Desorption characteristics of rare earth (R) hydrides (R=Y, Ce, Pr, Nd, Sm, Gd and Tb) in relation to the HDDR behaviour of R-Fe-based-compounds

Article abstract of DOI:10.1016/S0925-8388(96)03097-6

Vacuum desorption characteristics of binary hydrides of rare earth metals (R=Ce, Pr, Nd, Y, Sm, Gd, Tb) were studied. A complete decomposition of hydrides, confirmed by X-ray diffraction, was achieved at temperatures up to 820°C. When heating the hydrogenated material in vacuum, two distinct hydrogen desorption events were observed for all the hydrides under investigation. The hydrogen desorption behaviour is different for the two groups of metals, namely group I (Ce, Pr and Nd) and group II (Y, Sm, Gd and Tb). The hydrogenation-disproportionation-desorption-recombination (HDDR) process was shown to take place for the Nd₆Fe₁₃Ge and Nd₆Fe_12.8Ga_1.2 intermetallic compounds, both with the Nd₆Fe₁₃Si-type structure. The hydrogen absorption and desorption properties of these compounds were studied in relation to the desorption characteristics of neodymium hydride.

Full text of DOI:10.1016/S0925-8388(96)03097-6

Journal of Alloys and Compounds 253–254 (1997) 128–133
L
Desorption characteristics of rare earth (R) hydrides (R5Y, Ce, Pr, Nd,
Sm, Gd and Tb) in relation to the HDDR behaviour of R–Fe-based-
compounds
a,
b
a
b
*
V.A. Yartys , O. Gutfleisch , V .V . Panasyuk , I.R. Harris
a
Physico–Mechanical Institute of the National Academy of Sciences of Ukraine, 5, Naukova Str., Lviv, 290601, Ukraine
b
School of Metallurgy and Materials, The University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK
Abstract
Vacuum desorption characteristics of binary hydrides of rare earth metals (R5Ce, Pr, Nd, Y, Sm, Gd, Tb) were studied. A complete
decomposition of hydrides, confirmed by X-ray diffraction, was achieved at temperatures up to 820 8C. When heating the hydrogenated
material in vacuum, two distinct hydrogen desorption events were observed for all the hydrides under investigation. The hydrogen
desorption behaviour is different for the two groups of metals, namely group I (Ce, Pr and Nd) and group II (Y, Sm, Gd and Tb). The
hydrogenation–disproportionation–desorption–recombination (HDDR) process was shown to take place for the Nd Fe Ge and
6
13
Nd Fe12.8Ga1.2 intermetallic compounds, both with the Nd Fe Si-type structure. The hydrogen absorption and desorption properties of
6
6
13
these compounds were studied in relation to the desorption characteristics of neodymium hydride.
Keywords: Rare earth metals; Hydrides; Desorption; HDDR
1
. Introduction
The hydrogenation–disproportionation–desorption–
are far below atmospheric pressure [5]. Vacuum desorption
properties were studied in details only for neodymium
hydride where a two-step decomposition was observed by
mass-spectrometer studies at temperatures up to 800 8C
recombination (HDDR) process has been applied very
successfully to the production of magnetically coercive
powders of Nd Fe B [1,2] and Sm Fe N [3]. In order
[8]. In order to proceed with the recombination step of the
HDDR route [1,2], two experimental procedures can be
applied. First, a continuous hydrogen vacuum desorption at
pressures below the equilibrium pressure on heating (up to
2
14
2
17
3
to understand the HDDR process and to apply it to
different rare earth systems, it is essential to characterise
the stability and absorption and desorption behaviours of
the particular R–hydrides. Phase equilibria in the binary
systems of rare earth metals with hydrogen have been
studied in a number of works (see, for example, [4–6]).
For Ce, Pr and Nd the binary hydride covers the approxi-
mate range RH2 to RH3 whilst maintaining the face-
centred cubic structure [7]. In contrast, a transformation
from a cubic into a hexagonal crystal symmetry takes place
on transition from the di-hydride to the tri-hydride for
R5Y and rare earth metals Sm, Gd and Tb [7]. Measure-
ments of hydrogen desorption isotherms indicated that, in a
temperature range up to 1000 8C the plateaux of H2
1
000 8C), or secondly, application of a high temperature
regime (above 1000 8C) where a substantial increase of the
desorption equilibrium pressure will ensure the complete
removal of hydrogen from the metal matrix, even under a
hydrogen atmosphere. In the present work, vacuum desorp-
tion behaviours were determined for a number of RH|3
binary hydrides including R5Y, Ce, Pr, Nd, Sm, Gd and
Tb. The HDDR routes for the compounds Nd Fe Ge and
6
13
Nd Fe12.8Ga1.2, both forming neodymium hydride on
6
disproportionation, were also studied and are discussed in
relation to the vacuum desorption behaviour of NdH .
|
3
pressure on the P–C–T diagram for the RH –R systems
2
2
. Experimental details
*
The hydrides studied in the present work were obtained
from the initial rare earth metals by heating them in
Corresponding author. Fax:
yartys@ah.ipm.lviv.ua
1
380 322 649427; e-mail:
0
925-8388/97/$17.00

1997 Elsevier Science S.A. All rights reserved
PII S0925-8388(96)03097-6

V.A. Yartys et al. / Journal of Alloys and Compounds 253–254 (1997) 128–133
129
hydrogen in a hydrogen DTA (HDTA) chamber. The
sample chamber containing the rare earth metal sample
with an average weight of 60–80 mg, was evacuated, filled
subsequently with hydrogen (1 bar pressure) and then
typical example is presented in Fig. 1 showing the data for
Y. The hydrogenation was completed in the temperature
range from 270 8C (Pr) to 455 8C (Tb). Saturated hydrides
of rare earth metals belonging to group I (Ce, Pr and Nd)
were formed at lower temperatures compared with those of
group II (Y, Sm, Gd and Tb). As can be seen from the data
presented in Table 1, the decomposition of hydrides
proceeds over a wide temperature range from 100 8C
2
1
heated from room temperature with a rate of 5 8C min
.
On heating, a strong exothermic reaction was observed
indicative of hydride formation. The hydrogenation treat-
ment lasted 15–30 min and was, in all cases, completed at
temperatures below 500 8C. The hydrogen desorption
properties of the hydrides were studied by cooling the
material to room temperature and then evacuating the
chamber. In this manner, the risk of oxidising the material
was reduced significantly.
(SmH32x) to 820 8C (YH ). The measurements showed
2
the existence of two separate events of hydrogen desorp-
tion for all the hydrides (see Fig. 2a–g). These events are
associated with the low temperature desorption RH →RH
3
2
(approximate formulae) and the high temperature desorp-
2
1
Subsequently, the material was heated at 5 8C min and
the temperature eventually increased to 800–850 8C, pro-
viding the conditions necessary for a complete desorption
of the hydrides. The experimental details of the HDTA
technique have been reported previously [9]. Temperature
pressure analysis (TPA) was also performed [10], again
using an initial hydrogen pressure of 1 bar and a heating
tion RH →R. A clear difference in stability, both for the
2
tri- and di-hydrides, was indicated for the two groups. In
the case of the tri-hydrides of group I, the low temperature
RH →RH desorption consisted of two overlapping de-
3
2
sorption events. A peak of hydrogen desorption observed
at 340–380 8C, is preceded at lower temperatures by a
shoulder in the hydrogen pressure curve. The most pro-
nounced effects were observed for Pr which exhibited two
low temperature peaks. The main desorption takes place at
340 8C and a smaller but clearly indicated second peak can
be observed at 255 8C. In contrast, the low temperature
desorption of the group II metals consisted of a regular,
single-peak with an approximate Gaussian-type distribu-
tion. Consequently, the tri-hydrides of group II desorb over
significantly narrower temperature ranges (160 8C for Y,
180 8C for Gd and 170 8C for Tb) compared to those of
group I (230 8C for Nd, 270 8C for Pr).
2
1
rate of 5 8C min
.
The compounds, Nd Fe Ge and Nd Fe12.8Ga1.2 were
6
13
6
prepared by argon arc melting mixtures of the high purity
constituent elements, with a subsequent annealing at
6
00 8C for 6 weeks in an attempt to homogenise the alloys.
X-ray diffraction studies were performed with a Phillips
PW 1012/10 diffractometer using Cu Ka radiation. Cell
parameters were calculated using the least square refine-
ment program LATTIC.
3
. Results and discussion
3.2. RH|2 hydrides
3
.1. RH|3 hydrides
It can be seen from the data presented in Table 1 that,
with increasing atomic number from Ce to Tb, the position
of the high temperature peak rises gradually from 650 8C
for Ce to 750 8C for Tb. The highest stability of di-hydride
was observed in the case of the YH|2 (desorption peak at
The strongly exothermic behaviour of hydride formation
causes a pronounced temperature increase accompanying
hydrogenation (e.g. DT565 8C in the case of Nd). A
7
90 8C). The eventual formation of the initial R metals
Table 1
Vacuum desorption behaviour of rare earth hydrides
Phase transition
YH –YH
Start
Peak
Finish
170
650
150
500
150
540
220
550
100
550
170
550
150
620
260
790
340
650
340
675
380
720
235
740
275
720
255
750
330
820
400
730
420
730
450
800
370
800
350
800
320
800
3
2
YH –Y
2
CeH –CeH
3
2
CeH –Ce
2
PrH –PrH
3
2
PrH –Pr
2
NdH –NdH
3
2
NdH –Nd
2
SmH –SmH
3
2
SmH –Sm
2
GdH –GdH
3
2
GdH –Gd
2
Fig. 1. Hydrogenation of yttrium in the HDTA rig (initial H pressure 1
2
2
1
TbH
–TbH
3 2
bar; heating rate 5 8C min ). Hydrogen absorption starts at 380 8C and is
TbH –Tb
2
completed by 445 8C.

1
30
V.A. Yartys et al. / Journal of Alloys and Compounds 253–254 (1997) 128–133
Fig. 2. HDTA traces of hydrogen desorption from the RH|3 hydrides: (a) CeH|3 (an allotropic transformation b-Ce–g-Ce and melting of Ce are indicated
on the DTA curve at 7208C and 7908C, respectively, above the temperature of complete hydrogen desorption); (b) PrH|3; (c) NdH|3; (d) YH|3; (e) SmH|3
f) GdH|3; (g) TbH|3
;
(
.

V.A. Yartys et al. / Journal of Alloys and Compounds 253–254 (1997) 128–133
131
Fig. 3. TPA traces of hydrogen absorption for Nd Fe12.8Ga1.2 inter-
Fig. 4. XRD patterns for Nd Fe GeH hydride after decomposition in the
6 13 x
6
metallic compound.
TPA chamber (heating in hydrogen up to 800 8C).
after hydrogen desorption was confirmed by X-ray diffrac-
tion. For example, the cell parameters of the hexagonal
unit cells of Tb and Gd, after desorption, were found to be
alloys contained Nd Fe Si-type compounds as the main
6
13
constituents, with small amounts of Nd-rich phase and
Nd–Ge (Nd–Ga) binary compounds. TPA studies of the
hydrogen absorption behaviour revealed two major absorp-
tion events for both compounds (see Fig. 3 where the data
˚
˚
˚
Tb: a53.603(1) A; c55.682(4) A and Gd: a53.622(1) A;
˚
c55.772(4) A. These results are in good agreement with
˚
˚
for Nd Fe12.8Ga1.2 are presented). The first absorp-
the reference data for Tb (a53.59 A; c55.66 A) and Gd
(
6
˚ ˚
a53.63 A; c55.79 A) metals [11].
tion started at 120 8C (Nd Fe12.8Ga1.2
)
or 150 8C
6
(
Nd Fe12.8Ge) and corresponds to the formation of
6
Nd Fe12.8Ga1.2H20.2 and Nd Fe12.8GeH18.5 hydrides. This
6
6
3
.3. HDDR behaviour of Nd Fe Ge and Nd Fe12.8Ga1.2
absorption leads to the insertion of hydrogen atoms into
the tetragonal unit cells of the compounds. Such insertion
does not change their tetragonal symmetry. However, due
to an anisotropic expansion of the cells in the [001]
direction, the c/a ratio increased on hydrogen absorption
from the initial values of 2.833 (Ga) and 2.851 (Ge) to a
value of 3.116 (the latter was the same for both hydrides).
The crystallographic characteristics of the intermetallic
compounds and their hydrides are presented in Table 2.
On further heating, a second hydrogen absorption event
was observed at 425 8C and 440 8C, respectively, for the
Ge- and Ga-containing compounds. As indicated by XRD
studies, this effect is associated with the decomposition of
the metal matrix with the formation of NdH32x binary
hydride and a-Fe as the main decomposition products (Fig.
4). Hydrogen desorption studies indicated clearly the
nature of the hydrogenated material (insertion-type tetra-
6
13
6
In addition to h-Nd1.1Fe B , (Nd,Pr) Fe T (T-doping
4
4
6
13
element) compounds can be formed as grain boundary
phases when doping elements are added to (Nd,Pr)FeB
permanent magnet alloy, as observed in the case of a
Pr Fe B5.5Cu1.5 magnet [12]. Nd Fe B , (F-phase),
and the ternary B-rich compounds in the Nd–Fe–B
system, namely the h- and r-borides, all disproportionate
in hydrogen on heating to 800 8C [13,14]. These processes
were found to be reversible (Nd Fe B [1,2], Nd Fe B
1
7
7 6
2
14
2
2
14
5
2
6
[14]) or irreversible (Nd1.1Fe B [13], (probably due to
4 4
oxidation)). For the R Fe T (R5Pr, Nd; T5Cu, Sb, Bi)
6
13
compounds, studies of their hydrogenation properties have
been limited to a temperature range of up to 350 8C [15].
At these temperatures no signs of a disproportionation
reaction were observed.
In the present work, hydrogen absorption and desorption
behaviour of Nd Fe Ge and Nd Fe12.8Ga1.2 were investi-
gonal hydride in the low temperature region or NdH32x-
containing material on exposure of the alloy to hydrogen at
6
13
6
gated at temperatures up to 800 8C. X-ray diffraction
temperatures up to 800 8C). For the saturated low tempera-
studies and SEM investigations showed that both annealed
ture Nd Fe GeH
hydride, the desorption is completed
6
13
18.5
Table 2
Crystallographic characteristics of Nd Fe Ge and Nd Fe12.8Ga1.8 intermetallic compounds and their hydrides
6
13
6
3
˚
˚
˚
Compound/hydride
a (A)
c (A)
c/a
V (A )
Da/a (%)
Dc/c (%)
DV/V (%)
Nd Fe13Ge
8.030(10)
8.111(3)
8.056(6)
8.178(6)
22.89(4)
25.27(1)
22.82(2)
25.48(3)
2.851
3.116
2.833
3.116
1476.0
1662.5
1481.0
1704.1
2
2
2
6
Nd Fe13GeH18.5
1.0
2
10.4
2
12.6
2
6
Nd Fe12.8Ga1.2
6
Nd Fe13GaH20.2
1.5
11.7
15.1
6

1
32
V.A. Yartys et al. / Journal of Alloys and Compounds 253–254 (1997) 128–133
cases, the formation of the R metals on desorption was
confirmed by X-ray diffraction studies. The group I metals
Ce, Nd, Pr) exhibited a broad (two peak) desorption
behaviour at low temperatures whereas the group II metals
Y, Sm, Gd, Tb) only exhibited a single peak. For the
(
(
RH|2 phases an increased stability was observed with
increasing atomic number of the R-component.
The HDDR route was established for Nd Fe Ge and
6
13
Nd Fe12.8Ga1.2. A disproportionation reaction in hydrogen
6
atmosphere of these compounds at temperatures above
5
00 8C was followed by a recombination reaction on
desorption at the same temperature. Desorption from the
disproportionated mixture exhibited close similarity to that
of NdH32x. Nd Fe Ge and Nd Fe12.8Ga1.2 compounds
6
13
6
represent a new example of R-containing materials where
the HDDR technique can be applied in order to modify the
microstructure of the initial alloys.
Acknowledgments
Dr. I. Bulyk, Mr. J. Clarke, Mr. A. Riabov, Mr. D.
Willey and Dr. A.J. Williams are thanked for their invalu-
able help in the experimental work and in the preparation
of the manuscript. The School of Metallurgy and Materials
and the National Academy of Sciences of Ukraine are
gratefully acknowledged for their support. The authors
would like to thank the Royal Society for the provision of
research grant supporting a collaborative research program
between the PhMI NAS Ukraine and the School of
Metallurgy and Materials, The University of Birmingham.
This work was supported in part by ICF grant UCQ2OO.
Fig. 5. HDTA desorption spectra: (a) Nd Fe13GeH18.5 saturated hydride;
6
(
b) Nd Fe13GeHx decomposed hydride.
6
at 500 8C (see Fig. 5a) with the main desorption taking
place at 185 8C and another, much weaker one, at 440 8C.
After this treatment, return to the crystal lattice of the
initial compound was confirmed by X-ray diffraction. As
expected, in the case of the disproportionated
Nd Fe12.8GeHx material (NdH32x1a-Fe), the main de-
sorption events (peaks at 160–210 8C and 680 8C, see Fig.
6
References
5
b for details) are very similar to those observed for the
[
[
[
1] T. Takeshita and R. Nakayama, Proc. 10th Int. Workshop on Rare
Earth Magnets and their Applications, Kyoto, 1989, p. 551.
2] I.R. Harris, Proc. 12th Int. Workshop on Rare-Earth Magnets and
their Applications, Canberra, 1992, p. 347.
3] S. Sugimoto, H. Nakamura, M. Okada and M. Homma, Proc. 12th
Int. Workshop on Rare-Earth Magnets and Their Applications,
Canberra, 1992, p. 372.
NdH32x hydride (Fig. 2c). In both materials the major part
of hydrogen is removed in the high temperature region due
to desorption of NdH to Nd. XRD studies showed that, on
hydrogen desorption, the tetragonal Nd Fe Ge and
Nd Fe12.8Ga1.2 compounds are restored, thus indicating
that the HDDR route takes place for these compounds.
2
6
13
6
[
[
[
[
4] C.E. Messer and M.K. Park, J. Less-Common Metals, 26 (1972)
2
35.
5] C. Ohki, H. Uchida and E. Ko, J. Japan Inst. Metals, 54 (1990) No.
, 146.
6] M.H. Mintz, Z. Hadari and M. Bixon, J. Less-Common Metals, 37
1974) 331.
2
4
. Conclusions
(
The general desorption behaviour of the fully hydro-
7] K.M. MacKay, Hydrogen Compounds of the Metallic Elements. E.
genated rare earth metals (R5Ce, Pr, Nd, Y, Sm, Gd, Tb),
when heated in a vacuum, consists of the initial desorption
of hydrogen to form the RH|2 composition followed by
the complete desorption at much higher temperatures to
form the R metal. This results in two distinct desorption
events and these cover a wide temperature range from
and F.N. Spon, London, 1966.
[8] A.J. Williams, P.J. McGuiness and I.R. Harris, J. Less-Common
Metals, 171 (1991) 149.
[
9] D. Book, I.R. Harris, A. Manaf, I. Ahmad and H.A. Davies, J.
Alloys Comp., 221 (1995) 180.
[
[
10] D. Book and I.R. Harris J. Alloys Comp., 221 (1995) 187.
11] Metals Reference Book, C.I. Smithells (ed.), Butterworth, London
and Boston, 1976.
1
00 8C (SmH →Sm ) to 820 8C (YH →Y). In all
|3 H|2 |2

V.A. Yartys et al. / Journal of Alloys and Compounds 253–254 (1997) 128–133
133
[
12] T. Kajitani, K. Nagayama and T. Umeda, Proc. 12th Int. Workshop
on RE Magnets and Their Applications, Canberra, 1992, p. 574.
13] V.A. Yartys, O. Gutfleisch and I.R. Harris, J. Magn. Magn. Mater.,
[14] V.A. Yartys, O. Gutfleisch and I.R. Harris, J. Alloys Comp., 253–
254 (1997) 134–139.
[15] J.M.D. Coey, Quinian Qi, K.G. Knoch, A. Leithe-Jasper and P.
Rogl, J. Magn. Magn. Mater., 129 (1994) 87.
[
1
19 (1996) 157–158.