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Influence of hydriding on the magnetic properties of Gd3Ni6Al2
Journal
of Alloys
and Compounds
259
( 1997)
2-C-28
M. Mhrussanovah,
i,Al,
ies of
etic pro
J.L. Bobet”, B. Darriet”, P. Peshev’
M. Terzievaf
Abstract SUMMIT details compound ctoichiomctric irntilysis.
ratio.
peculiarities
be prepared However,
itftcr
even
As a result of LI kinetic
per formula increase
the synthesis
of
c;ln only
unit of Cd ,Ni,,AI?
after
study
annealing
this treatment
on hydroge
has been
of the unit cell p;lrametcr
of the intermetallic
prolonged
the samples
Lupkkc,
confirmed.
of about
I .45c/r
Gd ,Ni,,AI:
of a multiphase contain
the formaGon
In this hydride is obscrvcd.
compound product
some impurities, of it ternary
the original
Rcsistivity
have
obtained
studied. It has been shown that this metals in a by meltin, (I of the constituent
as proved
by X-ray
and electron
microprobe
hydride
with a maximum content of 8.6 l-l atoms and only an Ca lAgn -type struckmz is preserved
cubic
and low
been
field
magneti/.ation
have shown the
measurements
k,,/Gd is reached at B=4.8 T. which is ) K. The value of M,,,, -7.34) leads to an important decrease of the Curie very close to the si\turi\tion moment of 7.0 F,, ci~lculi\tcd for in Gd’ ’ ion. Elydrogcn absorption knpm~urc (7;. =60( I ) K for Gtl ,Ni,,AI,H, ,,), Similurly lo the lX)rM c~npound. a value 01’7.0( I ) (~l,,/Gclis obkGned for the saturation
mxurrc’ncc of fcrromagnctic ordering in Gd ,Ni,,AI,
I11tjI11cIU01 the hydride hydrided 4‘ IOW
MIIIPIL’ iIS t”.l~cvicr
;U 13-3.8
WCll
iIs
Scicrrcc
hy
T. An ilttcn\pc i\ w&!itkllit~l:
it1 7’,. = I I8( I
is mitdc to cxphlin 01’ rhc
(id-Ckl
thi,, bchaviour by i\ non-uniliwn~ sl>iiriill tlisrriburion
intl’r;k.Yionr
in thi\
hiInll9Ic
ccbnsisting
earth).
ternary
for the system
with
r0rcr\iblc
E-Ni
ii~ter~~~et~~llicsaimed
are exhaustively is available
intcrmotallic 121. Buschow study
CdNiAl
magnetic phases
compound. properties
wcrc
described
1.31 and Takeshita of Cd-Ni-Al
in 1078 by Rykhal
of some by
et al. 141.
ot’
cOnsli\nl.
IlydWgCll
lilr)JC
rulis
at establishing
storage materials.
discussed in
n equiatomic illld
absorption
e as hydrogen
( I 1I. On the contrary,
he tirst systematic ublishcd
Especially
in
ht et al. ( 11 for the first time establ the preparation
Ocstcrreichcr
of’W ;ki(jt~ls in the
01’ the Ii\tlicc
has stit~~ulate~l numerous studies OIB the properties
t1 recent yeiuh an impor9utlt rcsui\rch effort has been used on intermetallic compounds of’ the systems RE-
other
by lhc incrcasc
S A.
illll(9lltltS
recently
ciILI\cd
review
much le
concerning
nation
the
I--AI
their
These studies of Buschow in this respect ternary
phases.
et al. [ 121 have found
c~~inp(~~lnd with a hexagonal ydrogen
phase diagram
..tt room tem owever.
et al. [Sl. These authors
these cuthors have not establishe
the obtained
11ydride phase. A substimtii
ed the existence of eight ternary phases in the range of O-33.3 at% Gd. Gd,Ni,, ‘h,Ni 17mtypc lattice pound been pL]blislicd recently by et IS- lo\. e unusual property *Corresponding
of’ GC have been cvidenc
otnaka 171 ittlcf Gladyshevskii
of
author
0925-8388/971Sl7.W 0 PI/ SO925-8388(97)00106-O
temperature 1997 Elsevier
Science S.A. All rights reserved.
but at lower
gas pressures.
The quarrtity
of
S. Pechrv et al. I Jomnal of Alloys and Con~pounds 259 (1997) 24-28
25
Table I Electron microprobe analysis of as-cast and annealed Gd,Ni,Al, samples
absorbed hydrogen corresponds to 7-9 and 2.7- 11.4 atoms H/formula unit for each of the two intermetallic compound families, respectively. Hydride formation takes place without a change of the original phase structure (Ca,Ag,-type for RE,Ni,Al, and CeNi,-type for RE,Ni,Al, respectively). From the literature data one can conclude that some ternary intermetallics of the two latter families could find application as low-pressure hydrogen storage materials. In this connection some knowledge of their fundamental properties is needed. Thus, the present work is a study on the magnetic properties of Gd3Ni,Al, as a representative of the first family, and on the changes of these properties observed after hydriding. The choice of Gd,Ni,Al, is associated with the stability (proved by preliminary experiments) of its hydride when stored for a long period of time under normal conditions.
A standard four-probe d.c. technique was used in electrical resistivity measurements. surements were pe magnetometer.
2.
3.
A polycrystalline Gd3Ni,AlZ sample was prepared by direct melting of stoichiometric amounts of the constituent metals with a purity better than 99.9%. The process was carried out in a cold-crucible r.f. levitation furnace under a purified argon atmosphere. The alloy button was turned side down and the sample was remelted several times in er to obtain a more homogeneous product. Afterwards, ingot was subjected to vacuum annealing at 700 “9: fol 20 days. X-ray powder diffraction with t camera (Cu K
Sample
As-cast
Annealed
Experimental compositions (at%)
Assigned phase
Estimated ratio (%‘r)
38(5) 31(5) 31(S) 90(2) h(2) 4(21
Gd
Ni
Al
35.1(9) 25.7(3) 17.3(l)
49.4(7) X6(6) 60.9(7)
13.5(7) 15.6(7) 21.9(6)
tid,lw,Al Gd,Ni,Al GdNi, ,AI, 3
27.6( 3)
54.6( 3) 54.q 5) 49.2( 2)
17.8(3) 27.0(I) I.3 9)
Gd,Ni,Al, GdNi, ,A1 , , GdNi
18(l)
49.3(9)
During this study it has been establis
racteristic of Cd as-cast product mig three different compositions ( assigned to one of the following
ixt phases:
ternary diagra
Nevertheless, some wea cubic unit cell.
presence of traces of in~p~~ity
Ni
activation was dchie
a constant amount of absorbed hydrogen was established.
f
Fig. I. Part of the Gd-Ni-Al
ternary diagram.
01--.-0 40
80
Timr Fig.
2.
Gd,Ni,~l,
Time
.,...,,,.ai
I20
dependence of hydrogen
160
200
240
(min)
80
uptake of activated
at room temperature and a hydrogen pressure of
powdery
I MPa.
e.oo?.o
0.0016
0.0012
Irlr
also
confirmed by the results of microprobe determinations performed on the annealed sample (Table 1). The attempts to eliminate these impurities by increasing duration or temperature of annealing, have failed. In a recent crystallochemical investigation on the phases with Ce,Ni,Si,-type structure, Yartys et al. 1161 have shown the existence of several different interstitial cages able to accept hydrogen atoms and have proposed plausible morsels for the structure of the hydride. On the basis of a neutron diffraction study of deutcratcd Tb,Ni,AI, samples. these authors have concluded that the maximum number of m atoms per formula unit might on the choice bet An two most probable models &viAopccl, vidut2 ol’ this l~~~~xii~~~it~~ number is to he cx CilSd’ WhiZtl hOth InWlels i\rd‘ i\l7l7lid‘simultilll tl ‘he t~r~~~~ry intcrmctullic ~~~t~~l~~)ii~l Gd ,Ni,,AI,, whi& ~uhjd’ct of the present pal96’r, h isostructural with ,Al, and the diff~r~~~~~b~t~ve~l~fhc l~ltti~~ ters of both substances is very stmll 15,131.Thus, comparable amounts of hydro~et~ absor~d as a result of hydridin are to be excited. Some results of the l~ydridin~ expen~~~~~~ts scurriedout with p d,Ni,Al, after activation are shown in Fig. 2 as a plot of om temperature ancl a ‘he ~i~~~xii~~ull~ ~lfi~(~u~~t of hydrogen corresponds to the formula of the ydride Gd ,Ni,AI $, (,, which is in good agree-
0,0024
0.0028
(EC’)
Fig. 3. Hydrogen absorption plateau pressure vs. reciprocal temperature plot for Gd ,Ni,,AI
2 (Van’t
Hoff plot 1.
ment with the results of the model study mentioned above as well as with the experimental data on Gd,Ni,Al, compound published by Yartys and Pavlenko [ 131. The shapes of the dependences of hydrogen uptake on for the second time at 37 73 K and P,, = 5.2 series samp are similar to the shap measurement data are summarized i 2. tht the maximum amount of hydr L’V ,Ni,,AI, dccrcases with rising temperature, whcrcas bY the time needed to attain 90% of this amount sharply ‘) kWi1
CillTkd
Il1cilslII%-
out at the four t~!l~~l~eriltllr~s
isotherms ~~I~tililKXl plateau ~orr~s~~)ndi tion and a wide anch. With rising temperature, the plateau pressure w decreases and the plateau is not observed at T The relatively large determin~~tio~error of the correspondirl~ values, presented in nds on the small plateau width and the low the measurement ir~str~l~~e~itsat P,, 2<
, a
small
* 3
shows
the
R
In
,,,, = absorptio is plot, calle
S. Prchev et al. I Jowlal of Alloys arxl Corltpowds 259 (1997) 24-28
van’t Hoff plot, has been used to calculate the enthalpy AH= -33.6(3.2) kJ mol-’ and the entropy AS = 170 (3) J mol-’ K-’ of the process. These data have allowed estimating the plateau pressure at room temperature. The value found ( 20.9X lo-’ MPa) is much lower than the atmospheric pressure ( ~0.1 MPa), so that the hydride Gd3Ni,Al,H,., must be very stable, which is also confirmed by our observations. The X-ray data have shown that the original cubic crystal structure of Gd,Ni,Al? is preserved in the hydrided sample and only an expansion of the unit cell is observed. For Gd,Ni,Al,H,,, the cell parameter a =0.9111 (4) nm, which corresponds to an increase of about 1.45% as compared to the parameter of the sample before hydriding. The results of resistivity measurements performed on annealed Gd,Ni,Al? sample are presented in Fig. 4. They reveal the metallic character of this compound. A change of slope in the reduced resistivity vs. temperature dependence is observed at T= 1E5(3) K. That could be associated with the appearance of a magnetic order below this temperature, which induces the onset of coherency attributed to a decrease in spin disorder. The low field measurements of the magnetization as a function of temperature for dJNi,AIZ show the occurrence of a ferromagnetic transition at Tc = 118( 1) K (Fig. 5), which is in quite good agreement with the resistivity measurements. The fact that magnetization attains easily a alue with the increase of the applied field at
27
0.6
0
40
80
120
‘r W
Fig. 5. Temperature dependence of Gd,Ni,Alz magnetization at a field B =O.Ol T.
^““__ ”__r.__. -v- --
1.0
f-
*
1
---c_l‘ -
7-“--’
-
t
0.4b*-1-""a" 0
_._.._I
--200
I.00 T(K)
Fig. 4. Temperature dependence of reduced resistivity of Gd,Ni,,Alz.
and Gd,Ni,Al,
between the magnetic moments of gadolinium atoms
moment calculated for t is an indication lhitt in
^__, _ I
160
Fig. 6. Field dependenceof Gd,Ni,Al, at a temperature T= 5 K.
and Gd,Ni,Al,
?J-.?s
eing less sharp than in the case of non-hydrided sample. An Wdl value of 7.0( I ) pJGd at B =4.8 T, which is similar to that of the parent compound, is observed for the hydride (Fig. 6) but the magnetization shows some difficulties to reach saturation with increasing field values. Attempting to propose an explanation of the above effects, one has to mention again the neutron diffraction study of Yartys et al. [ 161 who have shown that as a result of hydriding of a compound isostructural with the intermetallic one considered in the present work, but containing Tb instead of Gd, the hydrogen is incorporated inequitably in three of the seven available sites. This non-uniform spatial distribution of hydrogen may lead to some disorder on the scale of a few atomic distances and to changes in strength of local magnetic interactions. Another effect of the hydrogen insertion is the increase in lattice constant and interatomic distances. Thus, the distance between the Cd-Gd nearest neighbours, which is 0.3707 nm in the parent intermetailic compound, becomes 0.3761 nm in the hydride. This larger separation between the gadolinium ions leads to weakening of the Gd-Gd interactions and causes a decrease in the Curie temperature. d also be mentioned that a behaviour similar to d,Ni,AIZ has been observed with other Gd-based r instance, hydrogen absorption by GdNi, to a ferromagnetic ordering temperature -8 K [ 191 and from 56-30 K [ I I 1. respectively.
The present study has been financed within the framework of PICS No 268 of the Centre National de la Recherche Scientifique and the Bulgarian Academy of Sciences. Some of the authors (M. Kh., M. T. and P. r) would also like to express their thanks to the National Fund of Scientific Investigations of Bulgaria for the financial support under contract No X-43 I/ 1994.
eferences A.E. Dwight,
M.H.
Mueller,
Trans. Met. Sot. AIME H. Oesterreicher.
R.A. Conner, I.W. Donney. H. Knott,
242 (1968)
J. Less-Common
2075.
( 1973)
Met. 31)
X5.
K.H.J. Buschow, J. Less-Common T. Takeshita. (197X)
S.K.
Mnlik. W.E. Walluce. J. Solid State Chem. 23
271.
R.M.
Rykhal’.
OS.
Zarechnyuk.
O.M.
Nauk Ukr. RSR. Ser. A 40 (1978)
Marich.
Dopovidi
Akad.
85.3.
1. Pop, N. Dihoiu. M. Coldea. C. Hagan. J. Less-Common Met. 64 (1979)
6.).
V.A. Romtika. Yu.N. Grin. Ya.P. Yarmolyuk. Skolozdra, Fiz. Met. Metalloved. R.E. Gladyhhevskii.
( 1991)
54
O.S. Zarechnyuk.
( 1982)
R.V.
691.
K. Cenr.unl. E. Pat-the. J. Solid State Chem. 100
9. E. R\nhe,
R.E. Gladyshevskii.
K. Cenfuul. H.D. Flack. E. Pa-the. Acta Cryst.
( 1993)
R 49 K.1I.J.
l%cvirr,
in:
K,A.
Ci\chneidncr,
on the PhyGcs and Chemistry
Ch. 47.
t I, Drulih. W ( 1‘IW41129.
lOl(4. p.
Pctryn\ki.
Fi\mell.W.E.
K.H.I.
SK.
171.
Jr..
I... Eyring
(Ed%.).
ol’ Rare Eurths. Vol. 6.
I. 13. Stidinski. J. Less-Common
VA, Ymy\, V,V I%vlcnko. Kocrrtl. Kllm.
J.
198
40X.
t3uschow.
t Iimdhook
Z. Kristallogr.
( 1992)
R.E. Gladyshevskii.
Willlitce. Inorg. Chem. 5
1t( ( 1902)
( 1966)
Met.
424.
105.
Buschow. Rep. Prop. Phyh. 40 ( 1977) 1179. Mrtlik. W.E. W&cc Solid State Comtn. 24 ( 1977) 283.
101
of Alloys
and Compounds
259
( 1997)
2-C-28
M. Mhrussanovah,
i,Al,
ies of
etic pro
J.L. Bobet”, B. Darriet”, P. Peshev’
M. Terzievaf
Abstract SUMMIT details compound ctoichiomctric irntilysis.
ratio.
peculiarities
be prepared However,
itftcr
even
As a result of LI kinetic
per formula increase
the synthesis
of
c;ln only
unit of Cd ,Ni,,AI?
after
study
annealing
this treatment
on hydroge
has been
of the unit cell p;lrametcr
of the intermetallic
prolonged
the samples
Lupkkc,
confirmed.
of about
I .45c/r
Gd ,Ni,,AI:
of a multiphase contain
the formaGon
In this hydride is obscrvcd.
compound product
some impurities, of it ternary
the original
Rcsistivity
have
obtained
studied. It has been shown that this metals in a by meltin, (I of the constituent
as proved
by X-ray
and electron
microprobe
hydride
with a maximum content of 8.6 l-l atoms and only an Ca lAgn -type struckmz is preserved
cubic
and low
been
field
magneti/.ation
have shown the
measurements
k,,/Gd is reached at B=4.8 T. which is ) K. The value of M,,,, -7.34) leads to an important decrease of the Curie very close to the si\turi\tion moment of 7.0 F,, ci~lculi\tcd for in Gd’ ’ ion. Elydrogcn absorption knpm~urc (7;. =60( I ) K for Gtl ,Ni,,AI,H, ,,), Similurly lo the lX)rM c~npound. a value 01’7.0( I ) (~l,,/Gclis obkGned for the saturation
mxurrc’ncc of fcrromagnctic ordering in Gd ,Ni,,AI,
I11tjI11cIU01 the hydride hydrided 4‘ IOW
MIIIPIL’ iIS t”.l~cvicr
;U 13-3.8
WCll
iIs
Scicrrcc
hy
T. An ilttcn\pc i\ w&!itkllit~l:
it1 7’,. = I I8( I
is mitdc to cxphlin 01’ rhc
(id-Ckl
thi,, bchaviour by i\ non-uniliwn~ sl>iiriill tlisrriburion
intl’r;k.Yionr
in thi\
hiInll9Ic
ccbnsisting
earth).
ternary
for the system
with
r0rcr\iblc
E-Ni
ii~ter~~~et~~llicsaimed
are exhaustively is available
intcrmotallic 121. Buschow study
CdNiAl
magnetic phases
compound. properties
wcrc
described
1.31 and Takeshita of Cd-Ni-Al
in 1078 by Rykhal
of some by
et al. 141.
ot’
cOnsli\nl.
IlydWgCll
lilr)JC
rulis
at establishing
storage materials.
discussed in
n equiatomic illld
absorption
e as hydrogen
( I 1I. On the contrary,
he tirst systematic ublishcd
Especially
in
ht et al. ( 11 for the first time establ the preparation
Ocstcrreichcr
of’W ;ki(jt~ls in the
01’ the Ii\tlicc
has stit~~ulate~l numerous studies OIB the properties
t1 recent yeiuh an impor9utlt rcsui\rch effort has been used on intermetallic compounds of’ the systems RE-
other
by lhc incrcasc
S A.
illll(9lltltS
recently
ciILI\cd
review
much le
concerning
nation
the
I--AI
their
These studies of Buschow in this respect ternary
phases.
et al. [ 121 have found
c~~inp(~~lnd with a hexagonal ydrogen
phase diagram
..tt room tem owever.
et al. [Sl. These authors
these cuthors have not establishe
the obtained
11ydride phase. A substimtii
ed the existence of eight ternary phases in the range of O-33.3 at% Gd. Gd,Ni,, ‘h,Ni 17mtypc lattice pound been pL]blislicd recently by et IS- lo\. e unusual property *Corresponding
of’ GC have been cvidenc
otnaka 171 ittlcf Gladyshevskii
of
author
0925-8388/971Sl7.W 0 PI/ SO925-8388(97)00106-O
temperature 1997 Elsevier
Science S.A. All rights reserved.
but at lower
gas pressures.
The quarrtity
of
S. Pechrv et al. I Jomnal of Alloys and Con~pounds 259 (1997) 24-28
25
Table I Electron microprobe analysis of as-cast and annealed Gd,Ni,Al, samples
absorbed hydrogen corresponds to 7-9 and 2.7- 11.4 atoms H/formula unit for each of the two intermetallic compound families, respectively. Hydride formation takes place without a change of the original phase structure (Ca,Ag,-type for RE,Ni,Al, and CeNi,-type for RE,Ni,Al, respectively). From the literature data one can conclude that some ternary intermetallics of the two latter families could find application as low-pressure hydrogen storage materials. In this connection some knowledge of their fundamental properties is needed. Thus, the present work is a study on the magnetic properties of Gd3Ni,Al, as a representative of the first family, and on the changes of these properties observed after hydriding. The choice of Gd,Ni,Al, is associated with the stability (proved by preliminary experiments) of its hydride when stored for a long period of time under normal conditions.
A standard four-probe d.c. technique was used in electrical resistivity measurements. surements were pe magnetometer.
2.
3.
A polycrystalline Gd3Ni,AlZ sample was prepared by direct melting of stoichiometric amounts of the constituent metals with a purity better than 99.9%. The process was carried out in a cold-crucible r.f. levitation furnace under a purified argon atmosphere. The alloy button was turned side down and the sample was remelted several times in er to obtain a more homogeneous product. Afterwards, ingot was subjected to vacuum annealing at 700 “9: fol 20 days. X-ray powder diffraction with t camera (Cu K
Sample
As-cast
Annealed
Experimental compositions (at%)
Assigned phase
Estimated ratio (%‘r)
38(5) 31(5) 31(S) 90(2) h(2) 4(21
Gd
Ni
Al
35.1(9) 25.7(3) 17.3(l)
49.4(7) X6(6) 60.9(7)
13.5(7) 15.6(7) 21.9(6)
tid,lw,Al Gd,Ni,Al GdNi, ,AI, 3
27.6( 3)
54.6( 3) 54.q 5) 49.2( 2)
17.8(3) 27.0(I) I.3 9)
Gd,Ni,Al, GdNi, ,A1 , , GdNi
18(l)
49.3(9)
During this study it has been establis
racteristic of Cd as-cast product mig three different compositions ( assigned to one of the following
ixt phases:
ternary diagra
Nevertheless, some wea cubic unit cell.
presence of traces of in~p~~ity
Ni
activation was dchie
a constant amount of absorbed hydrogen was established.
f
Fig. I. Part of the Gd-Ni-Al
ternary diagram.
01--.-0 40
80
Timr Fig.
2.
Gd,Ni,~l,
Time
.,...,,,.ai
I20
dependence of hydrogen
160
200
240
(min)
80
uptake of activated
at room temperature and a hydrogen pressure of
powdery
I MPa.
e.oo?.o
0.0016
0.0012
Irlr
also
confirmed by the results of microprobe determinations performed on the annealed sample (Table 1). The attempts to eliminate these impurities by increasing duration or temperature of annealing, have failed. In a recent crystallochemical investigation on the phases with Ce,Ni,Si,-type structure, Yartys et al. 1161 have shown the existence of several different interstitial cages able to accept hydrogen atoms and have proposed plausible morsels for the structure of the hydride. On the basis of a neutron diffraction study of deutcratcd Tb,Ni,AI, samples. these authors have concluded that the maximum number of m atoms per formula unit might on the choice bet An two most probable models &viAopccl, vidut2 ol’ this l~~~~xii~~~it~~ number is to he cx CilSd’ WhiZtl hOth InWlels i\rd‘ i\l7l7lid‘simultilll tl ‘he t~r~~~~ry intcrmctullic ~~~t~~l~~)ii~l Gd ,Ni,,AI,, whi& ~uhjd’ct of the present pal96’r, h isostructural with ,Al, and the diff~r~~~~~b~t~ve~l~fhc l~ltti~~ ters of both substances is very stmll 15,131.Thus, comparable amounts of hydro~et~ absor~d as a result of hydridin are to be excited. Some results of the l~ydridin~ expen~~~~~~ts scurriedout with p d,Ni,Al, after activation are shown in Fig. 2 as a plot of om temperature ancl a ‘he ~i~~~xii~~ull~ ~lfi~(~u~~t of hydrogen corresponds to the formula of the ydride Gd ,Ni,AI $, (,, which is in good agree-
0,0024
0.0028
(EC’)
Fig. 3. Hydrogen absorption plateau pressure vs. reciprocal temperature plot for Gd ,Ni,,AI
2 (Van’t
Hoff plot 1.
ment with the results of the model study mentioned above as well as with the experimental data on Gd,Ni,Al, compound published by Yartys and Pavlenko [ 131. The shapes of the dependences of hydrogen uptake on for the second time at 37 73 K and P,, = 5.2 series samp are similar to the shap measurement data are summarized i 2. tht the maximum amount of hydr L’V ,Ni,,AI, dccrcases with rising temperature, whcrcas bY the time needed to attain 90% of this amount sharply ‘) kWi1
CillTkd
Il1cilslII%-
out at the four t~!l~~l~eriltllr~s
isotherms ~~I~tililKXl plateau ~orr~s~~)ndi tion and a wide anch. With rising temperature, the plateau pressure w decreases and the plateau is not observed at T The relatively large determin~~tio~error of the correspondirl~ values, presented in nds on the small plateau width and the low the measurement ir~str~l~~e~itsat P,, 2<
, a
small
* 3
shows
the
R
In
,,,, = absorptio is plot, calle
S. Prchev et al. I Jowlal of Alloys arxl Corltpowds 259 (1997) 24-28
van’t Hoff plot, has been used to calculate the enthalpy AH= -33.6(3.2) kJ mol-’ and the entropy AS = 170 (3) J mol-’ K-’ of the process. These data have allowed estimating the plateau pressure at room temperature. The value found ( 20.9X lo-’ MPa) is much lower than the atmospheric pressure ( ~0.1 MPa), so that the hydride Gd3Ni,Al,H,., must be very stable, which is also confirmed by our observations. The X-ray data have shown that the original cubic crystal structure of Gd,Ni,Al? is preserved in the hydrided sample and only an expansion of the unit cell is observed. For Gd,Ni,Al,H,,, the cell parameter a =0.9111 (4) nm, which corresponds to an increase of about 1.45% as compared to the parameter of the sample before hydriding. The results of resistivity measurements performed on annealed Gd,Ni,Al? sample are presented in Fig. 4. They reveal the metallic character of this compound. A change of slope in the reduced resistivity vs. temperature dependence is observed at T= 1E5(3) K. That could be associated with the appearance of a magnetic order below this temperature, which induces the onset of coherency attributed to a decrease in spin disorder. The low field measurements of the magnetization as a function of temperature for dJNi,AIZ show the occurrence of a ferromagnetic transition at Tc = 118( 1) K (Fig. 5), which is in quite good agreement with the resistivity measurements. The fact that magnetization attains easily a alue with the increase of the applied field at
27
0.6
0
40
80
120
‘r W
Fig. 5. Temperature dependence of Gd,Ni,Alz magnetization at a field B =O.Ol T.
^““__ ”__r.__. -v- --
1.0
f-
*
1
---c_l‘ -
7-“--’
-
t
0.4b*-1-""a" 0
_._.._I
--200
I.00 T(K)
Fig. 4. Temperature dependence of reduced resistivity of Gd,Ni,,Alz.
and Gd,Ni,Al,
between the magnetic moments of gadolinium atoms
moment calculated for t is an indication lhitt in
^__, _ I
160
Fig. 6. Field dependenceof Gd,Ni,Al, at a temperature T= 5 K.
and Gd,Ni,Al,
?J-.?s
eing less sharp than in the case of non-hydrided sample. An Wdl value of 7.0( I ) pJGd at B =4.8 T, which is similar to that of the parent compound, is observed for the hydride (Fig. 6) but the magnetization shows some difficulties to reach saturation with increasing field values. Attempting to propose an explanation of the above effects, one has to mention again the neutron diffraction study of Yartys et al. [ 161 who have shown that as a result of hydriding of a compound isostructural with the intermetallic one considered in the present work, but containing Tb instead of Gd, the hydrogen is incorporated inequitably in three of the seven available sites. This non-uniform spatial distribution of hydrogen may lead to some disorder on the scale of a few atomic distances and to changes in strength of local magnetic interactions. Another effect of the hydrogen insertion is the increase in lattice constant and interatomic distances. Thus, the distance between the Cd-Gd nearest neighbours, which is 0.3707 nm in the parent intermetailic compound, becomes 0.3761 nm in the hydride. This larger separation between the gadolinium ions leads to weakening of the Gd-Gd interactions and causes a decrease in the Curie temperature. d also be mentioned that a behaviour similar to d,Ni,AIZ has been observed with other Gd-based r instance, hydrogen absorption by GdNi, to a ferromagnetic ordering temperature -8 K [ 191 and from 56-30 K [ I I 1. respectively.
The present study has been financed within the framework of PICS No 268 of the Centre National de la Recherche Scientifique and the Bulgarian Academy of Sciences. Some of the authors (M. Kh., M. T. and P. r) would also like to express their thanks to the National Fund of Scientific Investigations of Bulgaria for the financial support under contract No X-43 I/ 1994.
eferences A.E. Dwight,
M.H.
Mueller,
Trans. Met. Sot. AIME H. Oesterreicher.
R.A. Conner, I.W. Donney. H. Knott,
242 (1968)
J. Less-Common
2075.
( 1973)
Met. 31)
X5.
K.H.J. Buschow, J. Less-Common T. Takeshita. (197X)
S.K.
Mnlik. W.E. Walluce. J. Solid State Chem. 23
271.
R.M.
Rykhal’.
OS.
Zarechnyuk.
O.M.
Nauk Ukr. RSR. Ser. A 40 (1978)
Marich.
Dopovidi
Akad.
85.3.
1. Pop, N. Dihoiu. M. Coldea. C. Hagan. J. Less-Common Met. 64 (1979)
6.).
V.A. Romtika. Yu.N. Grin. Ya.P. Yarmolyuk. Skolozdra, Fiz. Met. Metalloved. R.E. Gladyhhevskii.
( 1991)
54
O.S. Zarechnyuk.
( 1982)
R.V.
691.
K. Cenr.unl. E. Pat-the. J. Solid State Chem. 100
9. E. R\nhe,
R.E. Gladyshevskii.
K. Cenfuul. H.D. Flack. E. Pa-the. Acta Cryst.
( 1993)
R 49 K.1I.J.
l%cvirr,
in:
K,A.
Ci\chneidncr,
on the PhyGcs and Chemistry
Ch. 47.
t I, Drulih. W ( 1‘IW41129.
lOl(4. p.
Pctryn\ki.
Fi\mell.W.E.
K.H.I.
SK.
171.
Jr..
I... Eyring
(Ed%.).
ol’ Rare Eurths. Vol. 6.
I. 13. Stidinski. J. Less-Common
VA, Ymy\, V,V I%vlcnko. Kocrrtl. Kllm.
J.
198
40X.
t3uschow.
t Iimdhook
Z. Kristallogr.
( 1992)
R.E. Gladyshevskii.
Willlitce. Inorg. Chem. 5
1t( ( 1902)
( 1966)
Met.
424.
105.
Buschow. Rep. Prop. Phyh. 40 ( 1977) 1179. Mrtlik. W.E. W&cc Solid State Comtn. 24 ( 1977) 283.
101