Hydrogenation Properties of Laves Phases LnMg2 (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Ho, Er, Tm, Yb)

Article abstract of DOI:10.1021/acs.inorgchem.7b02319

The hydrogenation properties of Laves phases LnMg₂ (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Ho, Er, Tm, Yb) were investigated by thermal analysis, X-ray, synchrotron, and neutron powder diffraction. At 14.0 MPa hydrogen gas pressure and 393 K, PrMg₂ and NdMg₂ take up hydrogen and form the colorless, ternary hydrides PrMg₂H₇ (P4₁2₁2, a = 632.386(6) pm, c = 945.722(11) pm) and NdMg₂H₇ (P4₁2₁2, a = 630.354(9) pm, c = 943.018(16) pm). The crystal structures were refined by the Rietveld method from neutron powder diffraction data on the deuterides (PrMg₂D₇, P4₁2₁2, a = 630.56(2) pm, c = 943.27(3) pm; NdMg₂D₇, P4₁2₁2, a = 628.15(2) pm, c = 940.32(3) pm) and shown to be isotypic to LaMg₂D₇. The LaMg₂D₇ type of hydrides decompose at 695 K (La), 684 K (Ce), 684 K (Pr), 672 K (Nd), and 639 K (Sm) to lanthanide hydrides and magnesium. The Laves phase EuMg₂ forms a hydride EuMg₂H_x of black color. Its crystal structure (P2₁2₁2₁, a = 664.887(4) pm, b = 1136.993(7) pm, c = 1069.887(7) pm) is closely related to the hexagonal Laves phase (MgZn₂ type) of the hydrogen-free parent intermetallic. GdMg₂ and TbMg₂ form hydrides GdMg₂H_x with orthorhombic unit cells (a = 1282.7(4) pm, b = 572.5(2) pm, c = 881.7(2) pm) and TbMg₂H_x (a = 617.8(3) pm, b = 1045.8(8) pm, c = 997.1(5) pm), presumably also with a distorted MgZn₂ type of structure. CeMg₂H₇ and NdMg₂H₇ are paramagnetic with effective magnetic moments of 2.49(1) μ_B and 3.62(1) μ_B, respectively, in good agreement with the calculated magnetic moments of the free trivalent rare-earth cations (μ_calc(Ce³⁺) = 2.54 μ_B; μ_calc(Nd³⁺) = 3.62 μ_B).

Full text of DOI:10.1021/acs.inorgchem.7b02319

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX
Hydrogenation Properties of Laves Phases LnMg2 (Ln = La, Ce, Pr,
Nd, Sm, Eu, Gd, Tb, Ho, Er, Tm, Yb)
Anton Werwein,† Florian Maaß,† Leonhard Y. Dorsch,† Oliver Janka,‡ Rainer Pottgen,‡
̈
Thomas C. Hansen,§ Justin Kimpton,∥ and Holger Kohlmann*,†
†Department of Inorganic Chemistry, Leipzig University, Johannisallee 29, 04103 Leipzig, Germany
‡Institut fur Anorganische und Analytische Chemie, Universitat Munster, Corrensstrasse 30, 48149 Munster, Germany
̈
̈
̈
̈
§Institut Laue-Langevin, 71 Avenue des Martyrs, 38000 Grenoble, France
∥Australian Synchrotron, 800 Blackburn Road, Clayton, Melbourne, Australia
S
* Supporting Information
ABSTRACT: The hydrogenation properties of Laves phases
LnMg2 (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Ho, Er, Tm, Yb)
were investigated by thermal analysis, X-ray, synchrotron, and
neutron powder diffraction. At 14.0 MPa hydrogen gas
pressure and 393 K, PrMg2 and NdMg2 take up hydrogen
and form the colorless, ternary hydrides PrMg2H7 (P41212, a =
632.386(6) pm, c = 945.722(11) pm) and NdMg2H7 (P41212,
a = 630.354(9) pm, c = 943.018(16) pm). The crystal
structures were refined by the Rietveld method from neutron powder diffraction data on the deuterides (PrMg2D7, P41212, a =
630.56(2) pm, c = 943.27(3) pm; NdMg2D7, P41212, a = 628.15(2) pm, c = 940.32(3) pm) and shown to be isotypic to
LaMg2D7. The LaMg2D7 type of hydrides decompose at 695 K (La), 684 K (Ce), 684 K (Pr), 672 K (Nd), and 639 K (Sm) to
lanthanide hydrides and magnesium. The Laves phase EuMg2 forms a hydride EuMg2Hx of black color. Its crystal structure
(P212121, a = 664.887(4) pm, b = 1136.993(7) pm, c = 1069.887(7) pm) is closely related to the hexagonal Laves phase (MgZn2
type) of the hydrogen-free parent intermetallic. GdMg2 and TbMg2 form hydrides GdMg2Hx with orthorhombic unit cells (a =
1282.7(4) pm, b = 572.5(2) pm, c = 881.7(2) pm) and TbMg2Hx (a = 617.8(3) pm, b = 1045.8(8) pm, c = 997.1(5) pm),
presumably also with a distorted MgZn2 type of structure. CeMg2H7 and NdMg2H7 are paramagnetic with effective magnetic
moments of 2.49(1) μB and 3.62(1) μB, respectively, in good agreement with the calculated magnetic moments of the free
trivalent rare-earth cations (μcalc(Ce3+) = 2.54 μB; μcalc(Nd3+) = 3.62 μB).
observed for Laves phases so far, while undergoing a metal−
semiconductor transition.20,21 The hydrogen atoms occupy four
different crystallographic sites and are coordinated tetrahedrally
by two Ln and two Mg atoms, [Ln2Mg2], by four Mg atoms,
[Mg4], or by two Ln and one Mg atom in a trigonal-planar
fashion, [Ln2Mg]. The crystal structures (LaMg2D7 type) show
a close relationship to the parent cubic Laves phase, which
becomes expanded and distorted upon hydrogen incorporation.
Europium is an exception both in the series of Laves phases
(Table 1) and in the hydrides. The hexagonal Laves phase
EuMg2 forms a hydride EuMg2H6. This is the only hydride of a
Laves phase LnMg2 containing divalent lanthanide ions. Its
crystal structure shows no resemblance to any of the Laves
phase structure types. It represents a tetragonal defect-variant
of the cubic perovskite type,22,23 which is another common
structure type for metal hydrides.24−26 Despite the favorable
environmental properties, the high molar content, and the good
volume efficiency for hydrogen,20 the hydrides LnMg2H7
known so far are not very promising candidates for hydrogen
1. INTRODUCTION
Lanthanide containing hydrides are a fascinating class of
compounds with a wide range of compositions, crystal
structures, and bonding properties, ranging from ionic1,2 to
metallic3−5 and complex hydrides.6,7 Laves phases are a large
group of intermetallic compounds with the composition AM2,
where A is the larger atom. The three most common structure
types are cubic MgCu2 (C15), hexagonal MgZn2 (C14), and
MgNi2 (C36). Many Laves phases can be used for hydrogen
storage, since they reversibly take up large amounts of
hydrogen.8,9 In the crystal structures of the hydrides, hydrogen
atoms often occupy tetrahedral voids surrounded by two atoms
each of A and M, [A2M2], and tetrahedral voids surrounded by
one A and three M atoms, [AM3].10−18 In lanthanide-
containing Laves phases LnMg2 the lanthanide atoms Ln
always occupy the A sites due to their large size.19 Laves phases
LnMg2 of the early lanthanides Ln = La−Gd with the exception
of europium crystallize in the cubic MgCu2 (C15) type; those
with Ln = Eu and Tb−Lu crystallize in the hexagonal MgZn2
(C14) type of structure (Table 1). LnMg2 (Ln = La, Ce, Sm)
were found to absorb large amounts of hydrogen. They form
isotypic hydrides LnMg2H7 with the highest hydrogen content
Received: September 12, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b02319
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Inorganic Chemistry
Table 1. Structure Types of Laves Phases LnMg2 and Their Hydrides
Article
a
b
LaMg2
CeMg2
PrMg2
NdMg2
SmMg2
EuMg2
GdMg2
20
20
21
LaMg2H7
CeMg2H7
PrMg2H7 [this work]
NdMg2H7 [this work]
SmMg2H7
EuMg2H6,22 EuMg2Hx [this work]
GdMg2Hx [this work]
TbMg2
TbMg2Hx [this work]
DyMg2
HoMg2
ErMg2
TmMg2
YbMg2
LuMg2
a
b
First row, Ln = lanthanides, MgCu2 structure type (C15) in bold, MgZn2 structure type in italics. Second row, LnMg2H7 in LaMg2H7 type of
structure, EuMg2H6 own structure type.
storage. They need high hydrogen pressures to form, exhibit
only moderate weight efficiencies, and release hydrogen at
rather high temperatures and irreversibly. In this work, the
hydrogenation behavior of Laves phases LnMg2 of the
remaining lanthanides Ln = Pr, Nd, Gd, Tb, Ho, Er, Tm, Yb
(Table 1) are investigated, which have not been reported yet to
form hydrides. The aim of this study is to rationalize the
formation and decomposition conditions of the hydrides, since
these properties are crucial for possible hydrogen storage
applications.
Table 2. Experimental Conditions and Products of
Hydrogenation Experiments
a
Laves
phase
p(H2),
MPa
Tmax, K
t, h
products
LaMg2
703
393
703
393
393
703
393
703
393
703
353
723
473
723
473
703
473
673
473
723
473
703
5.0
14
5
2
LaH3, MgH2
48 LaMg2H7
CeH3, MgH2
CeMg2
2
14
9.5
5
48 CeMg2H7
48 PrMg2H7
PrMg2
NdMg2
2
NdH2,NdH2+x,MgH2
14
5
48 NdMg2H7
2. EXPERIMENTAL SECTION
SmMg2
EuMg2
GdMg2
TbMg2
HoMg2
ErMg2
2
SmH2, SmH2+x,MgH2
14
5
48 SmMg2H7
Syntheses. Because of the air sensitivity of the compounds all
handlings were performed in an argon-filled glovebox. Lanthanide
metals (La, Ce, Nd, Eu: Chempur, Sm: Alfa Aesar, Gd, Tb, Er, Yb:
Kristallhandel Kelpin, all with a purity of 99.9% or better) were
surface-cleaned, cut into small pieces, and mixed with magnesium
powder (abcr, 99.8%) in a stoichiometric 1:2 ratio. These mixtures
were sealed in niobium ampules, which were heated under vacuum in a
tube furnace to 1173 K and subsequently quenched in water. The
products were annealed below the peritectic decomposition temper-
ature of the Laves phases LnMg2.27 For preparation of the hydrides
(deuterides), well-ground samples LnMg2 were placed in crucibles
2
EuMg2H6, EuMgH4
12
5
48 EuMg2Hx
2
GdH2,GdH2+x
14
5
48 GdMg2Hx
2
TbH2,TbH2+x,TbH3, MgH2
14
5
48 TbMg2Hx
2
HoH2,HoH2+x
14
5
48 HoH2,HoH2+x
2
ErH2,ErH2+x,ErH3
14
5
48 ErH2,ErH2+x
made from hydrogen-resisting Bohler L718-V alloy and reacted with
̈
TmMg2
YbMg2
2
TmH2,TmH2+x, TmH3, Mg
hydrogen (deuterium) gas (hydrogen: Air Liquide, 99.9%; deuterium,
Air Liquide, 99.8%) in autoclaves made from the same alloy. The
temperature was increased with 10 K/h and kept at the maximum for
48 h (see Table 2 for gas pressures and temperatures). After the
annealing time the autoclave was cooled naturally. Replacing 5 mol %
of neodymium by samarium or terbium in the synthesis procedure led
to substituted samples as described below. The hydrogenation reaction
was performed in analogy to NdMg2H7.
14
5
48 TmH2
2
YbH2, YbH2.63, Yb4Mg3H14,
Yb19Mg8H54
393
8
2
YbMg2, YbH2.63
a
Experiments were in a differential scanning calorimeter (italics) and
autoclaves (bold).
Thermal Analysis. Hydrogenation experiments were performed in
a differential scanning calorimeter (DSC) Q1000 from TA Instru-
ments equipped with a gas pressure cell.28 The powdered samples
(10−30 mg) were loaded in aluminum pans, which were subsequently
crimped. The experiments were done with a heating rate of 10 K/min
under a hydrogen atmosphere of 5.0 MPa at 298 K, which rose to 6.7
MPa at the final temperature of 703 K.
X-ray Powder Diffraction. X-ray powder diffraction data were
collected on a Huber G670 camera with image plate system using Mo
Kα1 or Cu Kα1 radiation. Flat transmission samples were prepared by
grinding the moisture-sensitive powders in an argon atmosphere,
mixing with apiezon grease, and putting the sample between two
sheets of capton foil.
In Situ X-ray Powder Diffraction. In situ X-ray powder
diffraction was performed on a Huber G670 diffractometer with
image plate system and Mo Kα1 radiation (10 min data collection time
per temperature and 10 K temperature steps). The sample was
enclosed in a quartz capillary of 0.3 mm diameter, which was
connected to a gas control system.29
Synchrotron Powder Diffraction. Synchrotron powder diffrac-
tion experiments were performed on samples in sealed glass capillaries
of 0.3 mm diameter at the Australian synchrotron powder diffraction
beamline with a wavelength λ = 58.9074 pm. Data collection time was
20 min.
high-flux diffractometer D20 in high-resolution mode. Powdered
samples were placed in indium-sealed vanadium containers with 6 mm
inner diameter. Data collection times were 75 min for NdMg2D7 and
15 min for PrMg2D7, and the wavelengths were calibrated using silicon
(NIST640b) as an external standard.
Rietveld Refinement.30,31 The crystal structures were refined
using the software package TOPAS32 for X-ray and synchrotron
diffraction data and with the FULLPROF software package for
neutron diffraction data.33,34 The crystal structures were visualized
with VESTA.35 Additional crystallographic information is available in
the Supporting Information.
Magnetic Measurement. The powdered samples of CeMg2H7
and NdMg2H7 were packed into polyethylene (PE) capsules in an
argon-filled glovebox and attached to the sample holder rod of a
Vibrating Sample Magnetometer unit (VSM) for measuring the
magnetization M(T, H) in a Quantum Design Physical Property
Measurement System (PPMS). The samples were investigated in the
temperature range of 2.5−300 K with magnetic flux densities up to 80
kOe.
3. RESULTS AND DISCUSSION
Hydrogenation Reactions. The synthesis of the Laves
phases yielded single-phase products according to X-ray
powder diffraction except for NdMg2 and GdMg2, which
Neutron Powder Diffraction. Neutron powder diffraction was
performed at the Institut Laue-Langevin in Grenoble, France, at the
B
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Article
exhibit up to 20 wt % byproducts (NdMg, GdMg). Lattice
parameters refined by the Rietveld method are in good
agreement with literature values. All samples take up
considerable amounts of hydrogen and form binary or ternary
hydrides (Table 2). Low to moderate hydrogen gas pressure
and high temperature seem to favor the formation of mixtures
of the binary hydrides LnH2−3 and MgH2. These samples
appear black after hydrogenation. In some cases no crystalline
magnesium species can be identified by X-ray powder
diffraction. Exceptions from this finding are EuMg2 and
YbMg2. The former yields red EuMg2H6, while the latter yields
a black powder consisting of four different binary and ternary
hydrides. Ternary Laves phase hydrides LnMg2H7 were
obtained only at higher hydrogen gas pressures (≥9.5 MPa)
and moderate temperatures (393 K). Among them are the new
compounds PrMg2H7 and NdMg2H7 (crystal structures and
chemical and physical properties vide infra). Under similar
conditions, EuMg2, GdMg2, and TbMg2 react to new hydrogen-
containing phases LnMg2Hx. The ternary Laves phase hydrides
LnMg2H7 are colorless (Ln = Pr, Nd), red (Ln = Ce), and
green (Ln = Sm) powders, while LnMg2Hx (Ln = Eu, Gd, Tb)
are black powders. Small impurities of the binary hydrides
LnH2+x or LnH3−x may also lead to a black appearance of the
samples. The ternary hydrides except for EuMg2Hx are stable in
air. The remaining Laves phases LnMg2 (Ln = Er, Ho, Tm, Yb)
react with hydrogen to form binary lanthanide hydrides, of
which ErH2 and HoH2 show very poor crystallinity, and no
trace of any crystalline magnesium-containing compound.
Crystal Structures of LnMg2H7 (Ln = La, Ce, Pr, Nd,
Sm). The X-ray powder diffraction patterns of the two new
phases PrMg2H7 and NdMg2H7 could be indexed with
tetragonal cells similar to those of CeMg2H7 and SmMg2H7.
This suggests the crystal structure to belong to the LaMg2D7
type, which is confirmed by the good correspondence of
calculated and measured X-ray powder diffraction data
(PrMg2H7, P41212, a = 632.386(6) pm, c = 945.722(11) pm;
NdMg2H7, P41212, a = 630.354(9) pm, c = 943.018(16) pm).
To locate the hydrogen positions, neutron powder diffraction
experiments on the deuterides were performed. Rietveld
refinement (Figure 1 and Figure S1 in the Supporting
Information) of the crystal structures confirmed the LaMg2D7
type and yielded precise structural parameters (Table 3 and
Table S1 in the Supporting Information). For the correspond-
ing investigations on PrMg2D7 and their evaluation see the
Supporting Information.
a
Table 3. Crystal Structure of NdMg2D7
Wyckoff
site
Biso,
atom
x
y
z
1 × 104 pm2
Nd
Mg
D1
D2
D3
D4
4a
8b
8b
8b
8b
4a
0.2699(3)
0.0287(4)
0.0395(4)
0.0824(4)
0.3107(4)
0.7547(5)
x
0
0.25(5)
0.88(6)
1.86(5)
1.64(5)
2.07(6)
1.80(7)
0.2629(5)
0.0264(4)
0.4339(4)
0.1553(4)
x
0.3514(3)
0.1274(3)
0.1815(2)
0.2512(4)
0
a
P41212, a = 628.15(2) pm, c = 940.32(3) pm.
The crystal structures of the hydrides (deuterides)
NdMg2H(D)7 are tetragonally distorted and hydrogen
(deuterium)-filled variants of the parent cubic Laves phases
LnMg2 (Figure 2). Hydrogen (deuterium) atoms occupy four
Figure 2. Crystal structures of NdMg2 (left, view along [010]) and
NdMg2D7 (right, view along [110], deuterium atoms omitted for
̅
clarity), emphasizing corner-sharing Mg4 tetrahedra.
different crystallographic sites (Table 3). Two of them
correspond to [Ln2Mg2] tetrahedral voids, one to an [Mg4]
tetrahedral void, and one exhibits a trigonal planar coordination
(Figure 3).
The mutual coordination spheres of neodymium and
magnesium are similar to that in the parent cubic Laves
phase. Neodymium atoms are coordinated 16-fold by a [La4]
Figure 1. Rietveld refinement (black: measured; red: calculated; blue:
difference diagram (Iobs − Icalc)) of the crystal structure of NdMg2D7
using neutron diffraction data (D20 at the Institut Laue-Langevin in
Grenoble, France; λ = 186.78(3) pm); Rp = 2.87%, Rwp = 3.81%; Bragg
positions from top to bottom: NdMg2D7 (73(1)%), NdD2 (23(1)%),
MgD2 (3.8(1)%); data point at 2θ = 125° excluded because of
detector failure.
Figure 3. Crystal structure of NdMg2D7 with coordination polyhedra
of the deuterium atoms.
C
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tetrahedron, which is capped by 12 magnesium atoms.
Magnesium atoms are icosahedrally coordinated by six
magnesium and six neodymium atoms.
In the hydrides (deuterides), both coordination polyhedra
are distorted with respect to the hydrogen-free parent cubic
Laves phases LnMg2 (Figure 4). In addition, lanthanide atoms
Figure 4. Coordination spheres of metal atoms in the crystal structure
of NdMg2D7.
are coordinated by 12 hydrogen (deuterium) atoms, and
magnesium is coordinated by seven hydrogen (deuterium)
atoms. These metal−hydrogen (deuterium) polyhedra are
strongly distorted and are related to an anti-cuboctahedron
and a monocapped trigonal prism, respectively.
The structural similarities of the cubic Laves phases LnMg2
and their hydrides (deuterides) LnMg2H(D)7 manifest
themselves in crystallographic group−subgroup relationships.
They are very useful tools for the analysis of ordering
phenomena in intermetallic compounds.38,39 The correspond-
Figure 5. Crystallographic group−subgroup relationships (Barnighau-
̈
sen tree36,37) showing the structural relation between the cubic Laves
phases LnMg2 (Ln = La, Ce, Pr, Nd, Sm; MgCu2 type, C15) and their
hydrides in the tetragonal LaMg2D7 type of structure. The evolution of
the positional parameters is shown only for the Ln and Mg atoms.
Zero (0) and fractional numbers (e.g., 1/2, 3/8) denote fixed, rational
numbers (e.g., 0.0, 0.5, 0.375) free positional parameters.
ing Barnighausen tree (Figure 5) shows two possible ways to
̈
reduce the symmetry of the parent structure in the MgCu2
type: (i) a translationengleiche transformation of index 3 (t3) to
space group I41/amd followed by a transition t2 and (ii) a
transition t2 to space group F4132 followed by a transformation
t3 to space group I4122. The final step in symmetry reduction is
a klassengleiche transformation of index 2 to space group P41212
values for LaH2 and LaH3 and suggest a considerable ionic
contribution to chemical bonding.40,41
(Figure 5). With the Barnighausen formalism, the refined
̈
atomic positions of the heavy atoms of LnMg2(H)D7 may now
be compared with the aristotype MgCu2 of LnMg2. Structural
parameters do not differ much between the cubic Laves phases
LnMg2 and their hydrides (deuterides) phases LnMg2H(D)7.
For example, the position of the Nd atom on the Wyckoff
position 4a (x, x, 0 with x = 0.2699(3)) differs only slightly
from the ideal value of 0.25 as transformed from the MgCu2
type (Figure 5). The translational symmetry, however, shows
substantial deviation from cubic with √2*a/c = 0.945 for
NdMg2D7; that is, the c axis expands considerably more upon
hydrogenation than the a axis. Because of the lanthanide
contraction the lattice parameters of the hydrides decrease with
atomic number of the lanthanide (Table 4). The interatomic
distances follow this trend closely (Table 5).
The hydrides LnMg2H7 differ considerably from other Laves
phase hydrides. They show the highest hydrogen content,
which is achieved by distorting the Laves phase structure in a
way as to allow hydrogen atoms to shift away from the
tetrahedral voids. In contrast to most other Laves phase
hydrides they exhibit an ordered hydrogen distribution at room
temperature and semiconducting behavior; that is, the hydro-
genation reaction is accompanied by a metal−semiconductor
transition. The volume increments of hydrogen in LnMg2H7
compounds are calculated to range from to 6.2 (for Ln = Sm)
to 6.7 cm3/mol (for Ln = La). They are thus between the
Hydrides LnMg2Hx of the Late Lanthanides (Ln = Eu,
Gd, Tb). The hydrogenation of the hexagonal Laves phase
EuMg2 under a pressure of 12.0 MPa and a temperature of 353
K resulted in the formation of a new phase. The X-ray
diffraction pattern resembles that of a hexagonal Laves phase
with expanded unit cell. A closer inspection, however, especially
of high-resolution synchrotron diffraction data, reveals
reflection splitting (Figure 6). The diffraction pattern was
indexed with an orthorhombic unit cell with a = 664.9 pm, b =
1137.0 pm, and c = 1069.9 pm. Systematic reflection absences
suggested space group P212121. A structure model was derived
by group−subgroup relationships (Figure 7). The symmetry
reduction to space group P212121 was conducted in such a way
as to properly describe the lattice parameters found by the
indexing procedure. Rietveld refinement with starting param-
eters as derived for space group P212121 (Figure 7) yielded a
good correspondence between observed and calculated
diffraction patterns (Figure 6). Trying the higher symmetric
models clearly revealed the absence of C-centering.
Upon hydrogenation of EuMg2 and formation of the hydride
EuMg2Hx, all lattice parameters increase, resulting in a volume
increase of 11% (Table 6). This clearly indicates hydrogen
incorporation. The volume increase, however, is somewhat
smaller than for LnMg2H7 (Table 4), suggesting a lower
hydrogen content. The crystal structure resembles a distorted
D
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Table 4. Lattice Parameters and Unit-Cell Volumes of the Ternary Deuterides LnMg2D7 (Ln = La, Ce, Pr, Nd, Sm) and Relative
a
Increase Δ with Respect to the Hydrogen-Free Laves Phases
unit cell
a, pm
LaMg2D7
CeMg2D7
PrMg2D7
NdMg2D7
SmMg2D7
639.00(2)
957.82(4)
391.10(3)
3%
635.01(3)
949.57(2)
382.90(3)
3%
630.56(2)
943.27(3)
375.05(2)
3%
628.15(2)
940.32(3)
371.03(2)
3%
624.10(1)
934.81(2)
364.11(1)
2%
c, pm
V, 1 × 106 pm3
Δa
Δc
9%
9%
9%
9%
8%
ΔV
15.8%
14.9%
14.3%
14.2%
13.8%
a
Accounting for lattice parameter transformation, see Figure 5.
Table 5. Selected Interatomic Distances in the Crystal
a
Structures of LnMg2D7
atom pair
D1-Ln
La
Ce
Pr
Nd
Sm
246.9(4)
240.9(4)
192.1(4)
246.6(4)
238.8(3)
213.1(4)
199.1(3)
255.4(3)
255.0(3)
211.6(3)
203.2(4)
206.2(3)
198.2(3)
246.3(4)
236.2(4)
190.4(4)
243.7(4)
236.0(3)
217.9(4)
199.0(4)
252.9(3)
250.7(4)
211.8(3)
201.2(4)
202.7(3)
198.0(3)
242.0(7)
234.1(7)
192.4(6)
241.1(8)
232.4(6)
216.4(6)
195.8(6)
251.3(8)
259.4(6)
211.1(6)
201.8(6)
202.4(6)
196.9(6)
242.3(4)
231.5(4)
194.1(5)
239.4(4)
231.5(3)
215.6(4)
195.5(4)
248.3(4)
247.3(4)
211.7(5)
201.5(5)
202.2(4)
195.3(4)
240.5(8)
231.6(8)
185.3(9)
242.4(9)
227.6(7)
219.7(9)
194.8(9)
246.8(8)
246.2(8)
207(1)
D1-Mg
D2-Ln
D2-Mg
D3-Ln
D3-Mg
D4-Mg
205(1)
198.2(8)
196.9(9)
a
In picometers. Ln = La, Ce,20 Pr, Nd (this work), and Sm21.
Figure 7. Crystallographic group−subgroup relationships (Barnighau-
̈
sen tree38,39) showing the structural relation (for the metal positions)
between the hexagonal Laves phase EuMg2 (MgZn2 type, C14) and its
hydride EuMg2Hx. Zero (0) and fractional numbers (e.g., 1/2, 3/8)
denote fixed, rational numbers (e.g., 0.0, 0.5, 0.375) free positional
parameters.
Table 6. Lattice Parameters and Unit Cell Volumes of the
a
Laves Phase EuMg2 and Its Hydride EuMg2Hx
V,
compound
a, pm
bortho, pm
c, pm
1 × 106 pm3
EuMg2
638.77(9)
1106.38
1032.23(19)
1069.887(7)
729.5(3)
EuMg2Hx
664.887(4)
1136.993(7)
808.80(1)
a
Orthogonalized, that is, bortho = 2 × cos 30° ahex
.
Figure 6. Rietveld refinement (black: measured; red: calculated; blue:
difference diagram (Iobs − Icalc)) of the crystal structure of EuMg2Hx
using synchrotron diffraction data (powder diffraction beamline,
Australian Synchrotron, Melbourne, Australia, λ = 58.9074 pm); Rp =
4.85%, Rwp = 6.64%; Bragg positions from top to bottom: EuMg2Hx
(98.24(5)%), EuH2 (1.76(5)%).
hexagonal Laves phase. In contrast to the parent hexagonal
Laves phase the faces shared by Mg4 tetrahedra are slightly
inclined toward each other in the hydride (Figure 8). Because
of the symmetry reduction, there are two crystallographically
different europium atoms and four crystallographically different
magnesium atoms in the crystal structure of the hydride. The
coordination of the atoms is similar to the atoms in the cubic
Laves phases. Both europium atoms are 16-fold coordinated
and located within distorted Frank−Kasper polyhedra. All four
magnesium atoms are coordinated by a distorted icosahedron.
Interestingly, so far there are only a few hydrides based on
hexagonal Laves phases known, for example, ZrCr2H3.8.42
Figure 8. Comparison of the crystal structures of EuMg2 (left) and
EuMg2Hx (right).
The compound EuMg2Hx was the first example in the system
Ln−Mg−H, which has a structural relationship to the crystal
structure of the parent LnMg2 compounds crystallizing in the
MgZn2 type. So it was obvious to investigate the hydrogenation
E
DOI: 10.1021/acs.inorgchem.7b02319
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
a
Table 7. Crystal Structure of EuMg2Hx
atom
Wyckoff site
x
y
z
Biso, 1 × 104 pm2
1.39(2)
iso(Eu1)
1.55(6)
Eu1
4a
4a
4a
4a
4a
4a
0.68629(11)
0.76379(12)
0.7441(8)
0.2282(9)
0.9735(8)
0.4413(8)
0.30514(7)
0.34032(7)
0.0019(5)
0.3332(5)
0.1149(5)
0.0738(5)
0.07208(9)
0.44317(8)
0.9813(5)
0.2721(6)
0.2501(7)
0.2401(7)
Eu2
B
Mg1
Mg2
Mg3
Mg4
B
B
B
iso(Mg1)
iso(Mg1)
iso(Mg1)
a
P212121, a = 664.887(4) pm, b = 1136.993(7) pm, c = 1069.887(7) pm.
behavior of hexagonal Laves phases LnMg2 of the heavier
lanthanides. The compounds GdMg2 and TbMg2 form new
phases under hydrogen pressure. The X-ray powder diffraction
pattern of GdMg2Hx was indexed with an orthorhombic cell
with a = 1282.7 pm, b = 572.5 pm, and c = 881.7 pm. The
lattice parameter ratios are similar to the tetragonal LnMg2D7,
but with a doubled a lattice parameter. The X-ray powder
diffraction pattern of TbMg2Hx was indexed with an
orthorhombic cell with a = 617.8 pm, b = 1045.8 pm, and c
= 997.1 pm. The lattice parameter ratios are very similar to
EuMg2Hx; however, a structure refinement using this structure
model was not successful. For neither of the phases GdMg2Hx
and TbMg2Hx a structure model was found, but Pawley fits
were performed yielding refined lattice parameters (Figures S2
and S3 in Supporting Information). The increase in volume is
3.53% for GdMg2Hx and 5.02% for TbMg2Hx. Compared to
EuMg2Hx (Table 7) and LnMg2H7 the volume increase upon
hydrogenation is lower and suggests a lower hydrogen content.
An in situ experiment was performed for GdMg2 under a
hydrogen gas pressure of 10.0 MPa. In situ investigations of
chemical reactions are very useful to reveal reaction pathways
and discover possible intermediates.43−46 Unfortunately,
GdMg2 did not form the ternary hydride under these
conditions, but first GdH2 at 463 K and then GdH3 at 543 K
formed (Figure S4 in Supporting Information).
indicating a reduced thermal stability of LnMg2H7 for the
heavier lanthanides. The decomposition reaction is irreversible
by and large as proven by the weak exothermic signal upon
cooling and the reaction products found by X-ray powder
diffraction after the experiment, being LnH2 and elemental Mg.
Despite their mainly ionic nature, the hydrides LnMg2H7 are
stable in air for several days. This is quite remarkable and very
uncommon for ionic hydrides.
The Laves phase NdMg2 was substituted with 5 atom % of
samarium or terbium. Powdered samples of Nd0.95Sm0.05Mg2
and Nd0.95Tb0.05Mg2 were hydrogenated under the same
conditions as for the unsubstituted compounds. The lattice
parameters as refined by the Rietveld method were smaller for
substituted than for the unsubstituted NdMg2 (Table 8), which
indicated a successful substitution. Excitation with UV light
(240 and 360 nm) did not yield emission visible for the
unaided human eye.
a
Table 8. Lattice Parameters of Pure and Substituted
Nd0.95Sm0.05Mg2H7 and Nd0.95Tb0.05Mg2H7
compound
NdMg2H7
Nd0.95Sm0.05Mg2H7
Nd0.95Tb0.05Mg2H7
a, pm
c, pm
630.134(9)
629.594(8)
629.352(8)
942.695(16)
942.307(15)
942.063(16)
a
In picometers.
Thermal Stability and Reactivity toward Air. The
thermal stabilities of the Laves phase hydrides LnMg2H7 were
investigated by thermal analysis under 0.1 MPa hydrogen gas
pressure up to 723 K (Figure 9). The onset of the signals
associated with decomposition of LnMg2H7 are at 639 K
(SmMg2H7), 672 K (NdMg2H7), 684 K (PrMg2H7), 684 K
(CeMg2H7), and 695 K (LaMg2H7). Using helium instead of
hydrogen atmosphere did not change the decomposition
temperatures significantly. The decomposition temperature
decreases with increasing atomic number of the lanthanides,
Magnetic Properties of CeMg2H7 and NdMg2H7. For
both compounds the temperature dependence of the magnetic
susceptibility (χ) was measured at 10 kOe (Figures 10 and 11,
top). The samples show paramagnetism over the whole
investigated temperature range. The measured susceptibility
was corrected for diamagnetic contributions (−8.3 × 10−4 emu
mol−1 for CeMg2H7 and −6.3 × 10−4 emu mol−1 for
NdMg2H7) caused, for example, by the capsule and traces of
impurities. These contributions were obtained by fitting the
magnetic data in the temperature range from 100 to 300 K
using the modified Curie−Weiss law. From the inverse
susceptibility data (χ−1) the effective magnetic moment was
determined. For CeMg2H7 μeff = 2.49(1) μB was calculated, and
for NdMg2H7 μeff = 3.62(1) μB was calculated; these values are
in good agreement with the calculated magnetic moments of
the free trivalent rare-earth cations (μcalc(Ce3+) = 2.54 μB;
μcalc(Nd3+) = 3.62 μB). Both measurements exhibit pronounced
curvatures at low temperatures, which can be attributed to
crystal field splitting of the magnetic ground states, a
phenomenon often observed in rare-earth compounds.47
Ternary and quaternary cerium hydrides have been summarized
recently in two review articles.48,49
In addition, low-field measurements with a flux density of
100 Oe were performed in a zero-field-cooled and field-cooled
modes (ZFC/FC). For NdMg2H7 (Figures 10 and 11, middle)
again only paramagnetism was observed. CeMg2H7 exhibits a
Figure 9. In situ thermal analysis (DSC, exothermic signal up,
endothermic signals down) of the decomposition of NdMg2H7 under
0.1 MPa hydrogen atmosphere.
F
DOI: 10.1021/acs.inorgchem.7b02319
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 11. Magnetic properties of NdMg2H7: (top) temperature
dependence of the magnetic susceptibility (χ and χ−1 data) measured
at 10 kOe; (middle) ZFC/FC measurements at 100 Oe; (bottom)
magnetization isotherms at 3, 10, and 50 K.
Figure 10. Magnetic properties of CeMg2H7: (top) temperature
dependence of the magnetic susceptibility (χ and χ−1 data) measured
at 10 kOe; (middle) ZFC/FC measurements at 100 Oe; (bottom)
magnetization isotherms at 3, 10, and 50 K.
slight anomaly at T = 15 K (Figures 10 and 11, middle).
Because of the rise in the susceptibility and the disappearance
in the high-field experiments, this feature must be caused by
traces of a ferromagnetic impurity. Ferromagnetic substances,
however, exhibit susceptibilities that are several orders of
magnitude higher than the ones of paramagnetic compounds.
Possible impurities that could form and are known from
literature are, for example, binary cerium hydrides, along with
CeMg, CeMg2, or CeMg3. While the latter one and CeMg
order anti-ferromagnetically at TN = 2.6 K50 and TN = 20 K,51,52
CeMg2 orders ferromagnetically at TC = 3.6 K.53 For binary
cerium hydrides CeH2 and the series of CeH2+x (0 ≤ x ≤ 0.65)
have been investigated. Susceptibility measurements indicate
that CeH2 orders anti-ferromagnetically at TN ≈ 7 K;54 for
CeH2+x (0 ≤ x ≤ 0.65) resistivity measurements were
conducted. Samples with x = 0.4, 0.6, and 0.65 all exhibit
ordering temperatures of T < 5 K.55
Finally magnetization isotherms at 3, 10, and 50 K were
recorded. The 10 and 50 K isotherms exhibit linear field
dependencies, consistent with paramagnetic behavior. The 3 K
isotherms both exhibit slight curvatures with tendencies for
saturation at high fields. For CeMg2H7 (Figure 10, bottom) a
saturation magnetization of μsat = 0.41(2) μB per Ce atom at 3
K and 80 kOe can be found, for NdMg2H7 (Figure 10, middle)
μsat = 0.53(2) μB per Nd atom at 3 K and 80 kOe is observed.
Both values are significantly lower than the expected saturation
magnetization of 2.14 μB (Ce3+) and 3.27 μB (Nd3+) according
to gJ × J.56
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DOI: 10.1021/acs.inorgchem.7b02319
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
4. CONCLUSION
Article
ACKNOWLEDGMENTS
■
The class of Laves phase-based hydrides LnMg2H7 was
extended by two members, that is, for Ln = Pr, Nd. Their
crystal structures as derived by neutron powder diffraction on
the deuterides are very similar to those known for Ln = La, Ce,
Sm. The structural relationship to the cubic MgCu2 type of the
parent Laves phases can be shown by crystallographic group−
The Institut Laue-Langevin, Grenoble, France, and the powder
diffraction beamline at the Australian Synchrotron, Australia are
acknowledged for providing beam time.
DEDICATION
■
Dedicated to Prof. Dr. Thomas Schleid on the occasion of his
60th birthday.
subgroup relationships (Barnighausen tree). Hydrides with the
̈
LnMg2D7 structure type are only formed by the earlier
lanthanides, which exhibit the MgCu2 structure type in Laves
phases LnMg2. These ternary hydrides form only at high
hydrogen gas pressures and low-to-moderate temperatures.
Lower hydrogen gas pressure and higher temperature favor the
formation of binary hydrides instead. New hydrides were also
found by hydrogenation of the hexagonal Laves phases LnMg2
(Ln = Eu, Gd, Tb). EuMg2Hx shows an orthorhombic
structure, which is a distortion variant of the parent hexagonal
Laves phase type of MgZn2. The lattice parameters suggest the
same to be the case for GdMg2Hx and TbMg2Hx. Despite the
high hydrogen capacity the Laves phases LnMg2 are not well-
suited for hydrogen storage. The formation condition with
hydrogen gas pressures of 9.0 MPa and higher are not practical,
and the decomposition temperatures of the hydrides above 660
K are rather high. Magnetic measurements show that the
ternary hydrides CeMg2H7 and NdMg2H7 exhibit no long-
range magnetic ordering and remain paramagnetic down to 2.5
K and that the rare-earth atoms are in the trivalent oxidation
state.
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S
* Supporting Information
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ORCID
Oliver Janka: 0000-0002-9480-3888
Holger Kohlmann: 0000-0002-8873-525X
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The manuscript was written through contributions of all
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DOI: 10.1021/acs.inorgchem.7b02319
Inorg. Chem. XXXX, XXX, XXX−XXX