Journal of Alloys and Compounds 299 (2000) L16–L20
L
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Letter
The crystal structures of EuH 2 and EuLiH 3 by neutron powder diffraction
H. Kohlmann*, K. Yvon
` , 24 Quai Ernest Ansermet, CH-1211 Geneve
` 4, Switzerland
Laboratoire de Cristallographie, Universite´ de Geneve
Received 30 November 1999; accepted 7 December 1999
Abstract
EuH 2 has been prepared by solid–gas and EuLiH 3 by solid–solid reaction. Their structures have been investigated by neutron powder
diffraction on the deuterides at a wavelength close to the minimum of the neutron absorption cross-section of a natural isotope mixture of
europium. EuD 2 crystallises with the orthorhombic PbCl 2 -type structure (space group Pnma, a5623.9(2), b5379.6(1), c5719.6(2) pm,
Z54), and EuLiD 3 with the cubic inverse perovskite type structure (space group Pm3¯m, a5378.75(5) pm, Z51). Both deuterides are
characterised by divalent europium and fully occupied deuterium sites. The Eu–D distances in EuD 2 are 255 pm (average over 9-fold Eu
co-ordination) and those in EuLiD 3 are 268 pm (12-fold Eu co-ordination).  2000 Elsevier Science S.A. All rights reserved.
Keywords: Metal hydrides; Neutron diffraction; Neutron absorption; Europium
1. Introduction
The structural analogy between europium and alkaline
earth elements in metal hydrides can be used for the search
for new hydrogen storage materials [1]. However, due to
the high absorption cross-section of natural isotope mixtures of europium ( nat Eu) for thermal neutrons, crystal
structure data on europium-based hydrides are scarce. The
only data known are those recently reported for EuMg 2 H 6
and EuMgH 4 [2,3]. In this paper we present data on two
more hydrides. Binary EuH 2 is of interest as a reference
substance and its structure [4], although likely to belong to
the PbCl 2 type, has never been confirmed. P–T –x measurements, for example, suggest a considerable range of
non-stoichiometry (EuH 1.8 – 1.95 [5,6]), thus raising questions about the possible presence of hydrogen defects and
the valence state of europium. The ternary hydride EuLiH 3
was assumed [7] to crystallise with the cubic perovskite
type structure but the hydrogen positions have never been
determined. Thus the perovskite type subbranch to which it
belongs (normal or inverse) is not certain. Atomic size
considerations suggest that it is of the inverse type, i.e., the
lower charged cation (Li 1 ) is expected to occupy the
octahedrally co-ordinated and the higher charged cation
(Eu 21 ) the cubo-octahedrally co-ordinated position. As in
*Corresponding author. Fax: 141-22-7026-108.
E-mail address: [email protected] (H. Kohlmann)
the previous studies [2,3], the neutron scattering measurements were performed on an advanced high flux diffractometer at a rather short neutron wavelength in order to
take advantage of the minimum in the wavelength dependence of the absorption cross-section of nat Eu for thermal
neutrons. In view of the rather high Eu content of the
present samples, absorption was expected to be even more
severe than for those studied previously.
2. Experimental details
2.1. Synthesis
The starting materials were Eu ingots (Arris International, 99.8%), LiH powder (Alfa, 98%) and LiD powder
(Strem Chemicals, .99% isotopic purity), H 2 gas (Carbagas, 99.9999%) and D 2 gas (AGA, 99.8%). All solids
were air sensitive and thus handled in an argon-filled glove
box. Binary EuH 2 (EuD 2 ) was prepared by hydrogenation
(deuteration) of europium metal at T5600 K and 2 MPa
hydrogen (deuterium) pressure during 2 days in an autoclave. The reaction product had the shape of the unreacted
metal and could be easily ground into a fine powder of
dark brownish-violet colour. Annealing an equi-molar
mixture of that powder with LiH (LiD) at T5600 K under
1 MPa hydrogen (deuterium) in an autoclave during 5 days
yielded EuLiH 3 (EuLiD 3 ) as a fine, bright red powder.
0925-8388 / 00 / $ – see front matter  2000 Elsevier Science S.A. All rights reserved.
PII: S0925-8388( 99 )00818-X
H. Kohlmann, K. Yvon / Journal of Alloys and Compounds 299 (2000) L16 –L20
L17
2.2. X-ray diffraction
For the EuH 2 and EuD 2 samples powder diffraction data
were recorded on a Bragg–Brentano diffractometer
(Philips PW1820, flat sample in a holder for air-sensitive
substances, Cu Ka radiation, 188#2u #1208, D2u 50.028,
10 s data collection time per step). The crystal structures
were refined (programme DBWS 9411 [8]) by neglecting
the hydrogen (deuterium) atoms. Refined cell parameters
for the hydride are: a5625.13(7), b5380.42(3), c5
721.31(9) pm. Refinement results on the deuteride are
listed in Table 1. For the EuLiH 3 sample data were
recorded on a Guinier camera (sealed glass capillary of 0.3
mm outer diameter, Co Ka 1 radiation) and the cell parameter was refined to a5379.63(2) pm by a least-squares
procedure from reflection positions corrected by an internal
standard silicon. This value is in excellent agreement with
that reported (379.6(1) pm [7]). For the EuLiD 3 sample,
temperature-dependent data were taken on a Guinier
powder diffractometer equipped by a closed cycle He
cryostat (Huber G 645, Cu Ka 1 radiation, 208#2u #928,
D2u 50.028, 20 s data collection time per step, sample
enclosed between two PET foils, internal standard silicon).
No structural phase transition was detected. The refined
cell parameter at T5293 K was a5378.80(2) pm, and that
at T512 K was 376.96(2) pm (programme DBWS-9411
[8], calibration of the 2u scale by fitting a second-order
polynomial against the reflections of an internal Si standard).
2.3. Neutron diffraction
The deuteride samples were filled into double-walled
vanadium cylinders (64 mm length, 9.15 mm outer diameter, 0.6 mm annular thickness) and mounted on the
diffractometer D4B (2 detectors) at the high-flux reactor at
ILL (Grenoble). The wavelength used ( l570.50(1) pm)
was close to the minimum of the absorption cross-section
of nat Eu (72 pm) for thermal neutrons, and was refined
from a measurement of a standard Ni sample. Due to the
relatively low resolution of the high angle data only those
of the first detector (6.158#2u #748) were used in the
structure refinements (FullProf [9]). The nuclear scattering
lengths were taken from Ref. [10]. For the EuD 2 sample
Fig. 1. Observed (top), calculated (middle) and difference (bottom)
neutron powder diffraction patterns of the EuD 2 sample (T5293 K,
l 570.50 pm). Vanadium reflections are due to the sample holder. No
absorption correction. Calculated patterns contain the refined background.
Intensity in total detector counts.
(2.1 g) data were recorded during 13.5 h. The good
correspondence between the observed and calculated patterns (Fig. 1) confirmed the PbCl 2 type structure. The
profiles were modelled by a pseudo-Voigt function and the
background was approximated by a polynomial of fifth
order. The following 27 parameters were allowed to vary:
one zero point correction, six background polynomial
parameters; one scale factor, three lattice, six positional,
three thermal displacement, one peak-shape and three halfwidth parameters for EuD 2 ; one scale factor, one lattice
and one thermal displacement parameter for vanadium
(sample holder). The peak shape and half-width parameters
of the latter were constrained to be equal to those of EuD 2 .
Refinement of the occupancy parameters of the two
deuterium sites did not result in values significantly
Table 1
Crystal structure data of EuD 2 refined from neutron powder diffraction and X-ray powder diffraction (in italics) (T5293 K); no absorption correction; form
of the temperature factor: T5exp [2Biso (sinu /l)2 ]
Space group Pnma (No. 62),
Eu
4c
D1
4c
D2
4c
R p 57.7%, R wp 56.9%, S52.3, R Bragg 54.5%
R p 52.3%, R wp 53.2%, S 51.5, RBragg 59.6%
a5623.9(2) pm, b5379.6(1) pm, c5719.6(2) pm
a5623.98(2) pm, b5379.71(1) pm, c5720.14(2) pm
x /a
y /b
0.2600(7)
1/4
0.2599(6)
1 /4
0.0284(7)
1/4
0.1444(5)
1/4
z /c
0.3873(4)
0.3877(2)
0.6785(4)
0.0721(6)
Biso (10 4 pm 2 )
0.99(5)
0.21(1)
1.75(8)
1.04(7)
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H. Kohlmann, K. Yvon / Journal of Alloys and Compounds 299 (2000) L16 –L20
different from 1.0. Thus they were fixed at unity. The
refinement results are listed in Table 1 and interatomic
distances are given in Table 3. Correcting the data for
absorption (program ABSOR [11], m (EuD 2 )520.67 cm 21
at l570 pm) did not change the results significantly,
except for a slight increase of the thermal displacement
parameters (Biso in 10 4 pm 2 : 1.14(5) for Eu, 1.89(8) for
D1, 1.14(7) for D2; see Table 1).
For the EuLiD 3 sample (2.0 g) data were collected
during 6 h. They showed good correspondence with a
calculated pattern (Fig. 2) based on the cubic inverse
perovskite-type structure. As minority phases some unreacted LiD and EuD 2 and traces of Li 2 O were detected.
The reflection profiles were modelled by a pseudo-Voigt
function and the background was approximated by a
polynomial of third order. The following 24 parameters
were refined: one zero point correction, four background
polynomial parameters; one scale factor, one lattice, three
thermal displacement, one peak shape and three halfwidth
parameters for EuLiD 3 ; one scale factor, one lattice and
two thermal displacement parameters for LiD; one scale
factor for EuD 2 ; one scale factor and one lattice parameter
for Li 2 O; one scale factor, one lattice and one thermal
Table 2
Crystal structure data of EuLiD 3 as refined from neutron powder
diffraction (T5293 K); no absorption correction; form of the temperature
factor as in Table 1
]
Space group Pm3 m (No. 221), a5378.75(5) pm
x /a
y /b
z /c
Biso (10 4 pm 2 )
Eu
1b
1/2
1/2
1/2
0.90(9)
Li
1a
0
0
0
0.1(1)
D
3d
1/2
0
0
1.39(4)
R p 59.2%, R wp 57.8%, S53.5, R Bragg 53.6%
displacement parameter for vanadium (sample holder). The
peak shape and halfwidth parameters of the secondary
phases and of vanadium were constrained to be equal to
those of EuLiD 3 . For EuD 2 the structural parameters were
fixed to those refined on the EuD 2 sample (see above).
Refinement results are listed in Table 2 and interatomic
distances are given in Table 3.
3. Results and discussion
EuH 2 appears to be a stoichiometric salt-like compound
that does not contain significant amounts of hydrogen
vacancies, at least not under normal conditions and in
equilibrium. This result is in accord with the semiconducting properties (Eg 51.85 eV [5]), the presence of localised
magnetic moments typical for divalent Eu [12], and the
volume increments [13], and justifies the classification of
this compound as a member of the ionic branch of the
PbCl 2 -type structure family [14]. As to the reports on its
possible non-stoichiometry [5,6], it is useful to point out
that europium metal, even if freshly distilled or sublimated,
tends to contain considerable amounts of hydrogen [15].
This means that measured hydrogen uptakes are generally
smaller than those expected for stoichiometric EuH 2 . The
Eu atoms in the structure are surrounded by nine deuterium
atoms forming a tri-capped trigonal prism (Fig. 3, left).
The mean Eu–D distance of 255 pm (Table 2) is close to
Table 3
Interatomic distances in EuD 2 and EuLiD 3 (in pm)
EuD2 :
Eu–D1
–2 D1
–2 D1
–D2
–2 D2
–D2
254.3(4)
265.8(4)
275.9(3)
238.3(5)
239.2(3)
241.6(5)
D1–Eu
–2 Eu
–2 Eu
D2–Eu
–2 Eu
–Eu
254.3(4)
265.8(4)
275.9(3)
238.3(5)
239.2(3)
241.6(5)
Eu–2 Eu
369.6(5)
–2 Eu
379.6(1)
]
d (Eu–D)5255 pm
Shortest D–D contact (D2–D2): 281.5(4) pm
Fig. 2. Observed (top), calculated (middle) and difference (bottom)
neutron powder diffraction patterns of the EuLiD 3 sample (for conditions
see Fig. 1).
EuLiD3 :
Eu–12 D:
Li–6 D:
Eu–6 Eu:
267.82(2)
189.38(2)
378.75(5)
D–4Eu:
D–2 Li:
D–8 D:
267.82(2)
189.38(2)
267.82(2)
H. Kohlmann, K. Yvon / Journal of Alloys and Compounds 299 (2000) L16 –L20
L19
Fig. 3. Deuterium co-ordination around europium in EuD 2 (left) and EuLiD 3 (right).
that observed in EuMgD 4 (253 pm [2,3]) where europium
is also 9-fold co-ordinated. The two deuterium sites have
5-fold nearly quadratic prismatic (D1) and 4-fold nearly
tetrahedral (D2) metal co-ordinations. These differences in
co-ordination numbers are reflected in the D–Eu distances
which are longer for D1 compared to D2 (Table 3), and
the thermal displacement parameters which are higher for
D1 compared to D2 (Table 1).
EuLiH 3 is also a stoichiometric salt-like hydride. Its
structure adopts the cubic inverse perovskite type structure
in which europium appears to be in the divalent oxidation
state. These findings are consistent with the colour (red),
the semiconducting behaviour (Eg 51.46 eV [16]),
¨
Mossbauer
spectroscopy experiments [17], volume increments [13] and the Goldschmidt’s tolerance factor, t50.94,
which is in the usual range 0.89,t,1 for cubic perovskitetype structures (see, for example, Ref. [18]). The Eu atoms
in the structure are cubo-octahedrally co-ordinated by
deuterium at a distance of 268 pm (Fig. 3, right). As
expected, this distance is considerably longer than the
average distance in EuD 2 (255 pm) in which Eu is only
nine co-ordinate. Interestingly, it is also longer than the
average Eu–D distance in EuMg 2 D 6 [2,3] in which Eu is
12 co-ordinate. The Eu–D distances in this defect perovskite structure are 266 pm toward the six-coordinate
deuterium site (4Eu12Mg) and 254 pm [2,3] toward the
four-coordinate deuterium site (2Eu12Mg). This contraction relative to the ideal perovskite structure is due to the
fact that in EuMg 2 D 6 every other Eu layer is missing,
which leads to a distortion of the deuterium cubo-octahedra and to deuterium co-ordination numbers that are lower
than that in EuLiD 3 (2Li14Eu). This underlines that metal
— deuterium distances in metal deuterides not only depend
on the co-ordination of the metal but also on that of
deuterium.
In conclusion, EuH 2 is confirmed to belong to the PbCl 2
structure type. It is isostructural to the alkaline earth
hydrides CaH 2 , SrH 2 , BaH 2 and to YbH 2 [19–22], in
contrast to the dihydrides of other rare earth elements that
adopt the fluorite-type structure. EuLiH 3 adopts the inverse
perovskite type structure such as the alkaline earth analogues SrLiH 3 and BaLiH 3 [23], rather than the normal
perovskite structure such as CsCaH 3 and RbMgH 3 [24].
The hydrogen storage efficiencies of EuH 2 and EuLiH 3 are
1.3 and 1.9 wt.%, and 78.1 and 91.8 g / l, respectively.
Acknowledgements
We thank Dr Henry Fischer and Dr Thomas Hansen,
ILL (Grenoble), for help with the neutron diffraction
experiment. This work was supported by the Swiss National Office of Energy and the Swiss National Science
Foundation.
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