High-pressure synthesis of novel hydrides in Mg-RE-H systems (RE = Y, La, Ce, Pr, Sm, Gd, Tb, Dy)

Article abstract of DOI:10.1016/j.jallcom.2005.04.093

The high-pressure synthesis of new hydrides of Mg-RE-H systems, where RE = Y, La, Ce, Pr, Sm, Gd, Tb and Dy, were conducted by using a cubic-anvil-type apparatus, and their crystal structure, thermal stabilities and hydrogen contents were investigated. In Mg-Y-H system, newly found MgY₂H _y with a FCC-type structure has been prepared. In MgH₂-x mol% REH (REH = LaH₃, CeH_2.5 and PrH₃), new hydrides with primitive tetragonal structure were synthesized around x = 25-33 under GPa-order high pressures. The lattice constants were a = 0.8193 nm, c = 0.5028 nm, a = 0.8118 nm, c = 0.4979 nm and a = 0.8058 nm, c = 0.4970 nm at x = 25 in Mg-La, Ce and Pr systems, respectively. The hydrogen contents of the novel compounds were 4.1, 3.7 and 3.9 mass% in Mg-La, Ce and Pr systems, respectively, and the chemical formulae were found to correspond to Mg ₃LaH₉, Mg₃CeH_8.1 and Mg ₃PrH₉. The new hydrides decomposed into Mg and rare-earth hydride at about 600 K (Mg₃LaH₉: 614 K, Mg ₃CeH_8.1: 609 K, Mg₃PrH₉: 630 K) with an endothermic reaction. In MgH₂-x mol% hREH (hREH = GdH ₃, TbH₃ and DyH₃), new hydrides with FCC-type structure were synthesized around x = 67.

Full text of DOI:10.1016/j.jallcom.2005.04.093

Journal of Alloys and Compounds 408–412 (2006) 284–287
High-pressure synthesis of novel hydrides in Mg–RE–H
systems (RE = Y, La, Ce, Pr, Sm, Gd, Tb, Dy)
A. Kamegawaa,∗, Y. Gotoa, H. Kakutab, H. Takamuraa, M. Okadaa
a
Department of Materials Science, School of Engineering, Tohoku University, Sendai 980-8579, Japan
b
COE fellow, IMR, Tohoku University, Sendai 980-8577, Japan
Available online 6 June 2005
Abstract
The high-pressure synthesis of new hydrides of Mg–RE–H systems, where RE = Y, La, Ce, Pr, Sm, Gd, Tb and Dy, were conducted by using a
cubic-anvil-type apparatus, and their crystal structure, thermal stabilities and hydrogen contents were investigated. In Mg–Y–H system, newly
found MgY2Hy with a FCC-type structure has been prepared. In MgH2–x mol% REH (REH = LaH3, CeH2.5 and PrH3), new hydrides with
primitive tetragonal structure were synthesized around x = 25–33 under GPa-order high pressures. The lattice constants were a = 0.8193 nm,
c = 0.5028 nm, a = 0.8118 nm, c = 0.4979 nm and a = 0.8058 nm, c = 0.4970 nm at x = 25 in Mg–La, Ce and Pr systems, respectively. The
hydrogen contents of the novel compounds were 4.1, 3.7 and 3.9 mass% in Mg–La, Ce and Pr systems, respectively, and the chemical
formulae were found to correspond to Mg3LaH9, Mg3CeH8.1 and Mg3PrH9. The new hydrides decomposed into Mg and rare-earth hydride at
about 600 K (Mg3LaH9: 614 K, Mg3CeH8.1: 609 K, Mg3PrH9: 630 K) with an endothermic reaction. In MgH2–x mol% hREH (hREH = GdH3,
TbH3 and DyH3), new hydrides with FCC-type structure were synthesized around x = 67.
© 2005 Elsevier B.V. All rights reserved.
Keywords: High pressure; Hydride; Magnesium; Thermal stability; Rare earth
1. Introduction
synthesis can be mainly classified into two groups according
to pressure transmitting medium, those are, autoclave-type
with gas media and the anvil-type apparatuses with solid
one. For example, new hydrides of CsMgH3 [3] and
REMg2H7 [4] (RE = La, Ce) have been prepared by using
the autoclave-type apparatus, while Mg3MnH7 [5] and
Sr6Mg7H26 [6] were reported to be synthesized under a high
pressure up to GPa range by using the anvil-type apparatus.
Moreover, Mg2Ni3H3.4 [7,8], (Ca1−xMgx)2NiHy (x ꢀ 0.4)
[9–12], MgCaHy [11,12], MgY2H8 [13–16], Mg3MnHy
[16], Mg4Ni [17], MgCu [17] and Mg51Cu20 [17] have been
synthesized by using the anvil-type apparatus in our previous
works.
In this study, high-pressure synthesis has been used to
explore novel Mg–RE-based hydrides by using a cubic-anvil-
type apparatus. Since rare-earth elements (RE) have large
compressibility of the atomic radii (e.g. the radius of La is
reduced more than 12% under 5 GPa), there seems to be pos-
sibility to obtain novel hydrides in the Mg–RE–H systems.
RE hydrides have also high moles of hydrogen. Therefore, the
Mg-based alloys are well known to exhibit a high hydro-
gen capacity. They are expected to play an important role
as hydrogen storage media, although Mg-based conventional
alloys require high working temperature. But there are some
problems of their high reaction temperature and low kinetics.
To reduce the problems, a number of studies exploring new
Mg–RE-based alloys and hydrides have been conducted with
conventional metallurgy technique such as melting, sintering
and ball milling [1,2]. It seems to reach their limits to develop
new compounds by conventional studies.
As another approach, a high-pressure synthesis is an
effective technique to explore new compounds in a variety
of research fields. In the field of a hydrogen storage media
development, this method is used to obtain new hydrides as a
different approach from traditional ones. The high-pressure
∗
Corresponding author.
E-mail address: kamegawa@material.tohoku.ac.jp (A. Kamegawa).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2005.04.093

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285
purpose of this study is to explore new hydrides of the Mg–RE
systems (RE = Y, La, Ce, Pr, Sm, Gd, Tb, Dy) using the cubic
anvil-typeapparatusandclarifythecrystalstructures, thermal
stabilities and hydrogen contents of newly found hydrides.
2. Experimental procedures
Raw materials were MgH2 powder and rare-earth metals.
Rare-earth hydride powders were prepared by hydrogenation
of the rare-earth metals in an autoclave filled with hydrogen
gas. The powders were mixed at nominal composition and
pressed into pellets and put into BN containers in argon
filled glove box. The mixtures were heated at 1073 K for
2 h under a quasi-hydrostatic GPa order pressure and then
quenched.
Fig. 2. X-raydiffractionpatternsofMgH2–x mol%LaH3 preparedat1073 K
for 2 h under 5 GPa (x = 10, 25, 33, 50).
A phase identification was performed by powder X-ray
diffraction (XRD) using Cu K␣ radiation. The crystal struc-
tures of new hydrides were estimated by ITO12 integrated
in the CRYSFIRE program. Then, lattice parameters were
refined by CELL program. Thermal stability was investigated
using a differential scanning calorimeter (DSC) under Ar-gas
flow. The hydrogen content was calculated from a weight loss
observed in a thermo-gravimetric analysis (TGA), and was
measured by means of fusion extraction analysis (LECO).
to be 0.51657(2) nm. This value is smaller than that of YH3
(0.5602 nm) showing same FCC structure.
Fig. 2 shows XRD patterns of MgH2–x mol% LaH3
(x = 10, 25, 33 and 50) samples prepared at 1073 K for
2 h under 5 GPa. For the samples of x = 25–33, unknown
phase appeared as a single phase. The crystal structure
was estimated to be primitive tetragonal-type structure
and was indexed as this figure. On the other hand, it had
been reported that Mg2LaH7 synthesized by using the
autoclave-type apparatus under 10 MPa also had the another
tetragonal-type structure [3], their cell parameters were
obviously different from this new tetragonal phase. This new
phase was only synthesized under 3 GPa or higher. The cell
parameters were a = 0.8193 nm, c = 0.5028 nm at x = 25 and
a = 0.8172 nm, c = 0.5015 nm at x = 33.
3. Results and discussion
In Mg–Y–H system, MgY2Hy with a FCC-type struc-
ture has been prepared at 800 ◦C under more than 3 GPa
as shown in Fig. 1. Fusion extraction analysis showed
3.7 mass% of the hydrogen amount in MgY2Hy, correspond-
ing to y = ∼7.8. The lattice constant of MgY2H∼7.8 was found
By fusion extraction analysis, for the sample of x = 25, the
amount of hydrogen was estimated to be 4.1 mass% and the
chemical formula of the tetragonal phase can be expressed as
Mg3LaH9. These samples showed yellowish color and were
unstable under the atmospheric pressure in air.
Fig. 3 shows the DSC curves of MgH2–x mol% LaH3
(x = 10, 25, 30, 33 and 50) samples prepared at 1073 K
for 2 h under 5 GPa. For the composition of x = 25–33,
Fig. 3. DSC curves of MgH2–x mol% LaH3 prepared at 1073 K for 2 h under
5 GPa (x = 10, 25, 30, 33, 50).
Fig. 1. X-ray diffraction patterns of MgH2–67 mol% YH3 prepared at
800 ◦C for 2 h under 2–5 GPa.

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Fig. 4. X-ray diffraction patterns of MgH2–25 mol% REH (RE = LaH3,
CeH2.5, PrH3) prepared at 1073 K for 2 h under 5 or 6 GPa.
Fig. 5. DSC curves of MgH2–25 mol% REH (REH = LaH3, CeH2.5, PrH3)
prepared at 1073 K for 2 h under 5 or 6 GPa.
an endothermic peak was observed at about 600 K. From
TG analysis, the amount of emitted hydrogen was esti-
mated to be 2.61 mass% for the sample of x = 25. The
reason of this small amount of emitted hydrogen was
found to be caused by partial dehydrogenation reaction
(Mg3LaH9 → 3 Mg + LaH3 + 3H2), which was confirmed
by X-ray diffraction analysis before and after DSC mea-
surement. The value of the amount of emitted hydrogen also
agreed to this equation. The endothermic temperature of the
new hydrides decreased with increasing La content in this
range.
newly phases were found in this study. To investigate the
thermal stability of the samples, DSC measurements were
performed. Fig. 5 shows the DSC curves of MgH2–25 mol%
REH (REH = LaH3, CeH2.5, PrH3) samples prepared at
1073 K under 5 or 6 GPa. An endothermic peak correspond-
ing to decomposition of new hydrides into Mg and REH
with partial dehydrogenation was observed at about 600 K
(Mg3LaH9: 614 K, Mg3CeH8.1: 609 K, Mg3PrH9: 630 K).
In these three systems, the difference of the synthesis con-
dition for new hydrides suggested that the ionic radius of
rare-earth element was related to the synthesis pressure. The
synthesis condition for new hydrides might be depended on
ionic radius ratio between hydrogen ion and each rare-earth
ion. If so, the system contained rare-earth element with the
smaller ionic radius would be required the higher synthesis
pressure to obtain new tetragonal-type hydride, since hydro-
gen ion has higher compressibility than that of rare-earth
ion.
Besides, in MgH2–x mol% REH systems excepting for
Pr system, these novel hydrides were observed in the range
of x = 25–33. The lattice constants of the hydride were
decreased with increasing of rare-earth content, although
rare-earth elements have larger ionic radius than that of Mg
element. Thismightbeduetothedefectstructuredescribedas
Mg3−δREH9−2δ. Judging from the lattice constant, Mg ions
would occupy no rare-earth ion sites and on the other hand,
rare-earth ions would occupy no Mg ion sites. Moreover,
from the reason, the hydride would become to be unstable
thermally with increase of rare-earth element.
Fig. 4 shows XRD patterns of MgH2–25 mol% REH
(REH = LaH3, CeH2.5, PrH3) samples prepared at 1073 K
under 5 or 6 GPa. As the result, the new tetragonal hydride
was synthesized in all of these systems. The new hydrides
with tetragonal structure were synthesized as single phase at
1073 K, over 3, 4 and 6 GPa in Mg–La, Mg–Ce and Mg–Pr
systems, respectively. For Mg–Ce–H system, the novel phase
had the solid-solution range similar to Mg–La–H system.
For the sample prepared under 3 GPa, raw material phases
also appeared with new tetragonal phase differently from
Mg–La–H system. On the other hand, for MgH2–25 mol%
PrH3 samples, tetragonal phase appeared as a single phase
at only composition of x = 25, and the hydride does not seem
to have the solid-solution range, which is different from the
other systems. For the sample prepared under 5 GPa, raw
material phases also appeared with new tetragonal phase
differently from the other systems. The cell parameters of
new phase are a = 0.8118 nm, c = 0.4979 nm at x = 25 and
a = 0.8109 nm, c = 0.4969 nm at x = 33, and a = 0.8058 nm,
c = 0.4970 nm at x = 25 in Mg–Ce and Pr systems, respec-
tively. The hydrogen content of new hydrides were estimated
to be 3.7 and 3.9 mass% in Mg–Ce and Pr systems by fusion
extraction analysis, respectively. Therefore, the chemical
formulae can be expressed as Mg3CeH8.1and Mg3PrH9.
Each samples in Mg–Ce and Pr system showed reddish
and yellowish color, and both samples were unstable under
the atmospheric pressure in air. However, in Mg–Sm–H
system, Mg and Sm hydrides were not reacted and no
Fig. 6 shows XRD patterns of MgH2–67 mol% hREH
(hREH = GdH3, TbH3, DyH3) samples prepared at 1073 K
under GPa pressure. As the result, the new FCC-type hydride
was synthesized in all of these systems. For Mg–Gd–H and
Mg–Dy–H systems, the novel phase was synthesized under
3 GPa or higher. For Mg–Tb–H system, the novel phase was
synthesized under 6 GPa or higher. GdH3, TbH3 and DyH3
have a hexagonal structure (cf. LaH3, CeH2.5, PrH2 have a
cubic one), and the newly found hydrides with hRE have

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287
to about 600 K under Ar flow, and decomposed into Mg and
rare-earth hydride. For MgH2–67 mol% hREH3 (hRE = Gd,
Tb, Dy), the new hydrides were also obtained and had FCC-
type structure.
Acknowledgement
This work has been supported in part by NEDO program
and 21st century COE (Center of Excellence) program.
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The synthesis of novel hydride in Mg–RE systems
(RE = Y, La, Ce, Pr, Sm, Gd, Tb, Dy) by using a high-
pressure apparatus has been conducted. In Mg–Y–H system,
the new hydride of MgY2Hy with a FCC-type structure has
been prepared at 800 ◦C under more than 3 GPa. The new
hydrides with tetragonal structure were synthesized as single
phase at 1073 K, over 3, 4 and 6 GPa in Mg–La, Mg–Ce
and Mg–Pr systems, respectively. The chemical composi-
tion of the new hydride was around MgH2–25 at.% REH
(REH = LaH3, CeH2.5 and PrH3). The chemical formulae and
lattice constants of the hydrides were found to be correspond-
ing to Mg3LaH9: a = 0.8193 nm, c = 0.5028 nm, Mg3CeH8.1
:
a = 0.8118 nm, c = 0.4979 nm and Mg3PrH9: a = 0.8058 nm,
c = 0.4970 nm, respectively. The hydrogen contents of the
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