Structure and magnetic properties of RNi (R = Gd, Tb, Dy, Sm) and R 6M1.67Si3 (R = Ce, Gd, Tb; M = Ni, Co) hydrides

Article abstract of DOI:10.1016/j.jallcom.2011.01.047

The paper reports the influence of hydrogen absorption on the structure and magnetic properties of intermetallic compounds RNi (R = Gd, Tb, Dy, Sm) and R₆M_1.67Si₃ (R = Ce, Gd, Tb; M = Ni, Co). It is demonstrated that the ternary hydrides GdNiH_3.2, TbNiH_3.4, DyNiH_3.4, and SmNiH_3.7 have similar CrB-type orthorhombic structures. In the structures of TbNiD_3.4 and DyNiD_3.4, deuterium atoms occupy tetrahedral 8f-interstices [R₃Ni], trigonal bipyramidal 4c-interstices [R₃Ni₂], and octahedral 4b-interstices [R₄Ni₂]. The hydrides of R ₆M_1.67Si₃ retain the hexagonal structure of their parent compound. In the structures of Tb₆Co _1.67Si₃D_11.2 and Ce₆Ni _1.67Si₃D_12.3, deuterium atoms occupy tetrahedral 4f-interstices [R₃M] and two types of tetrahedral interstices [R₃Si]: 12i₁ and 12i₂. The formation of the hydrides causes a substantial expansion of the metallic sublattice, a weakening of ferromagnetic interactions, and a decrease in the magnetic transition temperature.

Full text of DOI:10.1016/j.jallcom.2011.01.047

Journal of Alloys and Compounds 509S (2011) S830–S834
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Structure and magnetic properties of RNi (R = Gd, Tb, Dy, Sm) and R6M1.67Si3
(R = Ce, Gd, Tb; M = Ni, Co) hydrides
Yu.L. Yaropolova,∗, A.S. Andreenkoa, S.A. Nikitina, S.S. Agafonovb, V.P. Glazkovb, V.N. Verbetskya
a Moscow State University, GSP-3, Leninskie Gory, Moscow 119992, Russia
b Russian Research Centre Kurchatov Institute, 1, Kurchatov Sq., Moscow 123182, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 July 2010
Received in revised form 5 January 2011
Accepted 7 January 2011
Available online 14 January 2011
The paper reports the influence of hydrogen absorption on the structure and magnetic properties of inter-
metallic compounds RNi (R = Gd, Tb, Dy, Sm) and R6M1.67Si3 (R = Ce, Gd, Tb; M = Ni, Co). It is demonstrated
that the ternary hydrides GdNiH3.2, TbNiH3.4, DyNiH3.4, and SmNiH3.7 have similar CrB-type orthorhombic
structures. In the structures of TbNiD3.4 and DyNiD3.4, deuterium atoms occupy tetrahedral 8f-interstices
[R3Ni], trigonal bipyramidal 4c-interstices [R3Ni2], and octahedral 4b-interstices [R4Ni2]. The hydrides of
R6M1.67Si3 retain the hexagonal structure of their parent compound. In the structures of Tb6Co1.67Si3D11.2
and Ce6Ni1.67Si3D12.3, deuterium atoms occupy tetrahedral 4f-interstices [R3M] and two types of tetrahe-
dral interstices [R3Si]: 12i1 and 12i2. The formation of the hydrides causes a substantial expansion of the
metallic sublattice, a weakening of ferromagnetic interactions, and a decrease in the magnetic transition
temperature.
Keywords:
Rare earth alloys and compounds
Metal hydrides
Magnetization
Crystal structure
© 2011 Elsevier B.V. All rights reserved.
Magnetic measurements
Neutron diffraction
1. Introduction
tribution of the electron density in the bulk of the crystal lattice,
thus exerting a direct effect on the magnetism of the 3d-ions. Incor-
Intermetallic compounds (IMCs) containing rare earth metal
atoms have attracted considerable attention due to their high
potential for various applications. Considerable attention has
recently been focused on magnetocaloric materials used in mag-
netic refrigeration. After the observation of a giant magnetocaloric
effect in Gd5Si2Ge2 [1], interest in IMCs containing gadolinium and
other rare earth metals has increased.
The magnetic properties of an IMC depend on the strength
of the exchange interactions between magnetic atoms, which is
determined by the interatomic distances and electronic struc-
ture. The introduction of hydrogen atoms into an IMC usually
results in significant changes in its crystal and electronic structure
[2–5]. Different types of exchange interactions are possible in com-
pounds consisting of rare earth and 3d transition elements: indirect
exchange between the 4f-subshells of the rare earth ions through
the conduction electrons and 3d-electrons, and direct exchange
interaction between the 3d-electrons. The strength of these inter-
actions depends on the distances between the interacting atoms
and, therefore, on the geometrical parameters of the crystal lattice.
Lattice expansion causes not only an increase in the interatomic
distances, but also a change in the electronic structure and redis-
porating into the IMC structure, hydrogen can partially donate its
electron to the conduction band or, conversely, acquire a partial
negative charge Hı−. The change in the number of electrons in the
conduction band leads to a change in the electron density at the
Fermi level, and this produces a direct effect on the magnetic prop-
erties of the compound. When hydrogen acts as an electron donor,
it increases the electron density in the conduction band, which is
responsible for the exchange interaction. This leads to an increase
in the magnetization and Curie temperature of the compound. If
hydrogen partially withdraws electron density from the conduc-
tion band, the magnetization and Curie temperature of the hydride
decrease. Thus, the effect of hydrogenation on the structural and
magnetic properties of IMCs is of considerable interest.
Rare-earth intermetallides possess promising magnetic proper-
ties due to the large magnetic moments of the rare earth atoms.
The RNi and R6M1.67Si3 IMC series attracted our attention because
they have approximately equal atomic ratios of the “hydridable”
rare earth element to the “nonhydridable” transition elements. In
addition, these compounds react readily with hydrogen at room
temperature and relatively low hydrogen pressures. Finally, both
IMC series show a strong magnetocaloric effect near the magnetic
transition temperatures [6–9].
The magnetic properties of the RNi compounds and their
hydrides were investigated previously between 78 and 300 K [10].
The magnetic transition temperatures of the RNi compounds and
∗
Corresponding author. Tel.: +7 495 939 1413; fax: +7 495 932 8846.
E-mail address: yaropolov@inbox.ru (Yu.L. Yaropolov).
0925-8388/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2011.01.047

Yu.L. Yaropolov et al. / Journal of Alloys and Compounds 509S (2011) S830–S834
S831
Fig. 1. Neutron powder diffraction patterns of Ce6Ni1.67Si3D12.3 and Tb6Co1.67Si3D11.2
.
all of their hydrides turned out to be below the nitrogen boiling
point (∼78 K). However, the calculated temperature dependences
of the reciprocal susceptibilities are linear in the temperature range
78–300 K and obey the Curie–Weiss law ꢀ = C/(T − ꢁ) in all cases
except for SmNi and SmNiH3.7. This makes it possible to estimate
the paramagnetic Curie temperature ꢁP and to calculate the effec-
tive magnetic moments ꢂeff of the rare earth atoms.
Here, we report the effect of hydrogenation on the structural and
magnetic properties of the RNi (R = Gd, Tb, Dy, Sm) and R6M1.67Si3
(R = Ce, Gd, Tb; M = Ni, Co) compounds.
analysis of the Ce6Ni2Si3-type structure showed that hydrogen
atoms could occupy only [R3T] and [R2T2] tetrahedral interstices
(T = Ni, Co, Si) in its lattice. According to an empirical rule [16], the
preferable interstices to be occupied by hydrogen are those sur-
rounded by the maximum possible number of “hydridable” metal
atoms. This means that the [R3T] tetrahedral interstices are most
readily occupied by hydrogen atoms. In addition, the H–H distances
˚
must be longer than 1.2–2.0 A, so hydrogen atoms will not occupy
12i-[R2MSi] interstices with a lower R content. For the same rea-
son, only half the 4f-[R3M] and 6h-[R3Si] interstices, whose R3 faces
are in the (0 0 1) crystallographic plane, can be filled by hydrogen
atoms, because the hydrogen atoms sitting on these sites block the
adjacent sites of the same type. Moreover, the hydrogen atoms
in the 4f-[R3M] and 6h-[R3Si] sites can partially block two types
of [R3Si] interstices, namely, 12i1 and 12i2, whose R3 faces are in
another crystallographic plane (Fig. 2).
Based on these facts and the deuterium content calculated from
volumetric data, we suggest a structural model in which deuterium
atoms occupy half the 4f-[R3M] interstices and two types of the
[R3Si] interstices: 12i1 and 12i2 (Fig. 3). Based on this structural
model, the maximum deuterium content of the studied deuterides
could be D/f.u. = 13.
2. Experimental details
The intermetallic compounds RNi and R6M1.67Si3 were synthesized by arc melt-
ing under an argon atmosphere in a furnace with a nonconsumable tungsten
electrode and a water-cooled copper hearth. The starting components were nickel
(99.99%), cobalt (99.99%), silicon (99.99%), and rare earth metals (99.9%), and tita-
nium sponge was used as a getter. The amounts of rare earths in the stocks were
chosen so as to compensate for the metal losses. The samples were turned over and
remelted several times to ensure homogeneity.
The hydrides were synthesized in a Siverts-type volumetric apparatus at room
temperature and hydrogen pressures of up to 1 MPa. The composition of the result-
ing hydrides was derived from volumetric measurements using the van der Waals
equation.
The phase composition and unit cell parameters of the starting alloys and their
hydrides were determined by X-ray powder diffraction using Cu K␣ radiation. The
RNi compounds were single-phase according to their X-ray diffraction patterns,
while the R6M1.67Si3 compounds contained more than 35% impurities. For this rea-
son, these samples were annealed at 900 ◦C for 10 days in evacuated quartz tubes.
As a result, the impurity content decreased to 5% or below.
According to the neutron diffraction data, the deuterium content
of the deuterides corresponded to the formulas Tb6Co1.67Si3D11.2
and Ce6Ni1.67Si3D12.3. The introduction of deuterium atoms into the
4f sites results in a 5–6% increase in the R–R distances, while the
Several deuterides were prepared by the same technique in order to study their
structure by neutron powder diffraction. The neutron diffraction experiments were
performed with the DISK diffractometer at Kurchatov Institute using neutrons with
˚
a wavelength of 1.668 A.
Magnetization measurements were performed with 150–400 mg samples in
the form of pellets (hydrides) or ingots (starting alloys) in the temperature range
78–300 K using a vibration magnetometer.
3. Results and discussion
3.1. Crystal structure of Tb6Co1.67Si3D11.2 and Ce6Ni1.67Si3D12.3
According to the X-ray diffraction data, the R6M1.67Si3 com-
pounds have Ce6Ni2Si3-type hexagonal structures, space group
P63/m, with the unit cell parameters close to those reported pre-
viously [11–15]. The metal lattices of the hydride phases were
of the same structure type as the virgin IMCs, but a pronounced
anisotropic distortion of the unit cell was nevertheless observed:
the lattice was expanded along the x and y crystallographic axes
and contracted along the z axis (Table 1).
X-ray and neutron powder diffraction studies were performed
for Tb6Co1.67Si3D11.2 and Ce6Ni1.67Si3D12.3. The neutron data
(Fig. 1) were used to refine the atomic coordinates and occupancies
of the sites filled with deuterium atoms (Table 1). A crystallographic
Fig. 2. Tb6Co1.67Si3D11.2 structure and coordination polyhedra of deuterium atoms.

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Yu.L. Yaropolov et al. / Journal of Alloys and Compounds 509S (2011) S830–S834
Table 1
Refined structural parameters of Ce6Ni1.67Si3D12.3 and Tb6Co1.67Si3D11.2
.
*
Compound
Atom
x
y
z
Biso (Å2)
Occupancy
Ce6Ni1.67Si3D12.3
Space group P63/m
a = 13.00(2) (ꢃa/a0 = 7.7%)
c = 4.134(3) (ꢃc/c0 = −4.1%)
V = 605.0(4) (ꢃV/V0 = 11.3%)
RBragg = 2.11, RF = 2.42
Ce(1)-6h
Ce(2)-6h Ni(1)-4e
Ni(2)-2c
0.242(2)
0.513(4)
0
1/3
0.230(2)
0.130(1)
0
2/3
0
1/4
1/4
0.081(1)
1/4
1/4
1.26
1.96
1.11
0.61
1.27
2.10
3.01
2.45
2.53
0.87
1.02
0.60
0.63
0.60
1.01
3.51
3.95
2.11
0.6
1
1
0.25(5)
1
0.21(6)
1
0.48(2)
0.96(5)
0.93(7)
1.0
1.0
0.26(5)
1.0
0.20(5)
1.0
0.49(3)
0.87(8)
0.83(6)
1.0
Ni(3)-2a
Si-6h
0
D(1)-4f
0.170(1)
1/3
0.420(5)
0.207(1)
0.238(1)
0.511(3)
0
1/3
0.452(5)
2/3
1/4
D(2)-12i1
D(3)-12i2
0.609(6)
0.504(4)
0.495(4)
1/4
1/4
0
1/4
0.167(2)
1/4
0.629(5)
0.502(6)
0.496(4)
1/4
0.394(4)
0.350(3)
0.228(2)
0.130(1)
0
2/3
0
0.456(5)
2/3
0.385(3)
0.358(3)
0.132(1)
0.401(5)
0.307(5)
0.920(2)
1/2
Tb6Co1.67Si3D11.2
Space group P63/m
a = 12.353(7) (ꢃa/a0 = 5.9%)
c = 3.950(3) (ꢃc/c0 = −4.5%)
V = 522.0(3) (ꢃV/V0 = 6.7%)
RBragg = 1.54, RF = 1.61
Tb(1)-6h
Tb(2)-6h
Co(1)-2b
Co(2)-2c
Co(3)-4e
Si-6h
0
0.168(2)
1/3
0.423(4)
0.203(2)
0
0
0
0
0
D(1)-4f
D(2)-12i1
D(3)-12i2
Tb-4c
TbNiD3.3
Space group Cmcm
a = 3.740(2)
b = 11.435(6)
c = 4.671(3)
Ni-4c
1/4
0.497(7)
1/4
1.0
2.1
2.7
2.1
1.0
1.0
1.0
0.32(1)
D(1)-8f
D(2)-4c
D(3)-4b
0
V = 199.8(3) (ꢃV/V0 = 23.4%)
RBragg = 1.28, RF = 1.81
DyNiD3.3
Space group Cmcm
a = 3.719(2)
Dy-4c
Ni-4c
0
0
0
0
0
0.133(2)
0.408(7)
0.303(4)
0.929(1)
1/2
1/4
1/4
0.511(7)
1/4
0
1.02
1.06
2.38
2.41
2.43
1.0
1.0
1.0
1.0
D(1)-8f
D(2)-4c
D(3)-4b
b = 11.329(5)
c = 4.645(2)
0.38(1)
V = 195.7(2) (ꢃV/V0 = 22.4%)
RBrag = 2.07, RF = 1.51
*
Isotropic Debye-Waller factor.
R–M distances increase insignificantly. This is due to the fact that
the expansion of the R3 face in the ab plane is accompanied by the
reduction of the tetrahedron height owing to the cell contraction
along the z axis. The introduction of deuterium atoms into the 12i1
and 12i2 sites causes an anisotropic distortion of these interstices.
Two R–R distances in the tetrahedron face R3 increase by 3–4%, and
the third distance increases by 6–7%. Likewise, one R–Si distance
increases by 7–8%, whereas the two other R–Si distances increase
insignificantly (by ∼1%).
The R–D and T–D distances in the structures of Tb6Co1.67Si3D11.2
and Ce6Ni1.67Si3D12.3 are similar to the distances observed in other
IMCs of the respective rare earth metals. These hydrides have inter-
atomic distances typical of the binary hydrides of the constituent
metals. A similar situation was previously discussed for transition
metal IMC hydrides [17,18]. The D–Si distances in the structure
of Ce6Ni1.67Si3D12.3 are close to the corresponding distance in the
structure of CeNiSiD1.2 (2.02 A [19]). They are longer than the
Si–H bond lengths reported for different silicon-based compounds
with a largely covalent bonding between the hydrogen and sil-
˚
˚
icon atoms (dcov = 1.48 A). In the structure of Tb6Co1.67Si3D11.2
,
˚
these interatomic distances are slightly shorter (1.72–1.79 A). We
think that the difference between the Si–H bond lengths in the
above compounds and CaSiD1+x [20] is mainly due to the differ-
ence between the atomic radii of the metal atoms. In addition,
CaSi is a pure silicide phase with a higher silicon content and
another arrangement of the Si atoms. For this reason, we assume
that the D–Si distances obtained in this study are quite reason-
able and are indeed longer than the Si–H bonds reported for
silicon-based compounds with largely covalent hydrogen–silicon
bonding.
3.2. Crystal structure of TbNiD3.3 and DyNiD3.4
According to our X-ray diffraction data, the RNi compounds
have a CrB-type orthorhombic structure, space group Cmcm (GdNi,
SmNi); a FeB-type structure, space group Pnma (DyNi); or a low-
temperature TbNi-type monoclinic structure, space group P21/m
(TbNi) in agreement with results in the literature [21,22]. Irrespec-
tive of the crystal structure of the RNi compounds, their hydrides
have a CrB-type structure. DyNi and TbNi undergo a structural
transformation upon hydrogenation, turning into another struc-
ture type. The hydrogenation of GdNi and SmNi is accompanied by
Fig. 3. Coordination polyhedra of deuterium atoms in the 4f-[D(1)Tb3Co], 12i1-
[D(2)Tb3Si] and 12i2-[D(3)Tb3Si] sites of the Tb6Co1.67Si3D11.2 structure.

Yu.L. Yaropolov et al. / Journal of Alloys and Compounds 509S (2011) S830–S834
S833
Fig. 4. Coordination polyhedra of deuterium atoms in the 8f-[D(1)R3Ni], 4c-[D(2)R3Ni2] and 4b-[D(3)R4Si2] sites of the TbNiD3.3 structure.
anisotropic lattice expansion without reordering the metal sublat-
tice.
deuterium atom is a distorted trigonal bipyramid 4c-[R3Ni2], dis-
torted tetrahedron 8f-[R3Ni], and distorted octahedron 4b-[R4Ni2],
respectively. A coordination polyhedron analysis shows that deu-
terium atoms should successively occupy interstices in the order
of increasing coordination number: first the tetrahedral sites, then
trigonal bipyramidal sites, and finally octahedral interstices. As in
other superstoichiometric hydrides (D/f.u. > 3), the occupation of
the octahedral 4b-sites begins after the complete filling of the 4c
and 8f interstices.
Neutron powder diffraction measurements for TbNiD3.4 and
DyNiD3.4 were performed to determine the deuterium atom posi-
tions and their occupancies. The data obtained were used to refine
the structure parameters (Table 1) and interatomic distances. The
calculated deuterium contents of the hydride phases corresponded
to the formulas TbNiH3.3 and DyNiH3.4. The difference between
the deuterium content values calculated for TbNiH3.3 by different
methods (volumetric data and deuterium site occupancies) can be
due to the partial desorption of deuterium from the sample after
decompression. The estimated accuracy of the D/f.u. determination
from the neutron diffraction data is 3%.
3.3. Magnetic properties of the deuterides
An analysis of the neutron diffraction patterns revealed three
types of interstices occupied by deuterium atoms: 4c, 8f and 4b
(Table 1, Fig. 4). This structural model was earlier reported for the
ternary deuteride LaNiD3.7 [23]. The coordination polyhedron of a
The compounds Gd6Ni1.67Si3, Gd6Co1.67Si3, and Tb6Co1.67Si3 are
ferromagnetic materials with Curie temperatures of, respectively,
TC = 305, 298, and 178 K (Table 2) determined from the temperature
dependences of magnetization.
Table 2
Crystallografic and magnetic data for the IMCs and their hydrides.
*
**
***
Compound
Structure type
Unit cell parameters (Å)
ꢂeff (␮
)
TC (K)
ꢁP (K)
В
GdNi
GdNiH3.2
TbNi
TbNiH3.4
DyNi
DyNiH3.4
SmNi
SmNiH3.7
Ce6Ni1.67Si3
Ce6Ni1.67Si3H12.3
Gd6Ni1.67Si3
Gd6Ni1.67Si3H12.3
Gd6Co1.67Si3
Gd6Co1.67Si3H12.6
Tb6Co1.67Si3
Tb6Co1.67Si3H11.2
CrB
CrB
TbNi
CrB
FeB
CrB
CrB
CrB
Ce6Ni2Si3
Ce6Ni2Si3
Ce6Ni2Si3
Ce6Ni2Si3
Ce6Ni2Si3
Ce6Ni2Si3
Ce6Ni2Si3
Ce6Ni2Si3
a = 3.778(4), b = 10.337(6), c = 4.238(5)
a = 3.767(2), b = 11.576(7), c = 4.733(3)
a = 21.31(2), b = 4.211(4), c = 5.454(2), ˇ = 97.43◦
a = 3.740(2), b = 11.435(6), c = 4.671(3)
a = 7.025(4), b = 4.181(3), c = 5.445(2)
a = 3.719(2), b = 11.329(5), c = 4.645(2)
a = 3.782(3), b = 10.375(4), c = 4.301(2)
a = 3.791(2), b = 11.644(4), c = 4.761(2)
a = 12.071(3), c = 4.312(1)
a = 13.00(2), c = 4.134(3)
a = 11.736(6), c = 4.180(2)
a = 12.44(2), c = 3.983(8)
a = 11.76(1), c = 4.161(4)
8.3
7.5
10.0
10.3
10.3
10.1
–
–
–
–
6.4
–
–
–
–
–
–
–
–
–
–
–
80
21
56
−12
51
3
–
–
–
–
13
–
29
–
196
15
305
–
298
–
178
–
7.0
–
8.5
8.6
a = 12.54(1), c = 3.970(3)
a = 11.685(6), c = 4.136(2)
a = 12.353(7), c = 3.950(3)
*
Effective magnetic moment per a rare earth atom; ꢂB is Bohr magneton.
Curie temperature.
Paramagnetic Curie temperature.
**
***

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Yu.L. Yaropolov et al. / Journal of Alloys and Compounds 509S (2011) S830–S834
The hydrogenation of the R6M1.67Si3 compounds leads to the
temperature. The effective magnetic moments of the compounds
remain unchanged upon hydrogenation.
weakening of the ferromagnetic interaction and to a decrease
in the magnetic transition temperature. A similar effect was
observed for the RNi–H2 systems [10]. The calculated paramagnetic
Acknowledgment
Curie temperatures are ꢁP = 13, 29, and 15 K for Gd6Ni1.67Si3H12.3
,
Gd6Co1.67Si3H12.6 and Tb6Co1.67Si3H11.2, respectively. The calcu-
lated effective magnetic moments are close to the data reported
by other authors [13,15,22] and to the magnetic moments of the
free rare earth ions. In addition, the influence of hydrogen on the
magnetic moments is negligible. Therefore, the 3d-bands of nickel
and cobalt are filled in these compounds and the observed mag-
netization is solely due to the magnetic moments of the rare earth
atoms.
The 4f–4f exchange interactions between rare earth atoms
decrease with increasing R–R distance. In spite of the very
anisotropic lattice distortion, all R–R distances in the R6M1.67Si3
compounds increase upon hydrogenation. Although the unit cell
contracts along the z axis, the lattice expansion in the xy plane
leads to the increase in all interatomic distances. Neutron diffrac-
tion data revealed a hydrogen-induced increase in the interatomic
distances in RNi and R6M1.67Si3 compounds, too. Thus, the decrease
in the magnetization and magnetic transition temperature of these
compounds upon hydrogenation mainly results from the weak-
ening of the 4f–4f exchange interaction caused by the lattice
expansion.
The authors thank Dr. V.A. Somenkov for his critical reading of
the paper and helpful discussions and comments.
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4. Conclusions
The intermetallic compounds RNi (R = Gd, Tb, Dy, Sm) and
R6M1.67Si3 (R = Ce, Gd, Tb; M = Ni, Co) interact readily with hydro-
gen at room temperature and hydrogen pressures of up to 1.0 MPa.
The hydrides of the RNi compounds have a CrB-type orthorhombic
structure. The R6M1.67Si3 compounds retain their Ce6Ni2Si3-type
structure upon hydrogenation. In spite of the very anisotropic dis-
tortion of their crystal lattices, all of the hydrides demonstrate
increased interatomic distances in comparison with their parent
IMCs. The lattice expansion results in a substantial decrease in
the exchange interaction strength and in the magnetic transition