Magnetic properties of the intermetallic compounds RNi (R=Gd, Tb, Dy, Sm) and their hydrides

Article abstract of DOI:10.1134/S0020168510040084

Hydrogen interaction with RNi intermetallic compounds and the influence of hydrogen on magnetic properties of these compounds were investigated. Ternary hydrides GdNiH_3.2, TbNiH_3.4, DyNiH_3.4 and SmNiH_3.7 were pr

Full text of DOI:10.1134/S0020168510040084

ISSN 0020ꢀ1685, Inorganic Materials, 2010, Vol. 46, No. 4, pp. 364–371. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © Yu.L. Yaropolov, V.N. Verbetsky, A.S. Andreenko, K.O. Berdyshev, S.A. Nikitin, 2010, published in Neorganicheskie Materialy, 2010, Vol. 46, No. 4,
pp. 421–428.
Magnetic Properties of the Intermetallic Compounds RNi (R=Gd,
1
Tb, Dy, Sm) and Their Hydrides
Yu. L. Yaropolov, V. N. Verbetsky, A. S. Andreenko, K. O. Berdyshev, and S. A. Nikitin
Lomonosov Moscow State University
eꢀmail: yaropolov@yandex.ru
Received July 02, 2009
Abstract—Hydrogen interaction with RNi intermetallic compounds and the influence of hydrogen on magꢀ
netic properties of these compounds were investigated. Ternary hydrides GdNiH , TbNiH , DyNiH and
3.2
3.4
3.4
SmNiH3.7 were prepared by hydrogenation of the initial alloys at room temperature and hydrogen pressure
up to 0.1 MPa. Hydrides possess orthorhombic CrBꢀtype structure (S.G. Cmcm). The formation of hydrides
results in substantial expansion of the metallic sublattices, weakening of the ferromagnetic interactions and
decreasing of the paramagnetic Curie temperatures.
DOI: 10.1134/S0020168510040084
1
INTRODUCTION
powerful method for magnetic properties modification of
already known materials or new compounds.
Intermetallic compounds formed between rare earths
and transition metals have attracted considerable attenꢀ
tion owing to their potential for various applications.
The data available in the literature concerning the
interaction of some RNi IMC with hydrogen are insuffiꢀ
Influence of hydrogen on magnetic properties of cient. The hydrogen interaction with LaNi, YbNi, LuNi,
materials can be caused by different factors. On the one ErNi, PrNi and CeNi intermetallic compounds have
hand, at hydrogenation the cell volume of intermetallic been already investigated earlier [7–10]. It is known that
compounds considerably grows, distances between metal RNi compounds interact with hydrogen at rather soft
atoms increase. All types of exchange interactions, possiꢀ conditions (low hydrogen pressure and room temperaꢀ
ble in Rꢀ3d compounds: an indirect exchange between ture) and absorb about 3 hydrogen atoms per formula
rare earth ions’ 4fꢀsubshells through conduction electrons
and 3dꢀelectrons or direct exchange interaction between
unit. Hydrides of LaNi, CeNi and PrNi retain the strucꢀ
ture of the intermetallic compounds (structure type CrB,
S.G. Cmcm). Introduction of hydrogen atoms leads to
cell volumes increase in this case. IMC with FeBꢀtype
structure (S.G. Pnma) undergo structural transition durꢀ
ing the hydrogenation, their hydrides also possess CrBꢀ
type structure [7–11].
3
dꢀelectrons—depend on distances between electrons
and geometrical parameters of a crystal lattice. Nowadays
it is not clear how lattice expansion will affect the dꢀd, fꢀd
or fꢀf exchange interactions in the general case. Experiꢀ
mental investigations revealed that geometrical expansion
of lattice at hydrogenation results in the strengthening of
dꢀd exchange interaction in the intermetallic compounds
Intermetallic compounds RNi (R = Pr, Nd, Sm, Gd,
with high iron content (e.g. R Fe , R Fe Ti): the paraꢀ Tb, Dy, Ho, Er, Tm) are ferromagnetic materials with
2
17
2
11
magnetic Curie temperature considerably grows and the Curie temperatures below liquidꢀnitrogen temperature.
magnetic moment of Fe atoms increases [1–3]. On the The effective magnetic moments of these compounds are
contrary, in case of RFe2 compounds the decreasing of the close to the magnetic moment values of the free lanꢀ
unit cell volume results in the increasing of the Curie temꢀ
perature [4].
thanide ions. The magnetization magnitude of SmNi in
paramagnetic region was too small to estimate correctly
Electronic structure and electron density distribution the effective moment and paramagnetic Curie temperꢀ
can strongly depend on the hydrogen atoms’ charge state. ature values [12]. Besides, it is known that GdNi,
The charge state change also can lead to the change of HoNi, ErNi and DyNi have large values of the MCE
magnetic properties at hydrogenation. Electron density at near the transition temperature [5, 6]. Magnetic propꢀ
Fermi level is one of the major factors determining magꢀ erties of RNi hydrides (R
netism. Redistribution of electron density in a conduction
dꢀband at hydrogenation results in electron density
redistribution at Fermi level [5, 6]. Thus, hydrogenation
of intermetallic compounds can be considered as rather
−
Gd, Tb, Dy, Sm) were not
investigated earlier.
3
Thus, the basic purpose of the given work was the
research of hydrogenation process of the RNi compounds
and magnetic properties of these IMC and ternary
hydrides.
1
The article was translated by the author.
364

MAGNETIC PROPERTIES OF THE INTERMETALLIC COMPOUNDS
Table 1. Lattice parameters of the IMC RNi and ternary hydrides
365
Composiꢀ
tion
Structure type
Lattice parameters, Å
Z
V
, Å3
Δ
V/V, %
GdNi
CrB
CrB
a
a
a
= 3.778(4),
= 3.767(2),
= 21.31(2),
b
b
b
= 10.337(6),
= 11.576(7),
c
c
= 4.238(5)
= 4.733(3)
4
4
165.54(3)
206.45(2)
–
GdNiH3.2
TbNi
24.7
–
TbNi (lowꢀtemperature
= 4.211(4),
c
= 5.454(2),
β
= 97.43
°
12 485.48(4)
modification
)
TbNiH3.4
DyNi
CrB
a
a
a
a
a
= 3.742(2),
= 7.025(4),
= 3.759(2),
= 3.782(3),
= 3.791(2),
b
b
b
b
b
= 11.516(6),
c
= 4.707(3)
4
4
4
4
4
202.88(2)
159.94(2)
199.00(2)
168.76(3)
210.15(2)
25.4
–
FeB
= 4.181(3),
= 11.368(4),
= 10.375(4),
= 11.644(4),
c = 5.445(2)
DyNiH3.4
SmNi
CrB
c
= 4 .655(2)
= 4.301(2)
= 4.761(2)
24.4
–
CrB
c
c
SmNiH3.7 CrB
24.5
EXPERIMENTAL
The unit cell volume increase in all cases exceeds
24% (the given values were calculated considering the
~
The present work represents the hydrogenation process
study of GdNi (CrBꢀtype structure), DyNi (FeBꢀtype),
TbNi (TbNiꢀtype structure, lowꢀtemperature modificaꢀ
tion) and SmNi (CrBꢀtype). Also the magnetic properties
study for these compounds and their hydrides has been
carried out.
different number of formula units per unit cell).
Magnetization measurements were performed at
vibration magnetometer in temperature range 78 300 K.
Tabletꢀshaped samples (150–400 mg) were used in these
experiments.
−
The starting intermetallic compounds RNi were synꢀ
thesized by arc melting under argon atmosphere in a furꢀ
nace with a nonꢀconsumable tungsten electrode and
RESULTS AND DISCUSSION
The transition temperatures of the RNi intermetallic
waterꢀcooled copper tray. Nickel (purity of 99.99%) and compounds and ternary hydrides appeared to be lower
rare earth metals (99.9%) were used as a starting compoꢀ than liquidꢀnitrogen temperature (78 K). Therefore we
nents and titanium sponge as a getter. The starting focused on the analysis of the paramagnetic region. Calꢀ
amounts of rareꢀearths were corrected due to waste of culated temperature dependences of reciprocal susceptiꢀ
metal during the melting process. To ensure homogeneity, bilities are linear in temperature range 78–300 K and folꢀ
the samples were turned and remelted several times.
low the CurieꢀWeiss law
χ
=
C/(
T
–
θ)
in all cases except
SmNi and SmNiH samples. That allowes to estimate
3.7
Hydrogen sorption properties were investigated on a
Sievert type volumetric apparatus at room temperature
and hydrogen pressures up to 1 MPa. The composition of
the hydrides was calculated from volumetric measureꢀ
ments using Van der Vaals equation.
the values of paramagnetic Curie temperature
effective magnetic moment eff.
θ
and
p
μ
Magnetic moment – field (M–H) isotherms are linꢀ
ear in the temperature range 78–300 K both for DyNi and
its hydride (Fig. 1). It means, that they are paramagnetic
at temperatures above 78 K. Temperature dependences of
a magnetic moment (M) and reciprocal susceptibility
Xꢀray powder diffraction measurements for starting
alloys and hydrides were performed to establish the phase
composition of the samples and calculate the lattice
(
1/ ) for DyNi and DyNiH3.4 are represented at Fig. 2.
χ
parameters. The characteristic Xꢀray used was CuK .
α
On the basis of reciprocal susceptibility temperature
dependences the paramagnetic Curie temperature values
RNi alloys easily interact with hydrogen at room temꢀ
perature and hydrogen pressure about 0.1 MPa. Ternary
hydrides possess orthorhombic CrBꢀtype structure. In
case of GdNi and SmNi introduction of hydrogen atoms
leads to expansion of the unit cells without structure transꢀ
formation. In case of TbNi and DyNi the ternary
hydrides formation is accompanied by metal sublattice
structure transformation (FeB–CrB structure transition).
These results agree with the available literature data about
ternary hydrides of other RNi compounds [7–11]. Lattice
(θ
p) and the magnetic moment per formula unit were calꢀ
culated using the CurieꢀWeiss law. For DyNi the effective
magnetic moment ( eff) is found to be 10.3 and paraꢀ
magnetic Curie temperature
μ
μ
B
θ
p
=51 K. For hydride
μ
eff
=
1
0.1 B, e.i. the effective magnetic moment practically has
μ
not changed in comparison with initial intermetallic comꢀ
pound. Paramagnetic Curie temperature has fallen to
θ
p
= 3 K after hydrogenation.
IMC TbNi is a paramagnetic in the temperature range
parameters of ternary hydrides are summarized in Table 1. 78–300 K (Fig. 3). Hydride TbNiH3.4 is a paramagnetic at
INORGANIC MATERIALS Vol. 46
No. 4 2010

366
YAROPOLOV et al.
а)
2
2
M
, A m /kg
(
M
, A m /kg
.15
.10
(b)
0
0
0
1.0
DyNi
DyNiH3.4
0.5
.05
–
600 –400 –200
0
200
400
Н
600
, kА/m
–600 –400 –200
–
0
200
400
600
Н, kА/m
0.05
0.10
0.15
–
0.5
–
–
T = 297 K
T
= 78 K
–
1.0
Fig. 1. Magnetization curves of the DyNi and its ternary hydride at 78 and 297 K.
2
–1
m
–6
M
, A m /kg
χ
×
10 , mol
(а)
(b)
0.20
0.15
0.10
0.05
0
DyNiH3.4
DyNi
30
25
20
15
10
5
H
= 120 кА/м
50
100 150 200 250 300
, K
0
50 100 150 200 250 300
, K
T
T
Fig. 2. Temperature dependences of the magnetic moment (
M
) and reciprocal susceptibility (1/ ) for DyNi and DyNiH3.4 samples.
χ
room temperature, but at 78 K a small hysteresis in fields GdNiH3.2, accordingly) and paramagnetic Curie temperꢀ
up to 80 kA/m is observed (Fig. 4). The small deviation of
the (T) dependence from the CurieꢀWeiss law at temperꢀ
atures 78–120 K indicates the presence of ferromagnetic
component in ternary hydride. The reciprocal susceptibilꢀ
ity temperature dependence of the TbNi sample follows
the CurieꢀWeiss law down to 78 K (Fig. 5). Paramagnetic
atures
ingly).
θ
(80 and 21 K for GdNi and GdNiH3.2, accordꢀ
p
χ
Magnetic properties of SmNi and SmNiH3.7 differ
from the above described samples. In this case magnetic
moment of hydride sample is higher than the magnetic
moment of IMC. Magnetic moment ꢀ field dependences
of hydride sample are strongly nonlinear both at 78 K and
at room temperature (Fig. 7). Reciprocal susceptibility
temperature dependences of the SmNi and its ternary
hydride are also nonꢀlinear and cannot be described
within the CurieꢀWeiss law. Therefore there is no way to
calculate the values of the effective magnetic moments
and the paramagnetic Curie temperatures for these samꢀ
ples (Fig. 8). It is necessary to note, however, that the
Xꢀray powder diffraction pattern of the SmNiH3.7 sample
revealed the presence of SmH2 phase (Fig. 9), whereas
Curie temperatures
θ
p
and the effective magnetic
moments for TbNi and its hydride have been calcuꢀ
μ
eff
lated: for TbNi
μ
eff
= 10.0
μB,
θ
p
= 56 K, for hydride the
magnetic moment almost has not change (
μ
= 10.3
μ
B),
but paramagnetic Curie temperature has strongly
decreased ( = –12 K). This fact can indicate the presꢀ
eff
θ
p
ence of antiferromagnetic ordering at low temperatures.
GdNi and GdNiH3.2 are also paramagnetic in the
temperature range 78–300 K. It follows from the temperꢀ
ature dependences of magnetic moments and reciprocal
susceptibilities (Fig. 6). Reciprocal susceptibility temperꢀ
ature dependences are linear and can be described by
CurieꢀWeiss law. It allowes to calculate the effective magꢀ SmNi sample did not contain any impurity phase
netic moments values (8.3 and 7.5 μВ for GdNi and (Fig. 10).
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2010

MAGNETIC PROPERTIES OF THE INTERMETALLIC COMPOUNDS
367
(
а)
, A m /kg
.5
(b)
2
2
M
, A m /kg
.15
.10
M
1
1
0
0
0
TbNi
.0
.5
TbNiH3.4
0.05
–
600 –400 –200 0
200 400 600
, kА/m
–800 –600 –400 –200
0
–0.05
200 400 600
Н
Н
, kА/m
–0.5
–1.0
–1.5
–
0.10
0.15
T
= 297 K
T
= 78 K
–
Fig. 3. Magnetization curves of the TbNi and its ternary hydride at 78 and 297 K.
2
M
, A m /kg
0
0
0
.15
.10
.05
–200
–100
0
100
200
Н, kА/m
–
0.05
0.10
–
–0.15
Fig. 4. Magnetization curves of the TbNiH3.4 sample at T = 78K.
(
а)
(b)
25
20
15
10
5
0.8
0.6
0.4
0.2
0
TbNi
TbNiH3.4
H
= 320 кА/м
50
100 150 200 250 300
, K
0
50 100 150 200 250 300
, K
T
T
Fig. 5. Temperature dependences of the magnetic moment (M) and reciprocal susceptibility (1/
χ
) for TbNi and TbNiH3.4 samples.
INORGANIC MATERIALS Vol. 46 No. 4 2010

368
YAROPOLOV et al.
2
M
, A m /kg
(
а)
(b)
2.50
2.25
2.00
1.75
1.50
1.25
1.00
0.75
0.50
0.25
0
4
0
GdNi
GdNiH3.2
30
2
0
0
H
= 310 кА/м
1
50
100 150 200 250 300
, K
0
50 100 150 200 250 300
T
T, K
Fig. 6. Temperature dependences of the magnetic moment (
M
) and reciprocal susceptibility (1/ ) for GdNi and GdNiH3.2 samples.
χ
Fig. 7. Magnetization curves of the SmNi and SmNiH3.7 samples at 78 and 297 K.
–
3
2
M
×
10 , A m /kg
(а)
SmNiH3.7
–1
м
–6
(b)
χ
×
10 , mol
SmNi
(
H
= 390 kА/m)
2
1
1
0
5
0
5
SmNi (
H
= 405 kА/m)
0.6
0.5
SmNiH3.7
0
0
0
0
.4
.3
.2
.1
0
50 100 150 200 250 300
, K
0
50 100 150 200 250
, K
T
T
Fig. 8. Temperature dependences of the magnetic moment (M) and reciprocal susceptibility (1/
χ
) for SmNi and SmNiH3.7 samples.
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No. 4 2010

MAGNETIC PROPERTIES OF THE INTERMETALLIC COMPOUNDS
369
1.0
111
0.9
0
.8
.7
021
131
0
0
.6
.5
.4
.3
.2
0
110
40 002
1
30
0
0
41
0
0
0
0.1
0
0.1
0
10
20
30
40
50
, deg
60
70
80
90
2θ
Fig. 9. Xꢀray powder diffraction pattern of the SmNiH3.7 sample. The arrow indicates the reflection of impurity phase (probably SmH2).
1.0
0.9
0.8
0.7
0.6
1
11
0
41
131
0
21
040
130
0
02
110
0.5
0.4
0.3
0.2
0.1
0
0.1
0
10
20
30
40
50
, deg
60
70
80
90
2θ
Fig. 10. Xꢀray powder diffraction pattern of the SmNi sample.
INORGANIC MATERIALS Vol. 46 No. 4 2010

370
YAROPOLOV et al.
Table 2. Magnetic properties of the IMC RNi and ternary hydrides (m – effective magnetic moment per REM atom, μ(R) –
eff.
3+
free Ln ions magnetic moment)
Composition
μ
eff.,
μ
B
μ
(R),
μ
θp, K
TC , K
B
SmNi
–
–
–
0.71
–
–
–
–
45 [12]
SmNiH3.7
GdNi
–
–
8.3
7.8–8.1 [5,12]
7.0
9.0
80
21
56
–12
51
3
77 [12]
–
69–71 [5,12]
GdNiH3.2
TbNi
7.5
–
9.7 [12]
–
–
10.0
10.3
10.3
10.1
40 [12]
–
52 [12]
TbNiH3.4
DyNi
–
59–62 [6,12]
–
10.7 [12]
–
10.0
64 [12]
–
DyNiH3.4
CONCLUSIONS
of RFe Ti compounds, J. Alloys Comp., 2001, vol. 316,
11
no. 1–2, pp. 46–50.
The results of the present investigation showed that
hydrogenation of the RNi compounds leads to weakening
of the ferromagnetic interaction and decreasing of the
magnetic transition temperatures (Table 2). The paraꢀ
magnetic Curie temperature value characterizes the
intensity of exchange interactions. The effective magnetic
moments of the RNi compounds and hydrides practically
do not change during hydrogenation and are very close to
the magnetic moments values of free rare earth ions. We
suppose that nickel atoms are nonꢀmagnetic in these
compounds, and magnetic properties are caused by rareꢀ
earth metal atoms. Probably, the disappearance of the
magnetic moment in transition metal sublattice is conꢀ
nected with the filling of a 3dꢀband of nickel atoms by
2
. Tereshina, I.S., Nikitin, S.A., Verbetsky, V.N., and Salamꢀ
ova, A.A., Transformation of magnetic phase diagram as a
result of hydrogen and nitrogen atoms in crystalline lattice
of R Fe , J. Alloys Comp., 2002, vol. 336, pp. 36–40.
2
17
3
. Nikitin, S.A. and Tereshina, I.C., Influence of interstitial
atoms on effective exchange fields in a ferromagnetic comꢀ
pounds of the rare earths and 3dꢀtransition metals R Fe
2
17
and RFe Ti, FTT, 2003, vol. 45, no. 10, pp. 1850–1856 (in
1
1
Russian).
4
. Deryagin, A.V., Moskalev, V.N., Mushnikov, N.V., and
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8
nickel atoms (
3d
) is close to completed. The injection of
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pletion. As a result the 3dꢀsubsystem magnetic moment
becomes extremely small, exchange interaction strongly
decreases. Therefore the magnetic properties depend on a
subsystem of rareꢀearth metal atoms.
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