Hydrogenation process of the Gd3M (M = Ni or Co) intermetallics compound

Article abstract of DOI:10.1016/j.jallcom.2004.10.095

The reaction of the intermetallic Gd₃M (M = Co or Ni) compounds with hydrogen was studied. The transformation of the crystal structure of the parent Gd₃M sample after subsequent stages of the hydrogenation was investigated by X-ray diffraction. It was found that both intermetallic compounds (Gd₃Ni and Gd₃Co) are thermodynamically unstable during hydrogenation decomposing into the gadolinium hydrides and the transition metal. It is very probable that, in the case of Gd₃Ni, nickel plays the role of a catalyst in the formation of GdH₃ in the low temperature range, whereas in the case of Gd₃Co the transition metal (Co) is rather inactive.

Full text of DOI:10.1016/j.jallcom.2004.10.095

Journal of Alloys and Compounds 404–406 (2005) 584–587
Hydrogenation process of the Gd M (M = Ni or Co)
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intermetallics compound
a,∗
a
b
a
´
T. Palewski , N.V. Tristan , H. Drulis , J. Cwik
a
International Laboratory of High Magnetic Fields and Low Temperatures,
Wrocław, Poland
b
W. Trzebiatowski Institute of Low Temperature and Structure Research of PAS,
Wrocław, Poland
Received 6 May 2004; received in revised form 30 September 2004; accepted 9 October 2004
Available online 19 July 2005
Abstract
The reaction of the intermetallic Gd
3
M (M = Co or Ni) compounds with hydrogen was studied. The transformation of the crystal structure of
the parent Gd M sample after subsequent stages of the hydrogenation was investigated by X-ray diffraction. It was found that both intermetallic
compounds (Gd Ni and Gd Co) are thermodynamically unstable during hydrogenation decomposing into the gadolinium hydrides and the
transition metal. It is very probable that, in the case of Gd Ni, nickel plays the role of a catalyst in the formation of GdH in the low temperature
range, whereas in the case of Gd Co the transition metal (Co) is rather inactive.
2005 Elsevier B.V. All rights reserved.
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©
Keywords: Hydrogenation; Structural transformation; Structural instability
1
. Introduction
earth and d-electron elements when the hydrides have the
same crystal structure as the parent intermetallic compounds
were recently presented in [2,3]. It should be noted that
some intermetallic compounds are unstable during hydro-
genation and become amorphous or decompose yielding
hydrides of respective metallic elements [4,5]. R3M com-
pounds are able to absorb up to nine hydrogen atoms per
formula unit (R3M + xH; x ≤ 9) [6]. It was suggested ear-
lier that the hydrogenation under low pressure of hydrogen
(p < 100 kPa) and at low temperatures (243–253 K) could
lead to the synthesis of the R3NiHx hydrides which pre-
serve the crystal structure of the parent compounds [7] but
it is very probable that the quality of the previous chem-
ical syntheses and X-ray patterns had not been sufficient.
On the other hand it has not been known which products:
solid solution or individual hydrides are obtained during re-
action of such compounds with hydrogen. The aim of the
present investigation was to get insight into the influence of
absorbed hydrogen on the phase composition and the trans-
formation of the crystal structures of the parent compounds
It is known that not only rare earth elements but also many
rare earth intermetallic compounds absorb large amounts of
hydrogen. The reaction of hydrogen with intermetallic com-
pounds results in crystalline or amorphous solid solutions
of hydrogen in the respective compounds or in the forma-
tion of hydrides with a fixed chemical composition. In the
case of solid solutions, the incorporation of hydrogen atoms
into the crystal lattice usually gives rise to an increase of
the unit cell volume. Moreover, a charge transfer between
hydrogen and other atoms present in the crystal lattice is
possible, and very often drastic changes of some physical
properties are observed upon hydrogenation. For example,
a diamagnet becomes a ferromagnet or a metallic conduc-
tor turns into a semiconductor or an insulator [1]. Changes
of physical properties of hydrogenated intermetallics of rare
∗
Corresponding author. Fax: +48 71 361 27 21.
E-mail address: paltom@alpha.ml.pan.wroc.pl (T. Palewski).
0
925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2004.10.095

T. Palewski et al. / Journal of Alloys and Compounds 404–406 (2005) 584–587
585
Fig. 1. Diffractograms of Gd3Ni (x = 0) and Gd3Co (x = 0), its products after hydrogenation (x = 1–9) and both hydrides of gadolinium.

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T. Palewski et al. / Journal of Alloys and Compounds 404–406 (2005) 584–587
Gd3Ni and Gd3Co during hydrogenation, as a function of its
amount.
2
. Experimental
The parent Gd3Ni and Gd3Co compounds were obtained
by melting both metals in a purified argon atmosphere. The
purity of gadolinium was 99.9% whereas that of nickel and
cobalt was 99.99%. Hydrogen was obtained by thermal
decomposition of the LaNi hydrogen storage compound.
5
The samples were crushed into powder before hydrogenation
and heated in a reaction chamber under vacuum at about
7
00 K for 2 h. When the sample temperature was lowered
to room temperature, hydrogen was put into the reaction
chamber in small portions (the initial pressure of hydrogen
in the chamber did not exceed 4 kPa) in order to avoid a
violent reaction between the powder and the gas. The mass
of the Gd3M sample and the initial pressure of hydrogen
balanced such that the concentration of hydrogen (x) in the
samples (Gd3M + xH) varied gradually from 1 to 9. The
samples were then homogenized at room temperature for
Fig. 2. X-ray diffraction patterns of hydrogenated samples (Gd3Ni + 8.8 H):
a) obtained at room temperature and homogenized for 60 h; (b) obtained at
low temperature (T 220 K) without homogenization at room temperature.
(
In the hydrogenated Gd3Co samples the formation of GdH3
is more difficult and even for x = 9, the GdH2 and GdH3
phases coexist. In the X-ray patterns of both hydrogenated
compounds two to three very weak additional peaks belong-
ing to an unidentified phase are also visible, but in low in-
tensity does not allow an identification. In addition, no lines
of the transition metals (Co or Ni) or of any known crys-
talline alloys of M (Ni or Co) with gadolinium are observed.
Also the elemental analysis showed that nickel and cobalt
are distributed unifomly with no visible precipitation. An-
alyzing these facts one may suppose that nickel and cobalt
transform to a dispersed phase not detectable in the reported
X-ray investigation. Based on the results described above, we
conclude that Gd3Co and Gd3Ni decompose during hydro-
genation into the pure rare earth hydrides with precipitation
of the transition metals (Co or Ni) as dispersed phases. It is
widely known that nickel in powder form exhibits catalytic
properties, especially in hydrogenation. It is thus possible that
Ni plays the role of a catalyst in the formation of GdH3 at low
temperatures; so far this hydride was synthesized at tempera-
tures much higher than room temperature [9,10]. On the other
hand cobalt is rather inactive in reactions with hydrogen and
this may be the reason of differences between final products
after hydrogenation of Gd3Ni and Gd3Co; pure GdH3 or a
mixture GdH2 and GdH3. In the case of Gd3Ni some hydrides
were synthesized at low temperature (T ≈ 220 K) and it was
found that X-ray patterns were similar to the patterns of sam-
ples synthesized at room temperature (see Fig. 2a), however,
in the X-ray pattern of this sample (see Fig. 2b) one can find
the lines of GdH2 and GdH3 phases exhibiting significant
distortion typical of insufficiently annealed compounds.
6
0 h and examined using X-ray diffraction with a cobalt
radiation source. An additional verification of the elemental
composition of each hydride was performed using the
Philips SEM 515 and EDAX 9800 electron microscope.
3
. Results
Both parent compounds Gd3M crystallize in an or-
thorhombic Fe3C-type crystal structure (Pnma space group).
The lattice parameters of Gd3Ni amount to a = 0.694 nm,
b = 0.969 nm and c = 0.635 nm, whereas for Gd3Co a =
0
.705 nm, b = 0.950 nm and c = 0.631 nm. An analysis of
the X-ray patterns of hydrogenated samples (see Fig. 1) in-
dicates that, in addition to lines (peaks) characteristic of the
orthorhombic phase, lines of two other phases may be iden-
tified with cubic and hexagonal crystal structures. The lat-
tice parameter of the cubic structure a = 0.53–0.55 nm is in
good agreement with the value expected of the dihydryde
GdH2, and the lattice parameters of the hexagonal structure
(a = 0.65 nm, c = 0.67 nm) correspond to the lattice param-
eters of GdH3 [8]. It is also seen that for the hydrogenated
samples with x ≤ 4 the peak intensity of the Gd3M phase
decreases whereas the peak intensity of the GdH2 phase in-
creases. In Gd3Ni which absorbed four hydrogen atoms per
formula unit only peaks belonging to the GdH2 phase are ob-
served while in the case of Gd3Co some peaks of Gd3Co
are still seen. When more than four hydrogen atoms are
absorbed, the lines characteristic of the hexagonal (GdH3)
phase appear, their intensities increasing with increasing x
with those belonging to the cubic phase decreasing. At the
same time in the case of the hydrogenated Gd3Ni samples,
the replacement of GdH2 by GdH3 is more evident and, for
x near 9, practically only peaks of the GdH3 phase are seen.
4. Conclusions
The presented results point out that Gd3M (M = Co, Ni) is
unstable during hydrogenation, and a direct synthesis of the

T. Palewski et al. / Journal of Alloys and Compounds 404–406 (2005) 584–587
587
hydrides Gd3MHx is impossible using a standard reaction be-
tween gaseous hydrogen and Gd3M. Thus, it is suggested that
the analysis of X-ray patterns of hydrogenated intermetallic
compounds should be more accurate and complete to avoid
mistakes in the identification of the products. It may be also
interesting to confirm the catalytic role of metallic nickel in
the formation of other rare earth hydrides.
[3] S.A. Nikitin, I.S. Tereshina, N.Yu. Pankratov, Yu.V. Skourski, Phys.
Rev. B 63 (2001) 134420.
[4] G. Wiesinger, G. Hilscher, Magnetism of Hydrides, in: K.H.J. Buschow
(Ed.), Handbook of Magnetic Materials, vol. 6, North Holland, Ams-
terdam, 1991, p. 511.
[
[
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[
5] N.V. Tristan, T. Palewski, H. Drulis, L. Folcik, S.A. Nikitin, Mat. Sci.
(Poland) 21 (3) (2003) 357.
6] K.H.J. Buschow, R.C. Sherwood, J. Appl. Phys. 49 (1978)
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8] Landolt-Börnstein, Numerical Data and Functional Relationships in
Science and Technology, in: K.-H. Hellwege, A.M. Hellwege (Eds.),
New Series III/6, Berlin, 1971, p.143.
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