ce5Co19
Th2Ni17
NiAs
CoSb2
CoAs3
La2Sb
P62/mcm
I 4¯ 3d
0.9587
0.7160
0.4949
0.4952
0.49475
0.83779
0.3890
0.65051
0.90385
0.4552
0.931
0.9511
0.3975
0.628
1.2769
0.3878
0.6083
–
2.1825
446
1036
1130
1103
1134
1204
1220
936
874
1240
1440
1600
1800
1500
–
[19]
[19]
[19]
[19]
[19]
[19]
[19]
[19]
[19]
[19]
[19]
[19]
[19]
[19]
This work
[14]
This work
–
–
–
–
2.449
2.473
4.87434
0.81373
0.5187
0.65410
0.63833
Im3
I4/mmm
Mn5Si3
P4Th3
P4/mmm
Cmca
Pbcm
P4/nmm
C2221
1.784
0.652
Ce5Sb3
Ce4Sb3
CeSb
HgMn
–
0.3244
1.824
1.8435
0.9849
1.0459
CeSb2
Sb2Sm
CeNiSb3
ZrCuSi2
0.613
0.6060
CeCoSb3
CeCo1−xSb2
CeCo1.33 Sb2
–
–
-
0.6011
a
Temperatures listed refer to phase transition or to the melting temperature (bold).
Fig. 1. Rietveld refinement result of the XRD pattern of the sample consisted of the compound CeCoSb3 with small amount of CoSb2 impurity.
pounds are listed in Table 1. Buschow [13] and Wu et al. [10]
reported that the compound CeCo5 decomposes into Ce5Co19 and
of Ce–10Co–42Sb-48 consisted of the patterns of this new phase
and the compound CoSb. The structure of this new phase is still
unknown, but its XRD pattern has been successfully indexed
by Topas 3.0 software [16] with a possible space group Ccc2,
◦
Ce Co by eutectoid reaction at a lower temperature (750 C).
2
17
Although the compound CeCo5, as well as the compounds Ce Co
2
17
and Ce5Co19 , were found from XRD in the sample with composi-
◦
tion of Ce–16.7Co–83.3(CeCo5) at 400 C, the compound CeCo5 is
◦
believed to be metastable at 400 C and should not be presented in
this isothermal section.
Three ternary compounds, CeCoSb3, CeCo Sb2 and
1−x
CeCo
4
[
Sb , were found in this Ce–Co–Sb ternary system at
1.33
2
◦
00 C. The compound CeCo1 Sb2 has ZrCuSi -type structure
−x 2
14] with space group P4/nmm, a = 0.3878 nm and c = 0.9849 nm.
The Rietveld refinement performed using the Topas program for
the XRD patterns of the sample, consisted of the new ternary
compound CeCoSb3 mainly, together with a small amount of CoSb,
shows that the compound CeCoSb3 is of CeNiSb -type [15] struc-
3
ture with the space group Pbcm, a = 1.27697 nm, b = 0.60601 nm
and c = 1.8435 nm. The calculated data and differences of the
powder diffraction patterns of CeCoSb3 are shown in Fig. 1. For
another new ternary compound with the possible chemical for-
mula as CeCo1+xSb , we could not obtain its pure phase pattern.
2
Fig. 2. XRD patterns of the samples with composition of Ce–10Co–42Sb-48 con-
Fig. 2 shows the XRD patterns of the sample with composition
sisted of the compound CeCo1.33 Sb2 and the CoSb phase.

6
2
R.M. Luo et al. / Journal of Alloys and Compounds 471 (2009) 60–63
Table 2
Chemical composition of the sample Ce–10Co–42Sb-48 obtained by EDS
No.
Ce (at.%)
Co (at.%)
Sb (at.%)
a1
a2
a3
23.60
22.09
22.83
29.81
31.10
30.67
46.59
46.81
46.50
Average (a1, a2, a3)
b
22.84
0
30.53
50.58
46.63
49.42
an average value of 22.84 at.% Ce, 30.53 at.% Co and 46.63 at.% Sb,
which reveals that the chemical formula of the new compound is
CeCo1.33 Sb . This phase is also found in the samples located in the
2
other three-phase regions, such as the sample Ce–15Co–35Sb-50
located in the CoSb + CeCo1 Sb + CeCo1.33 Sb2 three-phase region
−x
2
(
Fig. 4). No movement of XRD pattern of this phase was found
in the samples with varying compositions, which indicated that
there is no solubility range for this compound. The structure types
and lattice parameters of the ternary compounds are also listed in
Table 1.
◦
3
.2. Isothermal section at 400 C of the Ce–Co–Sb ternary system
By comparing and analyzing the X-ray diffraction patterns of
all samples, and identifying the phases present in each sample, we
determined the phase equilibria in the Ce–Co–Sb ternary system
◦
and constructed its isothermal section at 400 C, as shown in Fig. 5.
It is consisted of 20 single-phase regions, 40 two-phase regions and
21 three-phase regions. Thetwentysingle-phase regions are: Ce, Co,
Sb, Ce24Co11 , CeCo , CeCo , Ce Co7, Ce5Co , Ce Co17 , CoSb, CoSb2,
2
3
2
19
2
CoSb , Ce Sb, Ce5Sb , Ce Sb , CeSb, CeSb , CeCoSb , CeCo Sb2
3
2
3
4
3
2
3
1−x
and CeCo1.33 Sb2.
The compound CoSb forms solid solutions by the way of sub-
stitution of Co atoms for Sb atoms at high temperature, but the
homogeneity range decreases with temperature decrease. The
◦
homogeneity range of CoSb is about 2.0 at.% Sb at 400 C [17]. The
maximum solid solubility of Ce in CoSb could not be detected (see
◦
Fig. 3(b)). The maximum solid solubility of Ce in CoSb3 at 400 C
is very small. The XRD pattern for the sample with composition of
Ce1.2 Co24.7Sb74.1 , shown in Fig. 6, consists of the patterns of a small
amount of CeSb2 and CoSb2 phases as well as CoSb3 main phase,
◦
indicating the maximum solid solubility of Ce in CoSb3 at 400 C is
Fig. 3. SEM micrographs of the sample with composition of Ce–10Co–42Sb-48 (a)
and EDS patterns for the dots labeled with b (b) and a1 (c) in (a).
a = 0.60830 (3) nm, b = 0.60118 (3) nm and c = 1.04596 (5) nm, the
figure of merit M(19) of this indexing is equal to 16.9. In order to
determine the exact chemical composition of this compound, a
SEM and EDS analysis are performed for the sample with com-
position of Ce–10Co–42Sb-48, shown in Fig. 3. The SEM image
shows that the alloy consists of two-phase regions, the darker grey
region and bright grey region, representing two phases existed in
this alloy. The EDS results show that the chemical composition of
the darker grey region is 50.58 at.% Co and 49.42 at.% Sb, without
any Ce detected (Fig. 3(b)), corresponding to CoSb phase. It also
reveals that the solid solubility of Ce in the CoSb phase could not
be detected. The chemical composition values, listed in Table 2,
from EDS analysis at a1 (Fig. 3(c)) to a3 three dots for brighter
grey region labeled in Fig. 3(a), are very close to each other with
Fig. 4. XRD pattern of the sample with composition of Ce–15Co–35Sb-50 located in
the CoSb + CeCo1−xSb2 + CeCo1.33 Sb2 three-phase regions showing the XRD pattern
of the compound CeCo1.33 Sb2.

R.M. Luo et al. / Journal of Alloys and Compounds 471 (2009) 60–63
63
4. Conclusion
We have investigated and constructed the isothermal sec-
◦
tion of the Ce–Co–Sb ternary system at 400 C. It is consisted
of 20 single-phase regions, 40 two-phase regions and 21 three-
phase regions. 14 binary compounds are: Ce24Co11 , CeCo , CeCo ,
2
3
Ce Co7, Ce5Co , Ce Co , CoSb, CoSb , CoSb , Ce Sb, Ce5Sb ,
2
19
2
17
2
3
2
3
Ce Sb , CeSb and CeSb . Three ternary compounds were found,
4
3
2
they are CeCoSb3 (Pbcm, a = 1.27697 nm, b = 0.60601 nm and
c = 1.8435 nm) CeCo1 Sb2 (space group P4/nmm, a = 0.3878 nm
−x
and c = 0.9849 nm), and CeCo1.33 Sb2 (possible space group Ccc2,
a = 0.6083 nm, b = 0.60118 nm and c = 1.04596 nm). The homogene-
ity range of CoSb phase is about 2.0 at.% Sb. The solubilities for the
other single-phase regions were not observed.
Acknowledgements
The work was supported by the National Natural Science
Foundation of China (No. 50471108) and Shenzhen Science and
Technology Research Grant (No. 200726).
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Fig. 5. Isothermal section of the phase diagram of the Ce–Co–Sb ternary system at
◦
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4
12
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