Boron nonstoichiometry, hardness and oxidation resistance of perovskite-type CeRh3Bx (x = 0-1)

Article abstract of DOI:10.1016/j.jallcom.2006.01.102

We report synthesis of perovskite-type boride CeRh₃B_x using the arc melting method. The samples are characterized by powder X-ray diffraction which shows that the crystal structure of CeRh₃B_x belongs to the cubic system (space group: Pm3m) in the range of 0 < x ≤ 1. Furthermore, CeRh₃ (x = 0) has the ordered AuCu₃-type structure (space group: Pm3m). As a result, CeRh₃B_x exists in a wide range of nonstoichiometric boron concentration 0 ≤ x ≤ 1 with the space group Pm3m. The lattice parameter, a, changes almost linearly with x, varying from 0.4015(1) nm (x = 0) to 0.4180(1) nm (x = 1.0). The micro-Vickers hardness of the CeRh₃B_x for x = 0 (0 at.%B), 0.210 (5 at.%B), 0.444 (10 at.%B), 0.706 (15 at.%B) and 1.0 (20 at.%B) are 2.0 ± 0.1, 4.3 ± 0.2, 1.8 ± 0.2, 5.5 ± 0.1 and 6.5 ± 0.2 GPa, respectively. Thermo-gravimetric analysis was done to study the oxidation resistance of the samples in air. It reveals that oxidation begins at 502 and 513 K for x = 0 and 1.0, respectively. A mixed phase of CeO₂ + Rh is identified as an oxidized product for all samples.

Full text of DOI:10.1016/j.jallcom.2006.01.102

Journal of Alloys and Compounds 426 (2006) 304–307
Boron nonstoichiometry, hardness and oxidation resistance of
perovskite-type CeRh3Bx (x = 0–1)
T. Shishidoa,∗, M. Okua, J. Yeb, S. Okadac, K. Kudoud, K. Iizumie, Y. Sawadae, Y. Ishizawaf,
A. Nomuraa, T. Sugawaraa, K. Obaraa, R. Saharaa, K. Yubutaa, H. Kojimaa, V. Kumara,
M. Tanakab, K. Shimamurab, S. Oishig, S. Kohikih, Y. Kawazoea, K. Nakajimaa
a Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
b National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan
c Faculty of Engineering, Kokushikan University, 4-28-1 Setagaya-ku, Tokyo 154-8515, Japan
d Faculty of Engineering, Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa-ku, Yokohama 221-8686, Japan
e Faculty of Engineering, Tokyo Institute of Polytechnics, 1583 Iiyama, Atsugi 243-0297, Japan
f Department of Environmental Science, Iwaki Meisei University, 5-5-1 Iino, Chuohdai, Iwaki 970-8551, Japan
g Department of Environmental Science and Technology, Faculty of Engineering, Shinshu University, Wakasato, Nagano 380-8553, Japan
h Department of Materials Science, Kyusyu Institute of Technology, Kita-Kyusyu, 804-8550, Japan
Received 14 December 2005; accepted 20 January 2006
Available online 18 April 2006
Abstract
We report synthesis of perovskite-type boride CeRh3Bx using the arc melting method. The samples are characterized by powder X-ray diffraction
which shows that the crystal structure of CeRh3Bx belongs to the cubic system (space group: Pm3m) in the range of 0 < x ≤ 1. Furthermore, CeRh3
(x = 0) has the ordered AuCu3-type structure (space group: Pm3m). As a result, CeRh3Bx exists in a wide range of nonstoichiometric boron
concentration 0 ≤ x ≤ 1 with the space group Pm3m. The lattice parameter, a, changes almost linearly with x, varying from 0.4015(1) nm (x = 0)
to 0.4180(1) nm (x = 1.0). The micro-Vickers hardness of the CeRh3Bx for x = 0 (0 at.%B), 0.210 (5 at.%B), 0.444 (10 at.%B), 0.706 (15 at.%B)
and 1.0 (20 at.%B) are 2.0 0.1, 4.3 0.2, 1.8 0.2, 5.5 0.1 and 6.5 0.2 GPa, respectively. Thermo-gravimetric analysis was done to study
the oxidation resistance of the samples in air. It reveals that oxidation begins at 502 and 513 K for x = 0 and 1.0, respectively. A mixed phase of
CeO2 + Rh is identified as an oxidized product for all samples.
© 2006 Elsevier B.V. All rights reserved.
Keywords: CeRh3B; Perovskite-type compound; Nonstoichiometry; Hardness; Oxidation resistance
1. Introduction
In the present work, we have synthesized perovskite-type
borides [5–9], CeRh3Bx and focused attention on the study of
their mechanical and chemical properties. At first, we studied
borides with nonstoichiometric compositions and measured the
change in the lattice parameter when x was varied. Subsequently,
therelationshipbetweentheBcontentandhardnessofCeRh3Bx,
as well as between the B content and resistance to oxidation of
CeRh3Bx when heated in air, were studied.
Rare earth element (R)–Rhodium (Rh)–boron (B) ternary
systems have interesting magnetic and superconducting prop-
erties. The discovery of the ErRh4B4 compound, which is a
re-entrant superconductor, has created much interest in the study
of these ternary systems [1,2]. We have previously grown single
crystals of several compounds by a slow cooling method using
molten copper as a flux. They are RRh3B, RRh3B2, RRh4B4 and
RRh5B2 [3,4]. The magnetic and superconducting properties of
these crystals have been reported.
2. Experimental details
2.1. Sample preparation
∗
Corresponding author.
E-mail address: shishido@imr.tohoku.ac.jp (T. Shishido).
Polycrystalline samples of CeRh3Bx were synthesized by the arc melting
method using 99.9% pure Ce, Rh and B as raw materials. These were weighed in
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2006.01.102

T. Shishido et al. / Journal of Alloys and Compounds 426 (2006) 304–307
the atomic ratio 1:3:x, where x = 0 (0 at.%B), 0.210 (5 at.%B), 0.444 (10 at.%B),
305
0.706 (15 at.%B) and 1.0 (20 at.%B). The mixture of the starting materials, about
2 g for each sample, was placed in a water-cooled copper hearth in reaction
chamber. Argon was used as a protective atmosphere. The pressure inside the
chamber was approximately 1 atm. A small amount of residual oxygen in argon
was eliminated by fusing a button of titanium as a reducing agent. The starting
materials were then melted for 3 min by an argon arc plasma flame with dc power
source at 20 and 100 A. The samples were then turned over and melted three
times under the same conditions. The synthesized samples were wrapped in a
tantalum foil and annealed at 1573 K for 20 h in vacuo to ensure homogeneity
and accurate comparison of results.
2.2. Characterization
For chemical analysis, the samples were fused using NaHSO4 as a flux
reagent, and the obtained material was dissolved into HCl. The chemical com-
position of each solution was analyzed by the induction coupled plasma atomic
emission spectrometry (ICP-AES) method, using Zn as internal standard. Crys-
tal structure characterization of the samples was performed by X-ray powder
diffraction (XRD). The micro-Vickers hardness (Hv) for the samples was mea-
sured at room temperature. A load of 300 g was applied for 15 s and 10 impres-
sions were recorded for each sample. The obtained values were averaged and the
experimental error was estimated. Thermo-gravimetric (TG) analysis and differ-
ential thermal analysis (DTA) were performed to study the oxidation resistance
of the samples in air. A sample of about 25 mg was heated at a rate of 10 K/min
up to 1473 K. The oxidation products were analyzed by powder XRD method.
Fig. 1. Powder XRD patterns for CeRh3Bx: (a) x = 0 (0 at.%B), (b) x = 0.210
(5 at.%B), (c) x = 0.444 (10 at.%B), (d) x = 0.706 (15 at.%B), and (e) x = 1.0
(20 at.%B).
decreased in the order of Gd, Lu and Sc. The B nonstoichiometry
range is the widest, 0 ≤ x ≤ 1, for Sc [12] which has the smallest
atomic radius among these R elements. However, an exceptional
result of full range of B nonstoichiometry, 0 ≤ x ≤ 1, is obtained
for CeRh3Bx from the view point of the atomic radius of R.
Further studies have been done on the hardness of CeRh3Bx,
which reflects the nature of the chemical bonding. Fig. 4 shows
the relationship between the B content x and hardness. The
micro-Vickers hardness, Hv of CeRh3Bx for x = 0, 0.210, 0.444,
0.706 and 1.0 are 2.0 0.1, 4.3 0.2, 1.8 0.2, 5.5 0.1 and
6.5 0.2 GPa, respectively. These results show that for x = 0.444
(10 at.%B), an anomalously low value of hardness is observed
compared to the values for other samples with boron concentra-
3. Results and discussion
The chemical compositions of the samples before and after
synthesis are almost the same. In addition, contamination from
the electrode (tungsten) and hearth (copper) could be disre-
garded. All samples of CeRh3Bx obtained by varying the B
content x were silvery in color and exhibited metallic luster.
The crystal structure analysis of CeRh3Bx with x = 1.0 by
powder XRD reveals that CeRh3B is a perovskite-type com-
pound (space group: Pm3m) in which Ce is positioned at the
eight corners of a cube while Rh occupies the face-centers and
B, the body-center. By varying x, the nonstoichionmetry of B
and the resulting change in the crystal structure were studied.
Fig. 1 shows the changes in the powder XRD pattern when x
is varied. With decreasing x, the powder XRD peaks of the
perovskite-type phase shift to higher angles. Fig. 2 shows the
relationship between x and the lattice parameter a of CeRh3Bx.
The lattice parameter changes almost linearly with x, varying
from a = 0.4015(1) nm (x = 0) to 0.4180(1) nm (x = 1.0). The
ordered AuCu3-type structure, which belongs to the space group
Pm3m, is adopted by CeRh3 (x = 0). Fig. 3 shows the crystal
structures of CeRh3 and CeRh3B. Here it is assumed that the
cubic system of CeRh3 is the host lattice and B is the guest atom
intheperovskite-typeCeRh3Bx. Accordingly, CeRh3Bx existsin
a wide nonstoichiometric range 0 ≤ x ≤ 1. Experimental results
[10] for LaRh3Bx show that this perovskite-type compound does
not have B nonstoichiometry, and the preferred composition is
x = 1. In GdRh3Bx [11] and LuRh3Bx [10], B nonstoichiome-
try ranges between 0.55 ≤ x ≤ 1 and 0.3 ≤ x ≤ 1, respectively.
The aforementioned results are listed in Table 1 and show that
B exhibits no nonstoichiometry only in the case of R = La. La
has the largest atomic radius among all the R elements. Boron
graduallybecomesnonstoichiometricastheatomicradiusofRis
Fig. 2. The lattice parameter a as a function of boron content x in CeRh3Bx.

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T. Shishido et al. / Journal of Alloys and Compounds 426 (2006) 304–307
Fig. 4. Hardness as a function of boron content x in CeRh3Bx.
imum temperature is 704, 708, 707, 673 and 754 K for x = 0,
0.210, 0.444, 0.706 and 1.0, respectively. The TG analysis
reveals that oxidation begins at 502, 511, 488, 514 and 513 K
for x = 0, 0.210, 0.444, 0.706 and 1.0, respectively. In the case
of x = 0.444 (10 at.%B), oxidation starts at a relatively low tem-
perature. As mentioned earlier, an anomalously low value of
hardness is observed for x = 0.444 (10 at.%B). It is interesting
that the anomaly appears at the same boron content of x = 0.444.
Thefinalweightgainsduetooxidationwhenheatedupto1473 K
are 7.0, 8.0, 9.2, 9.6 and 11% for x = 0, 0.210, 0.444, 0.706 and
Fig. 3. Schematic crystal structure of CeRh3 and CeRh3B.
tioninthevicinityofthisvalueof x. Thecauseforthisanomalous
hardness value when x = 0.444 remains unclear and further char-
acterization of these samples is in progress to clarify this.
Studies of the resistance to oxidation in air reflect the nature
of the chemical bonding in these compounds as B content is
varied. As shown in Fig. 5, the DTA result reveals no sharp
exothermic peak for any of the samples. The exothermic max-
Table 1
Boron nonstoichiometry in the RRh3Bx (R = La, Ce, Gd, Lu and Sc)
RRh3Bx R
Boron nonstoichiometry
range of x
Atomic radius of
R (nm)
Reference
La
Ce
Gd
Lu
Sc
x = 1
0.188
0.183
0.180
0.174
0.164
[10]
This study
[11]
[10]
[12]
0 ≤ x ≤ 1
0.55 ≤ x ≤ 1
0.3 ≤ x ≤ 1
0 ≤ x ≤ 1
Fig. 5. TG–DTA curves for CeRh3Bx (x = 0–1.0).

T. Shishido et al. / Journal of Alloys and Compounds 426 (2006) 304–307
307
Table 2
Results of TG–DTA measurements on CeRh3Bx
x (nominal composition)
Oxidation onset,
K by TGA
Exotherm maximum,
K by DTA
Weight gain at
1273 K (%)
Weight gain at
1473 K
Oxidized products
1.000
0.706
0.444
0.210
0
513
514
488
511
502
754
673
707
708
704
19.7
18.4
21.1
19.3
19.2
11.0
9.6
9.2
8.0
7.0
CeO2 + Rh
CeO2 + Rh
CeO2 + Rh
CeO2 + Rh
CeO2 + Rh
1.0, respectively. A study of the oxidized products by the pow-
der XRD method shows that the products are CeO2 + Rh for
all samples. Phenomenal temperatures of oxidation onset and
weight gain after measurement of TG–DTA are summarized in
Table 2.
Acknowledgements
The present study was performed under the inter-university
cooperative research program of the ARCMG-IMR, Tohoku
University. The authors are grateful to Dr K. Takada and Mr.
Y. Murakami and Mr. M. Ishikuro of IMR, Tohoku University,
for chemical analyses.
4. Conclusions
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(4) TG curve shows that oxidation begins at 502 and 513 K
for x = 0 and 1.0, respectively. In the case of x = 0.444
(10 at.%B), oxidation starts at a relatively low temperature.
A mixed phase of CeO2 + Rh is identified as an oxidized
product for all samples.