High-pressure synthesis of novel europium magnesium hydrides

Article abstract of DOI:10.1016/S0925-8388(01)01217-8

Three new ternary metal hydrides of composition Eu₂MgH₆, Eu₆Mg₇H₂₆ and Eu₂Mg₃H₁₀ have been synthesised from mixtures of the binary hydrides in a multi-anvil press at quasi-hydrostatic pressures of up to 3.2 GPa and temperatures of up to 870 K. Crystal structures of the deuterides were determined by joint refinements of neutron and synchrotron powder diffraction data. All structures have alkaline earth and fluorine analogues. Eu₂MgD₆, K₂GeF₆ type structure, space group P3?m1, a=550.644(6) pm, c=410.054(6) pm; Eu₆Mg₇D₂₆, Ba₆Zn₇F₂₆ type structure, I2/m, a=1409.39(4) pm, b=564.55(2) pm, c=1150.90(3) pm, β=90.683(2)°; Eu₂Mg₃D₁₀, Ba₂Ni₃F₁₀ type structure, C2/m, a=1748.41(5) pm, b=572.31(2) pm, c=736.47(2) pm, β=111.478(2)°. The coloured compounds are saline and show volume contractions of up to 7.3% upon formation from the binary hydrides. Eu₂MgH₆ shows Curie-Weiss paramagnetic behaviour (μ_eff=7.88 μ_B) and orders ferromagnetic at T_C=31.6 K.

Full text of DOI:10.1016/S0925-8388(01)01217-8

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Journal of Alloys and Compounds 322 (2001) 59–68
L
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High-pressure synthesis of novel europium magnesium hydridesq
H. Kohlmanna,1, B. Berthevillea, T. Hansenb, K. Yvona ,
*
a
´
`
`
Laboratoire de Cristallographie, Universite de Geneve, 24 Quai Ernest Ansermet, CH-1211 Geneve 4, Switzerland
bInstitut Laue-Langevin, Avenue des Martyrs, BP 156X, F-38042 Grenoble Cedex, France
Received 19 February 2001; accepted 28 February 2001
Abstract
Three new ternary metal hydrides of composition Eu2MgH6, Eu6Mg7H26 and Eu2Mg3H10 have been synthesised from mixtures of the
binary hydrides in a multi-anvil press at quasi-hydrostatic pressures of up to 3.2 GPa and temperatures of up to 870 K. Crystal structures
of the deuterides were determined by joint refinements of neutron and synchrotron powder diffraction data. All structures have alkaline
¯
earth and fluorine analogues. Eu2MgD6, K2GeF6 type structure, space group P3m1, a5550.644(6) pm, c5410.054(6) pm; Eu6Mg7D26
,
Ba6Zn7F26 type structure, I2/m, a51409.39(4) pm, b5564.55(2) pm, c51150.90(3) pm, b590.683(2)8; Eu2Mg3D10, Ba2Ni3F10 type
structure, C2/m, a51748.41(5) pm, b5572.31(2) pm, c5736.47(2) pm, b5111.478(2)8. The coloured compounds are saline and show
volume contractions of up to 7.3% upon formation from the binary hydrides. Eu2MgH6 shows Curie–Weiss paramagnetic behaviour
(meff57.88 mB) and orders ferromagnetic at TC531.6 K.
2001 Elsevier Science B.V. All rights reserved.
Keywords: Rare earth compounds; Hydrogen absorbing materials; High pressure; Crystal structure; Neutron diffraction
1. Introduction
2. Experimental details
Recently, the first ternary europium magnesium hydrides
EuMgH4 and EuMg2H6 have been reported [1,2]. The
compounds were synthesised by hydrogenation of inter-
metallic compounds at 600 K under a hydrogen gas
pressure of 5 MPa and structurally characterised by
neutron powder diffraction at a wavelength close to the
minimum of neutron absorption by a natural isotope
mixture of europium. In view of the known existence of
ternary alkaline earth hydrides such as A2MgH6 (A5Sr
[3], Ba [4]), A6Mg7H26 (A5Ba [5]) and A2Mg3H10 (A5
Sr [6], Ba [7]), some of which (Sr2MgH6) were only
obtained by application of high quasi-hydrostatic pressure,
it was tempting to try the latter method also on the
Eu–Mg–H system. Here we report on the successful high
pressure synthesis and crystal structure of three novel
europium magnesium hydrides of composition Eu2MgH6,
Eu6Mg7H26 and Eu2Mg3H10.
2.1. Synthesis
The starting materials were europium ingot (natural
isotopic mixture, natEu, Arris International, 99.9%), mag-
nesium powder (Cerac, 99.6%, 400 mesh) and hydrogen
(Carbagas, 99.9999%) and deuterium gas (AGA, 99.8%).
All materials were handled in an argon-filled glove box
since they were sensitive to air. In a first step EuH2
(EuD2) powders were prepared by direct hydrogenation
(deuteration) of a piece of europium metal at T5600 K and
2 MPa hydrogen (deuterium) pressure during 2 days in an
autoclave. MgH2 (MgD2) was synthesised by hydrogena-
tion (deuteration) of magnesium powder at T5750 K and 9
MPa hydrogen (deuterium) pressure during 8 days in an
autoclave. To complete the reactions the products were
reground and the procedure repeated. In a second step
mixtures of EuH2 (EuD2) and MgH2 (MgD2) at various
proportions were compacted to pellets and placed in a
high-pressure cell. The cell consisted of a boron nitride
tube with boron nitride lid as sample container and was
surrounded by a graphite tube as external resistance
heating and pyrophyllite as pressure transmitting medium
[8]. The assembled cube-shaped cells were pressurised by
an octahedral anvil press. First, the pressure was increased
at a constant rate of typically 50 MPa/min followed by
qDedicated to Professor Dr. Horst P. Beck on the occasion of his 60th
birthday.
*Corresponding author.
E-mail address: klaus.yvon@cryst.unige.ch (K. Yvon).
1Present address: Department of Physics, 4505 South Maryland Park-
way, University of Nevada, Las Vegas, NV 89154-4002, USA.
0925-8388/01/$ – see front matter
PII: S0925-8388(01)01217-8
2001 Elsevier Science B.V. All rights reserved.

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H. Kohlmann et al. / Journal of Alloys and Compounds 322 (2001) 59–68
rapid heating (100 K/min) up to pressures and tempera-
tures of 3.2 GPa and 870 K, respectively (see below). The
samples were kept under these p2T conditions for 2 or 3 h
and then quenched by switching off the resistance heating
while releasing the pressure at a rate of about 50 MPa/
min.
Preliminary experiments suggested the formation of at
least three new ternary hydride phases of likely com-
position Eu2MgH6, Eu6Mg7H26 and Eu2Mg3H10 (see
Section 2.5). Depending on the molar ratio of the starting
materials these phases coexisted in various proportions and
were associated with more-or-less non-reacted EuH2 and
MgH2. Samples containing Eu2MgH6 without Eu6Mg7H26
and Eu2Mg3H10 were prepared from mixtures of either
EuH2 and MgH2 or EuH2 and EuMgH4 by using an excess
of EuH2. Samples containing Eu6Mg7H26 in which the
Eu2Mg3H10 content was minimised were prepared at
relatively low pressures (1.5–1.7 GPa). However, they
always contained Eu2MgH6 whose content was reduced by
capillaries (0.2 mm outer diameter) and measured in the
range 38#2u#438, D2u50.0058 and a wavelength of l5
60.054(1) pm during 6 h. They came from the same
batches as those investigated later by neutron diffraction
(see Section 2.3) and contained three ternary deuteride
phases, binary EuD2 and a(g)-MgD2, and traces of MgO
(for diffraction patterns see Fig. 1a).
2.3. Neutron diffraction
In view of the high absorption cross-section of natural
isotope mixtures of europium (natEu) for thermal neutrons
[9,10], the wavelength was chosen close to the minimum
of the absorption cross-section at lmin572.9 pm [1,2,10].
Data were taken on the high intensity diffractometer D4b
installed at the hot source of the reactor at ILL (Grenoble,
France). Three deuteride samples were investigated.
Whereas the Eu2MgD6 sample (0.9 g) was measured at
full intensity, the Eu6Mg7D26 (1.7 g) and Eu2Mg3D10 (1.6
g) samples could only be measured at 30% intensity due to
the non-availability of the hot source. In order to reduce
absorption further the samples were filled into double-
walled vanadium cylinders (64 mm length, 9.15 mm inner
diameter of the outer tube, 7.95 mm outer diameter of the
inner tube, 0.6 mm annular sample thickness). Wavelength
and zero-shift corrections were determined from a nickel
standard and found to be 70.50(1) pm and 20.1361(2)8
(Eu2MgD6 sample), and 70.647(8) pm and 20.1713(3)8
(Eu6Mg7D26 and Eu2Mg3D10 samples), respectively. Total
data collection times were 11, 32 and 24 h for the
Eu2MgD6, Eu6Mg7D26 and Eu2Mg3D10 samples, respec-
tively. Due to the low resolution at high diffraction angles
only data of the first detector (48#2u#708) were used in
the structure refinements (for diffraction patterns see Fig.
1b).
using
a
slight MgH2 excess. Samples containing
Eu2Mg3H10 in which the Eu6Mg7H26 content was mini-
mised were prepared by using an excess of MgH2. The
simultaneous presence in most samples of all three high-
pressure phases is presumably due to pressure and tem-
perature gradients in the high pressure cell. Attempts to
improve the homogeneity of the starting mixtures did not
yield better samples. For the structural and magnetic
studies three hydride samples were synthesised at the
following conditions (nominal composition, maximum
applied pressure and temperature in parentheses):
‘Eu2MgH6 sample’ (EuH2 /MgH252.1/1, 3 GPa, 870 K),
‘Eu6Mg7H26 sample’ (EuH2 /MgH256/7.7, 1.7 GPa, 790
K), ‘Eu2Mg3H10 sample’ (EuH2 /MgH252/4, 3.2 GPa,
760 K). Corresponding deuteride samples were synthesised
under the same conditions. Their colour was brown
(Eu2MgH6), red–brown (Eu6Mg7H26) and orange–light
brown (Eu2Mg3H10). The thermal stability was tested by
heating the samples under hydrogen atmosphere in an
autoclave. The Eu2MgH6 sample was found to be stable up
to 700 K and to decompose at 750 K into EuH2 and Mg.
The Eu6Mg7H26 and Eu2Mg3H10 samples decomposed at
700 K into Eu2MgH6 and EuMgH4.
2.4. Magnetic susceptibility measurements
The magnetic susceptibility of a Eu2MgH6 sample (7.7
mg powder pellet, containing 7% EuH2 as determined by
X-ray phase analysis) was measured on a SQUID magneto-
meter in the following ranges of temperature and magnetic
fields: 10 K#T#100 K (B50.01 T) and 50 K#T#300 K
(B52 T). Curie–Weiss paramagnetism and ferromagnetic
ordering at low temperatures was observed (Fig. 2). From
the slope and the intercept of the 1/X versus T plot in the
paramagnetic region a Curie temperature of TC531.6(2) K
and an effective magnetic moment for europium of meff5
7.88(1) mB was derived by correcting for the magnetic
EuH2 impurity.
2.2. X-ray diffraction
All reaction products were examined by X-ray powder
diffraction on a Guinier camera (samples enclosed in
sealed glass capillaries of 0.3 mm outer diameter, Co Ka1
radiation, internal Si standard) or on a Bragg–Brentano
diffractometer (Philips PW1820, flat samples in a holder
for air-sensitive substances, Cu Ka radiation, internal Si
standard). For structure refinement high-resolution
synchrotron powder diffraction data were recorded for the
deuteride samples Eu6Mg7D26 and Eu2Mg3D10 on the
Swiss–Norwegian beamline (BM1B) at ESRF (Grenoble,
France). The samples were enclosed in sealed glass
2.5. Structure determination
The X-ray diffraction patterns of the various samples
could be indexed on mixtures of trigonal, orthorhombic
and monoclinic unit cells and were consistent with calcu-

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H. Kohlmann et al. / Journal of Alloys and Compounds 322 (2001) 59–68
61
Fig. 1. Graphical representation of a joint Rietveld refinement on two synchrotron (a) and three neutron (b) powder diffraction patterns taken on the
deuteride samples Eu2MgD6, Eu6Mg7D26 and Eu2Mg3D10. The patterns show the following phases: s-Eu6Mg7D26 (1–5, 8); n-Eu6Mg7D26 (1–6, 8);
s-Eu2Mg3D10 (1–5, 7, 8); n-Eu2Mg3D10 (1–8); n-Eu2MgD6 (1–3, 7, 8). Phases: (1) Eu2MgD6, (2) EuD2, (3) Eu6Mg7D26, (4) Eu2Mg3D10, (5) a-MgD2,
(6) V, (7) g-MgD2, (8) MgO. Observed (crosses), calculated (solid line) and difference patterns (bottom) and Bragg markers of phases 1–8 (from bottom to
top). The inserts give a more detailed view of the d range 351.5 pm$d$185.0 pm, which corresponds to 9.808#2u#18.688 for the synchrotron patterns
and 11.58#2u#22.08 for the neutron patterns. Wavelengths 70.5–70.6 pm (neutrons), 60.05 pm (synchrontron).
lated patterns based on K2GeF6, Ba6Mg7D26 and
Ba2Ni3F10 type structures, respectively. This suggested the
presence of ternary hydride phases of composition
Eu2MgH6, Eu6Mg7H26 and Eu2Mg3H10 isoptypic to the
corresponding barium magnesium hydrides [4,5,7]. For the
orthorhombic pattern of Eu6Mg7H26, however, the
synchrotron data revealed a splitting of certain diffraction
lines thus indicating a lowering of symmetry. A satisfac-
tory fit was obtained by assuming a monoclinic distortion
(b590.68) of the orthorhombic lattice (b5908) similar to
that in the fluoride Ba6Zn7F26 [11]. For the sake of
comparison with the orthorhombic Ba analogue
Ba6Mg7D26 the non-standard space group setting I2/m
instead of standard C2/m was chosen. Refined cell param-
eters of the hydrides as obtained from laboratory X-ray
data are given in Table 3.
as performed with FullProf [12] resulted in satisfactory fits
and accurate metal positions for the synchrotron data, but
in unstable refinements and inaccurate deuterium positions
for the neutron data, even when applying severe con-
straints. In order to make use of the complementary nature
of both types of data a joint structure refinement was
performed by simultaneous use of all five data sets.
Depending on the data set up to eight phases were refined
(see Fig. 1). A recent version of GSAS [13] was used that
allowed to include the imaginary part of the neutron
scattering length, b0, of the highly absorbing natEu. In
order to test the programme the crystal structure of EuD2
which was previously refined [14] without taking into
account anomalous dispersion was refined again, but no
significant changes for the structure parameters were
observed. Starting values for the various atomic parameters
were obtained from preliminary refinements on the in-
Preliminary Rietveld refinements on individual patterns

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H. Kohlmann et al. / Journal of Alloys and Compounds 322 (2001) 59–68
Fig. 1. (continued)
dividual patterns (Eu2MgD6, Eu6Mg7D26, Eu2Mg3D10) or
from the literature (EuD2 [14], a(g)-MgD2 [16]). The joint
refinement converged and yielded structure parameters of
satisfactory precision (Table 1). The background of the
synchrotron data was described by a power series with
three coefficients and that of the neutron data by interpola-
tion between 10 points. Absorption corrections were
applied for the synchrotron data according to Hewat’s
formula [17,18] and for the neutron data by calculating
transmission factors for annular samples by using the
programme ABSOR [19]. For the ternary phases all
(isotropic) displacement parameters were constrained to be
equal for atoms of the same kind. In the final refinement
the following 182 parameters were refined: 60 positional
(three for Eu2MgD6, 33 for Eu6Mg7D26, 24 for
Eu2Mg3D10), 11 thermal displacement (three each for