DSpace at VNU: Magnetic properties of LaNi5-based compounds

Journal of Magnetism and Magnetic Materials 262 (2003) 485–489

Magnetic properties of LaNi5-based compounds
L.T. Taia,b,*, B.T. Hangb, N.P. Thuya,b, T.D. Hienb
a

Cryogenic Laboratory, Faculty of Physics, College of Natural Science, Vietnam National University,
334 Nguyen Trai Road, Hanoi, Viet Nam
b
International Training Institute for Materials Science (ITIMS), ITIMS Building, Dai Hoc Bach Khoa, 1 Dai Co Viet Road,
Hanoi, Viet Nam

Abstract
Magnetic properties of the La(Ni4.5M0.5), (La1ÀxRx)Ni5 (M=Fe, Co, Mn, Cu, Cr, Si and R=Nd, Pr) and their
hydride compounds have been studied by magnetization measurement using a VSM. The susceptibility (w) of the
samples increases with Co, Mn, Fe, Nd, Pr but decreases with Cu, Cr, Si additives. The hydrogenation changes
magnetic properties of the La(Ni4.5M0.5), (La1ÀxRx)Ni5 parent compounds. Specially, the milling process changes
magnetic properties of LaNi5. This work indicates that besides electrochemical measurements, the study of magnetic
properties also gives important information on the quality of rechargeable sealed nickel–metal hydride batteries.
r 2003 Published by Elsevier Science B.V.
PACS: 82.47.Cb; 75.20.En
Keywords: Metal hydrides; Chemical hydrogenation; Magnetic properties

1. Introduction
The intermetallic RT5 compounds (R: rare
earth, T: transition metals) can absorb and desorb
a large hydrogen quantity at atmospheric pressure
and near room temperature. This property makes
them attractive to be used as negative electrode
for nickel-metal hydride (Ni–MH) rechargeable
batteries [1,3]. The use of a hydride-forming

electrode has enabled the development of rechargeable Cd-free batteries, which have a long
*Corresponding author. Cryogenic Laboratory, Faculty of
Physics, College of Natural Science, Vietnam National University, 334 Nguyen Trai Road, Hanoi, Viet Nam. Tel.: +84-4585281; fax: +84-4-8584438.
E-mail addresses: (L.T. Tai), hang
@itims.edu.vn (B.T. Hang).

life and high-energy storage capacity (30–50%
higher than Ni–Cd batteries) [1,2]. Many studies
have been performed on RT5 compounds for
battery application. Main attention, however,
has been focused on electrochemical measurements [4–7], whereas the magnetic properties of
hydride material have been insufficiently studied.
In this paper, we present results of magnetic
measurements on La(Ni4.5M0.5), (La1ÀxRx)Ni5
(M=Fe, Co, Mn, Cu, Cr, Si and R=Nd, Pr)
compounds and their hydrides.

2. Experimental
All samples were prepared by conventional arcmelting under argon atmosphere. All starting

0304-8853/03/$ - see front matter r 2003 Published by Elsevier Science B.V.
doi:10.1016/S0304-8853(03)00082-9


L.T. Tai et al. / Journal of Magnetism and Magnetic Materials 262 (2003) 485–489

0.08

M (emu/g)

materials (La, Nd, Pr, Ni, Co, Fe, Mn, Cu, Cr and
Si) are of 99.9 wt % or more purity. A slight excess
of La, Nd, Pr was used to compensate the weight
loss during the melting process. Prior to melting
the samples, Ti–metal ingot was melted as a getter.
The ingots were turned over and remelted several
times in order to ensure the homogeneity. Powder
samples with an average particle size of about
50 mm were obtained by pulverizing the as-melted
compounds in an agate mortar. The samples were
checked by X-ray diffraction, which reveals the
samples to be of single phase of CaCu5 structure.
Hydride samples were prepared by a chemical
hydrogenation method. The parent alloy powders
were mixed with NaBH4 in a proper ratio. This
mixture was put in a spherical jar and soaked in
50 ml of distilled water. The whole system was
tightly sealed for at least 24 h for the following
chemical reaction to complete.

LaNi5 powder

0.00

-10000

0

10000

H (Oe)
2

La0.75Nd0.25Ni5
La0.75Nd0.25Ni5Hx

0

-2
-10000

LaðNi2MÞ5 þ NaBH4 þ 3H2 O

0

10000

H (Oe)

¼ LaðNi-MÞ5 Hx þ NaH2 BO3 þ ð4-x=2ÞH:

0.04
LaNi4.5Co0.5

M (emu/g)

LaNi4.5Co0.5Hx

0.00

-0.04
-10000

0

10000

H (Oe)
0.1

LaNi4.5Mn0.5
LaNi4.5Mn0.5Hx

M (emu/g)

The produced powder was then washed several
times by distilled water in order to get rid of the
reacted by products. Finally it was washed in
alcohol and dried in air.
Magnetic properties of the samples have been
studied by measuring magnetization curves and
the thermomagnetization using a sensitive vibrating sample magnetometer (VSM) system. Magnetization curves at room temperature were
measured on the bulk samples, powder samples
and their hydrides. The maximum applied field
was 1.3 T. The temperature dependence of the
magnetization MðTÞ of the hydride samples was
measured in a nitrogen gas environment. The
temperature range was from 300 K to above 700 K.
The maximum applied field was 0.1 T. During the
measurement, the temperature was raised at a rate

estimated to be about 5 C/min.

LaNi5
LaNi5Hx

-0.08

M (emu/g)

486

0.0

-0.1
-10000

0

10000

H (Oe)

3. Results and discussion
The magnetization curves of the as-prepared
samples are shown in Fig. 1 for comparison with
those of the hydride samples. As can be clearly
seen from the magnetization curves, all parent

Fig. 1. Magnetization curves of La(Ni4.5M0.5) and (La1ÀxRx)Ni5 (M=Fe, Co, Mn and R=Nd, Pr) and their hydrides, at
room temperature.

L.T. Tai et al. / Journal of Magnetism and Magnetic Materials 262 (2003) 485–489

487

Table 1
Susceptibility (w) at 300 K of parent samples and Curie temperature (Tc ) of hydride samples
No.

Parent samples

w (10À6)

Hydride samples

Tc (K)

1
2
3
4
5
6
7
8
9
10
11
12

13
14

LaNi5
LaNi4.5Co0.5
LaNi4.5Fe0.5
LaNi4.5Mn0.5
LaNi4.5Cu0.5
LaNi4.5Cr0.5
LaNi4.5Si0.5
La0.99Nd0.01Ni5
La0.95Nd0.05Ni5
La0.90Nd0.1Ni5
La0.99Pr0.01Ni5
La0.95Pr0.05Ni5
La0.90Pr0.1Ni5

3.80
7.40
35.06
11.05
3.01
2.35
2.49
5.02
5.43
6.14
4.58
5.13
5.88

LaNi5Hx
LaNi4.5Co0.5Hx
LaNi4.5Fe0.5Hx
LaNi4.5Mn0.5Hx
LaNi4.5Cu0.5Hx
LaNi4.5Cr0.5Hx
LaNi4.5Si0.5Hx
La0.99Nd0.01Ni5Hx
La0.95Nd0.05Ni5Hx
La0.90Nd0.1Ni5Hx
La0.99Pr0.01Ni5Hx
La0.95Pr0.05Ni5Hx
La0.90Pr0.1Ni5Hx
LaNi5 powder

570
755
700
565
485
495
465
570
570
570
570
570
570
570

samples are paramagnets. From these curves the
susceptibility (w) was determined. Results are
shown in Table 1.
The susceptibility of the LaNi5 compound
increases when La is partly replaced by Nd, Pr
and Ni is partly replaced by Co, Fe, Mn, but it
decreases when Ni is partly replaced by Cu, Cr and
Si. This can be understood by taking into account
that, in the same valence state of the metals, the
paramagnetic moments of the Co, Fe and Mn ions
are all larger than that of a Ni ion [8], and that Nd,
Pr are magnetic while Cu, Cr and Si are nonmagnetic. In contrast to those of the parent
samples, the magnetization curves of all hydride
samples show a ferromagnetic behavior. This can
be explained as being due to the ferromagnetism of
pure Ni (or/and Co, Fe, Mn) phases that were
decomposed from the compounds during the
hydrogenation process. The difference in ferromagnetic behavior of the hydride samples, with
different element substitutions, can also be seen in
Fig. 1.
The thermomagnetization measurements were
performed on all hydride samples in two different
ways. At first, the magnetization of the asprepared samples was measured with increasing
temperature from 300 K to above 700 K. After
that, the samples were cooled down to room
temperature. Then the magnetization was measured again with increasing temperature in the

same temperature interval. Results are shown in
Fig. 2.

In Fig. 2 the filled symbols are results taken on
as-hydrided samples and the empty symbols are
those for the same samples in the second measuring cycle. One can easily see a clear difference
between
the
two
magnetization
curves.
The thermal cycle thus has a pronounced effect
on the thermal magnetization behavior of the
materials.
The anomaly observed in the curves of the asprepared samples can be explained as being due to
the presence of free Ni (or/and Co, Fe, Mn) metal
phases that were decomposed by the hydrogenation process and exist in a nearly amorphous or
cluster form [9,10]. The onset temperatures of the
maxima at ToTc observed for all hydride samples
can be considered as recrystallization temperatures.
From the thermomagnetic curves obtained in
the second measurement cycle, the Curie temperatures (Tc ) were determined for the hydride
compounds (see Table 1). It is clear that the Curie
temperatures of La(Ni4.5M0.5) compounds differ
very remarkably for different M metals. In
contrast, the Tc values of (La1ÀxRx)Ni5 compounds with R=La, Nd and Pr are all the same.
This indicates that in the hydrogenation process
only the transition metals were decomposed from
the compounds.


L.T. Tai et al. / Journal of Magnetism and Magnetic Materials 262 (2003) 485–489

488

0.8

LaNi5Hx

LaNi5 powder
0.6

0.06
M (emu/g)

M (arb.units)

0.12

0.00

0.4

0.2

200

400

600

800

T (K)

0.0

0.18

300

M (arb.units)

La0.75Nd0.25Ni5Hx

600

700

Fig. 3. The temperature dependence of the magnetization of
LaNi5 powder measured at H ¼ 1 kOe.

0.09

200

400

600

800

T (K)

0.24

M (arb.units)

500

T (K)

0.00

LaNi4.5Co0.5Hx
0.12

0.00
200

400

600

800

T (K)
0.30

LaNi4.5Mn0.5Hx
M (arb.units)

400

powder has the same behavior as that of the
hydride sample (see Fig. 3). It can be due to the
fact that during the milling process Ni atoms were
also decomposed from the sample.
We note that the effect of the chemical hydrogenation process of the (La1ÀxRx)Ni5 and La(Ni4.5M0.5) compounds is quite similar to that of
the charging process with negative electrode in
Ni–MH batteries. We observed the same results in
all samples, when they are subjected to a charging–
discharging process. The transition metals are
decomposed by the hydrogenation process and
precipitated in cluster form on the surface of
particles, leading to the increase of the reactivity of
these surfaces. This effect can be related to the
larger value of the double-layer capacitance as
found in electrochemical measurements.

0.15

4. Conclusion

0.00

In this work, the magnetic properties of
LaNi4.5M0.5 and LaNi4.5M0.5Hx have been studied. The main results can be summarized as
follows:

200

400

600

800

T (K)
Fig. 2. The temperature dependence of the magnetization of
hydride samples measured at H ¼ 1 kOe.

Specially, after milling, the LaNi5 sample
changes from paramagnetic to ferromagnetic (see
Fig. 1), and the thermomagnetic curves of LaNi5

(1) The susceptibility of LaNi5-based compounds
increases when partly replacing Ni by Co, Fe,
Mn and La by Nd, Pr and decrease upon
replacing by Cr, Cu, Si additive.
(2) The LaNi5-based compounds change from
paramagnetic to ferromagnetic after chemical
hydrogenation and milling.


L.T. Tai et al. / Journal of Magnetism and Magnetic Materials 262 (2003) 485–489

(3) The magnetic measurements on La(Ni–M)5
powders provide valuable information on the
nature of the hydrogenation during the
electrochemical process in a Ni–MH rechargeable battery.

Acknowledgements
This work is supported by National Research

Program under the Grant KC02/13/02 and the
State Program on Fundamental Research of
Vietnam under the Grant No. 421001.

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