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Formation and magnetic properties of crystalline and amorphous SmCo2 hydrides
Journal
of the Less-Common
Metals,
147
(1989)
9
- 18
FORMATION AND MAGNETIC PROPERTIES OF CRYSTALLINE AND AMORPHOUS SmCo, HYDRIDES
K. KANEMATSU, Physical
Science
T. SUGIYAMA, Laboratories,
M. SEKINE
Nihon
University
and T. OKAGAKI at Narashino,
Funabashi,
Chiba
(Japan)
K. I. KOBAYASHI Mitsubishi
Electric
Coorporation,
(Received
January
9, 1988
Sagami
Works, Sagamihara,
Kanagawa
(Japan)
; in revised form March 10, 1988)
summary Ferromagnetic C15-type compounds SmCo, are found for 1.74 < x < 2.04. The values of the magnetizations vary from 1.21 I.cs(SmCol.,,J1 to 1.34 ~s(SmCo,+.,-‘. The Curie temperatures are 230 K, independent of 3~. The initial hydrogenation of SmCo, leads to a C14-type hydride which contains about four hydrogen atoms per SmCo,. Below 373 K, the C14-type hydride changes into a PuNi,-type hydride having four hydrogen atoms per SmCo,. Above 453 K, the C14-type hydride transforms into an amorphous hydride having fewer than three hydrogen atoms per SmCo?, which in turn decomposes into SmH, and cobalt above 573 K. The Cl4-type and the PuNi,-type hydrides are considered to be paramagnetic. The amorphous hydride is ferromagnetic up to the decomposition with 2.2 I.cs(SmCoJ’.
1. Introduction Many Laves phase compounds AB, form a hydride and,the absorbed hydrogen atoms are considered to occupy the tetrahedral interstitial sites A2B2, AB, and Bq. The hydrogen atom site preference has been discussed, taking into account the spatial size of the site [l], or considering affinity of the hydrogen for the constituent metals A and B [2]. These models do not allow any contribution from the absorbed hydrogen in the construction of crystal, assuming that the hydrogen atoms are small. By contrast, it was suggested recently that the hydrogen atoms are in an H--like state and play an important part in the construction of the crystal, comparable with that of the metals A and B [ 31. According to this model, it is possible that the compounds change their structures as a result of the hydrogen absorption. The cubic Laves phase compound SmCo, is an example. The hydrogenation of SmCo, at room temperature induces a change from the Cl5 type to the Cl4 type of structure. Heating promotes a further structural change from 0022-5088/89/$3.50
@ Elsevier
Sequoia/Printed
in The Netherlands
10
the Cl4 type to the PuNis type with lapse of time. At higher temperature, the C14-type hydride transforms into an amorphous hydride, and further heating causes the amorphous hydride to decompose giving SmH, and cobalt. The purpose of the present paper is to report the process of the transformation of SmCoz induced by the hydrogenation. We include details of experimental results of X-ray and magnetic studies on the cubic Laves phase compounds SmCo, and their hydrides.
2. SmCo, 2.1. Experimental details Samples of SmCo, were prepared by arc melting stoichiometric mixtures of samarium (purity, 99.9%) and cobalt (purity, 99.6%) in an argon atmosphere. Arc metling was repeated several times. The samples were sealed in evacuated silica tubes and annealed at 1073 K for more than 360 ks. Crystal structure and lattice constants were determined by X-ray diffraction with Fe Ka! radiation. The magnetization measurements were performed using a vibrating sample magnetometer in a magnetic field of 12.5 kOe. 2.2. Crystal structure and magnetization The crystal structure of SmCo, was ascertained by X-ray diffraction to be of Cl5 type in the composition range 1.74 < x < 2.04 and no other line was found in the X-ray pattern. The lattice constants are about 7.25 A, almost independent of the composition x, and the value is very close to the value of SmCo*, a = 7.248 A, reported by Harris et al. [4]. The magnetizations at 0 K, extrapolated from the thermomagnetic curves, and the Curie temperatures are plotted as a function of x in Fig. 1. The magnetizations change from 1.21 ~z(SmCol.,J1 to 1.33 pz(SmCo,,,)-‘, but the Curie temperatures are about 230 K, independent of the composition. Magnetization measurements on SmCoz have been carried out
-r--l -730
SmCox a
.u.
0
00
-
000
m
1.25
-250
TC
= 5”
.
0
E
.
0.. 000
“0 7
_'
I
O: 0
-;; . -
I-’
200
0
lJ----J 1.7 I.8 I.8x 2.0 Fig. 1. Composition atures for SmCo,.
2.1
dependence
of lattice
constants,
magnetizations
and Curie
temper-
11
earlier by Farrel and Wallace [ 51, and their thermomagnetic curve shows that their SmCoz is a mixture of two ferromagnets with Curie temperatures about 230 K and higher than 280 K respectively. The magnetization of the ferromagnet with 230 K is estimated to be about 1.1 I.cB(SmCoz)-‘. As a representative example of SmCo,, the thermomagnetic curve of SmCo, is shown in Fig. 3. 2.3. Explanation of the magnetic properties based on molecular field model In SmCoz, 8a and 16d sites of the Cl5 structure are occupied by 8Sm and 16Co atoms respectively. Excess samarium atoms in SmCo, of x < 2 are supposed to occupy the 16d sites with cobalt atoms, and excess cobalt atoms in SmCo, of 2 < x are supposed to occupy the 8a sites with samarium atoms. According to molecular field model, the magnetism of SmCo, of x < 2 is due to a magnetization and an exchange parameter, i.e. a molecular field coefficient of a sublattice consisting of atoms at 16d sites. The magnetizations of the 16d sublattice calculated from the experimental results are 7.3 X lo3 - 7.4 X lo3 e.m.u. (ABJ’, nearly independent of the composition x. Values of the molecular field coefficient are also considered to be nearly independent of the composition x because the lattice constants are independent of the composition. These sublattice magnetizations and molecular field coefficients lead to Curie temperatures independent of the compositions which agree with the experimental results. There is no explanation of the composition-independent sublattice magnetization and lattice constants, and the explanation may be given by an energy band calculation.
3. Hydrides 3.1. Hydrogen absorption Hydrogen absorption for SmCo, was carried out in hydrogen pressures of 0.1 - 0.3 MPa at several temperatures, after degassing in vacuum at about 670 K. The amount of hydrogen absorbed was determined by gravimetry. The absorption isotherms of the SmCo,-H system, just after degassing, were taken at several temperatures. A C14-type hydride was formed as a first step of the hydrogenation and was obtained when the hydrogenation was performed below 373 K for reaction times less than about 40 ks. A PuNi,-type hydride was observed to occur together with the C14-type hydride when the sample was hydrogenated for about 1 Ms. An amorphous hydride appeared in the sample hydrogenated above 413 K, which decomposed into SmHz and cobalt above 673 K. 3.2. Cl4
type hydride
All hydrides of SmCo, prepared at room temperature have the Cl4 type of structure, and the amount of the absorbed hydrogen is about four atoms per SmCo,, independent of the composition x. The absorption iso-
12
therms of the SmCo,-H system at 313 K are shown in Fig. 2, as a representative example of the Cl6type hydride. The positions of the diffraction lines of this sample are given in Table 1; all the diffraction lines in the corresponding X-ray pattern are broadened. Indexing was made on the basis of the Cl4 type of structure, but the atomic arrangement is considered to be quite different from the arrangement of C14-type inter-metallic compounds because the intermetallic compounds have many lines characteristic of the Cl4 type other than those of the Cl4 type hydride and they do not have P
(MPI\
’
t
Fig. 2. Absorption isotherms of the SmCos-H system at 313 K, 373 K and 453 K. Samples: (313 K), C14-type hydride; (373 K), mixture of C14-type and PUN&type hydrides; (453 K), amorphous hydride. TABLE 1 X-ray pattern of SmCos and SmCozH4.e with Cl4 t ype of structure (Fe Ko radiation) SmCoz H4.0 (C14)
SmCoz (Cl 5) hkl
28
intensity
111
26.60
m
220 311 222 422 333 440 620 533
44.24 52.44 54.94 81.50 87.64 97.84 114.90 121.84
vs ws S
hkl
28
Intensity
003 110 112 004
37.3 40.7 48.2 50.5
m-vw
006 220
79.6 88.4
S
vs S
s S S
W W
m S
The diffraction lines of hydride are very broad. The intensity of the (003) line varies and disappears in a few samples.
13
the (0 0 3) line. The relative intensity of the (0 0 3) line is different in the various samples and a few samples do not have this line. No relation between the relative intensity and the preparation process can be given at present. The lattice constants are a = 5.60 A, c = 9.12 A, independent of SmCo, composition. They decrease with increasing hydrogenation temperature, reaching the values a = 5.50 A, c = 9.08 A for the sample hydrogenated at 413 K. The magnetic field dependence of the C14-type hydride shows this hydride to be paramagnet, a weak ferromagnetic contribution possibly being due to small amounts of decomposed free cobalt, as described later. The thermomagnetic curve of the sample (313 K, 1.06 MS) after the isothermal experiment at 313 K (Fig. 2) is shown as a representative example of the Cl4 type hydride in Fig. 3. 3.3. PuNi3-type hydride The PuNis-type hydride always coexists with the C14-type hydride. As an example, Table 2 shows the diffraction lines of the sample (373 K, 0.274 MS) after the isothermal absorption experiment performed at 373 K for 0.274 MS (Fig. 2). The relative intensities of the PuNis-type hydride are similar to those of YXZrl-XCoZ~Hr [6] and Y,Zr,_,FezgH, [7]. They are different from those of the PuNis-type compounds YxZr,_,Co,,, [8] and YxZr,_,Fe,, [7]. The PuNis-type hydride is supposed to contain about four hydrogen atoms per SmCo,, because the hydrogen content of the sample (373 K, 0.274 MS) is four atoms per SmCoz. The thermomagnetic curve of this sample is nearly the same as that of the (313 K, 1.06 MS) sample, and the PuNi,-type hydride on the magnetic properties is supposed to be similar to the C14-type hydride. 3.4. Amorphous
hydrides
The possibility of preparing amorphous hydride was discussed by Buschow and Beekmans [9]. These authors argued that the disappearance
0
100
200
300 TJKl 400
Fig. 3. Temperature dependence of the magnetizations of the hydrogenated SmCoT samples. Samples: (313 K, 1.06 MS), C14-type hydride; (413 K, 1.03 MS), mixture of C14-type and PuNis-type hydrides; (423 K, 74 ks), mixture of C14-type, PuNis-type and amorphous hydrides; (523 K, 8 ks), mixture of C14-type and amorphous hydrides; (453 K, 0.46 MS); amorphous hydride; (673 K, 0.24 MS), mixture of SmHz and cobalt.
14 TABLE 2 X-ray pattern of SmCozH4_0 (373 K, 0.274 MS) (Fe KCYradiation) Cl4 type PuNia type
Intensity
28 37.3 38.00 40.7 41.46 42.42 48.2 49.82 50.5 52.7 57.92 64.56 77.2 81.4
W
m m s m s s m
h
k
I
0
0
3
h
k
00
1 9,
10
7
11 0, 0 0 10, 0 0 11, 20 3
10 11
8 2
20 5, 11 9, 1 1 11 2 0 13, 30 6
1 0 11 20
7
110
1 1 2 0 0 4
W
VW m W
VW
30 2 0 14
3
The diffraction lines of the Cl4 type are broad and those of PuNia type are sharp. Lattice constants of the PuNis type hydride are a = 5.454 A, c = 26.74 A.
of sharp diffraction lines after hydrogen absorption is due to the formation of microcrystalline decomposition products rather than to the formation of amorphous hydride. However, the sample (453 K, 0.457 MS) was concluded to be amorphous, because the magnetization measurements shown in Fig. 4 reveal that the formation of the amorphous hydride and the de-
It
0
I
H/L
3
Fig. 4. Temperature dependence of magnetization. Increasing magnetization between 430 K and 520 K appearing in the sample (313 K, 1.06 MS) is due to a change from the Cl4 type to the amorphous state. Increasing magnetizations between 570 K and 620 K appearing in the samples (313 K, 1.06 MS) and (453 K, 0.46 MS) are due to decomposition of the amorphous hydride to SmHa and cobalt.
15
composition to SmHz and cobalt occur over definite temperature ranges. The X-ray pattern taken with Fe KCYradiation exhibits only a diffuse peak at 28 = 30” - 45“. The isotherms show that the amorphous hydride contains less than three hydrogen atoms per SmCoz (Fig. 2). The magnetization measured in 12.5 kOe is slightly temperature dependent and the value is 2.2 pn(SmCo,)-’ (Fig. 3). The Curie temperature was not determined because the sample decomposed at about 600 K. The amorphous hydride appears in the samples hydrogenated at 413 K and 523 K. This temperature range is nearly the same as the range in which the transformation from the Cl4 type to the amorphous state was found in the magnetization measurements (Fig. 4). The formation of the amorphous hydride depends also on the hydrogenation time. When hydrogenated at 523 K the sample prepared with a reaction time of 8.1 ks is a mixture of the Cl4 type and the amorphous hydrides, and the sample prepared for 70.2 ks is amorphous. 3.5. SmH2 and cobalt In the samples hydrogenated above 573 K, the X-ray patterns were analysed as a mixture of the lines indexed as f.c.c. structure with a = 3.573 A and a broad line at 28 = 56.5” (Table 3). The former lines agree well with those of SmH, reported as a = 3.563 a [6]. The line at 26 = 56.5” agrees with the (1 1 1) line of f.c.c. cobalt. The isotherms at 673 K in Fig. 5 shows TABLE X-ray
3 pattern
of SmCozHz.
2
36.32 42.24 56.5 61.28 73.32 77.22 92.3 101.1 103.52 107.44
1hydrided
at 573
K (Fe Kcu radiation)
In te nsi ty
SmHz hkl
WS
111 200
vs m
co hkl
111 220 311 222 400
S S W ww
220
ww
m m
331 420
22
y
:
~!?y&t*,, ,
300
400
500
Fig. 5. Absorption isotherms mixture of SmHz and cobalt.
(
600 T(K) 700
of SmCoz-H
system
at 573
K and 673
K. Samples
are a
16
that the hydrogen content is two atoms per SmCo;? supporting the formation of SmH,. The magnetization is nearly independent of temperature and the value is 3.4 ~a(SmCoJ1, which is nearly the same as the value of free cobalt decomposed from SmH,.
4. Structural change and decomposition of hydrides 4.1. Structural change from Cl4 type to amorphous state The sample (523 K, 8.1 ks) is a mixture of the C14-type and the amorphous hydrides, and the sample (523 K, 1.03 MS) is amorphous. The amorphous hydride is also found in the sample (413 K, 1.03 MS). Therefore the transformation from the Cl4 type to the amorphous state progresses with time in the temperature range between 413 and 523 K. This transformation was also ascertained using magnetization measurements. The thermomagnetic curve of the sample (313 K, 1.06 MS) shows two steps of increasing magnetization. The first step which starts near 430 K and finishes near 520 K is ascribed to the transformation from the Cl4 type to the amorphous state, as was ascertained by X-ray examination. 4.2. Structural change from Cl4 type to PuNi, type The C14-type hydride is formed as a first step of the hydrogenation, and the PuNi,-type hydride is formed after a lapse of time from the C14type hydride below 373 K. This was found in the samples (413 K, 1.03 MS) and (423 K, 74 ks) consisting of the C14-type and the amorphous hydrides. The critical temperature to separate the two types of transformations could not be determined. It is noteworthy that the structures of SmCo* occuring after hydrogenation are analogous to those of YxZr,_,Co,,, i.e. the C15, the four-layered hexagonal and the PuNis type obtained in the composition ranges 3t= 0, 0.1 < x < 0.3 and 0.3 < x respectively. The structure of the C14-type hydride may be the same as the four-layered hexagonal structure because the X-ray diffraction patterns of both structures are similar except for a few weak lines indicative of the four layered structure. The structural changes from the Cl5 type to the PuNi, type in YxZr,_xCo2.9 are considered to relate to the decreasing number of 4d electrons caused by substitution of the zirconium for the yttrium. The similarity between the structures of both systems suggests that the structural changes induced by hydrogenation are due to a decrease in the numbers of electrons at the metal sites which is probably caused by a transfer of electrons (less than one per SmCo*) from the metal atoms to the hydrogen atoms. This inference suggests that the hydrogen atoms accepting the small number of electrons change from a neutral valence to an H--like state and the hydrogen atoms in the PuNi&ype hydride accept more electrons than those in the Cl4-type hydride.
17
4.3. Decomposition of amorphous hydride to SmH, and cobalt The sample (453 K, 0.457 MS), a typical amorphous hydride, exhibits an increasing magnetization which starts near 570 K and finishes near 620 K (Fig. 4). The cooling curve is nearly the same as the sample (673 K, 0.27 MS), a mixture of SmH2 and cobalt. After magnetization measurements, the sample was ascertained to be a mixture of SmH2 and cobalt by X-ray examination. The same decomposition was also observed in the thermomagnetic curve of the sample (313 K, 1.06 MS) as the second step which starts near 570 K and finishes near 620 K (Fig. 4). The sample (413 K, 1.03 MS), a mixture of the C14-type, the PuNis-type and the amorphous hydrides, shows increasing magnetization which starts near 430 K and finishes near 650 K. The curves exhibit no distinct steps to indicate transformations from the Cl4 type to the amorphous state and from the amorphous state to SmHz and cobalt. Furthermore, the transformations seem to be affected by the presence of the PuNis-type hydride. 4.4. Dehydrogenation of SmH,-type and PuNi,-type hydrides The sample (673 K, 0.27 MS) shows a remarkable decrease in magnetization near 875 K, which is ascribed to a decrease in the free cobalt due to the formation of Sm-Co compounds with samarium obtained from dehydrogenated SmH2. According to X-ray examination, the sample obtained after the measurement contains SmCo*, SmCo, and other unidentified compounds in addition to small amounts of SmH, and cobalt. The PuNi,-type hydride seems to release hydrogen above 640 K. The sample (423 K, 74 ks), a mixture of the PuNi,-type and the amorphous hydrides, shows increasing magnetization between 430 K and 640 K (Fig. 6). According to X-ray examination, the sample corresponding to maximum magnetization at 640 K contains the PuNis-type hydride together with SmH, and cobalt, and the sample obtained after the measurement up to 900 K is a mixture of SmH2, cobalt, SmCo*, SmCo, and other unidentified
I 0
I
I
200
I
,
400
I
I
,
1
600 TiKl800
Fig. 6. Temperature dependence of magnetization. Decreasing magnetization above 875 K appearing in the sample (673 K, 0.24 MS) is due to the formation of Sm-Co compounds from cobalt and samarium dehydrogenated SmHz. Increasing magnetization above 430 K in the sample (423 K, 74 ks) is due to the transformation from the C14-type hydride to SmHz and cobalt, and the decreasing magnetization above 640 K is due to the formation of Sm-Co compounds from cobalt and samarium formed by pure hydrogenation of SmHz and the dehydrogenation of the PuNis-type hydride.
18
compounds. In this sample, the simultaneous presence of the three hydrides seems to affect the transformation, the decomposition and the dehydrogenation. 4.5. Positions of metal atoms in the PuNi,-type hydride Generally intermetallic compounds have a wide homogeneous range of composition. For example, ZrCo, has a Clfi-type structure for 1.9 < 3c< 3.0 [lo], and the excess cobalt atoms are supposed to occupy the 8a site with the zirconium atoms. Similarly, excess atoms in the PUN&-type hydride, SmCo2H4, are supposed to occupy the cobalt site. In a previous were discussed on the basis of the paper, the structures of YxZrl_,Co,, model that the structures consist of stacks of Laves-type A,B4 slabs and CaCus-type ABs slabs [S]. The PuNi, type of structure AB, is considered to be the alternate stacking [A,B,] [AB,], and excess atoms in the PuNistype hydride, SmCo,H4, are considered to be in the AB, slab as [SmzCo41 [ Sm( SmCo4)]. We consider that the hydrogen atoms occupy the interstitial sites forming tetrahedra around the samarium atoms. However, there is no experimental evidence to support this suggestion at present, although currently an analysis of the X-ray results is progressing.
References 1 2 3 4 5 6 7 8 9 10
D. G. Westlake, J. Less-Common Met., 90 (1983) 251. D. Shaltiel, J. Less-Common Met., 62 (1978) 407. K. I. Kohayashi and K. Kanematsu, J. Phys. Sot. Jpn., 55 (1986) 1336. I. R. Harris, R. C. Mansey and G. V. Reynor, J. Less-Common Met., 9 (1965) 270. J. Farrel and W. E. Wallace, J. Znorg. Chem., 5 (1966) 105. K. I. Kohayashi and K. Kanematsu, unpublished results. K. Itoh, T. Okagaki and K. Kanematsu, unpublished results. K. I. Kohayashi and K. Kanematsu, J. Phys. Sot. Jpn., 55 (1986) 4435. K. H. J. Buschow and N. M. Beekmans, Phys. Status Solidi, a, 60 (1979) 193. H. Fujii, F. Pourarian and W. E. Wallace, J. Magn. Magn. Mater., 24 (1981) 93.
of the Less-Common
Metals,
147
(1989)
9
- 18
FORMATION AND MAGNETIC PROPERTIES OF CRYSTALLINE AND AMORPHOUS SmCo, HYDRIDES
K. KANEMATSU, Physical
Science
T. SUGIYAMA, Laboratories,
M. SEKINE
Nihon
University
and T. OKAGAKI at Narashino,
Funabashi,
Chiba
(Japan)
K. I. KOBAYASHI Mitsubishi
Electric
Coorporation,
(Received
January
9, 1988
Sagami
Works, Sagamihara,
Kanagawa
(Japan)
; in revised form March 10, 1988)
summary Ferromagnetic C15-type compounds SmCo, are found for 1.74 < x < 2.04. The values of the magnetizations vary from 1.21 I.cs(SmCol.,,J1 to 1.34 ~s(SmCo,+.,-‘. The Curie temperatures are 230 K, independent of 3~. The initial hydrogenation of SmCo, leads to a C14-type hydride which contains about four hydrogen atoms per SmCo,. Below 373 K, the C14-type hydride changes into a PuNi,-type hydride having four hydrogen atoms per SmCo,. Above 453 K, the C14-type hydride transforms into an amorphous hydride having fewer than three hydrogen atoms per SmCo?, which in turn decomposes into SmH, and cobalt above 573 K. The Cl4-type and the PuNi,-type hydrides are considered to be paramagnetic. The amorphous hydride is ferromagnetic up to the decomposition with 2.2 I.cs(SmCoJ’.
1. Introduction Many Laves phase compounds AB, form a hydride and,the absorbed hydrogen atoms are considered to occupy the tetrahedral interstitial sites A2B2, AB, and Bq. The hydrogen atom site preference has been discussed, taking into account the spatial size of the site [l], or considering affinity of the hydrogen for the constituent metals A and B [2]. These models do not allow any contribution from the absorbed hydrogen in the construction of crystal, assuming that the hydrogen atoms are small. By contrast, it was suggested recently that the hydrogen atoms are in an H--like state and play an important part in the construction of the crystal, comparable with that of the metals A and B [ 31. According to this model, it is possible that the compounds change their structures as a result of the hydrogen absorption. The cubic Laves phase compound SmCo, is an example. The hydrogenation of SmCo, at room temperature induces a change from the Cl5 type to the Cl4 type of structure. Heating promotes a further structural change from 0022-5088/89/$3.50
@ Elsevier
Sequoia/Printed
in The Netherlands
10
the Cl4 type to the PuNis type with lapse of time. At higher temperature, the C14-type hydride transforms into an amorphous hydride, and further heating causes the amorphous hydride to decompose giving SmH, and cobalt. The purpose of the present paper is to report the process of the transformation of SmCoz induced by the hydrogenation. We include details of experimental results of X-ray and magnetic studies on the cubic Laves phase compounds SmCo, and their hydrides.
2. SmCo, 2.1. Experimental details Samples of SmCo, were prepared by arc melting stoichiometric mixtures of samarium (purity, 99.9%) and cobalt (purity, 99.6%) in an argon atmosphere. Arc metling was repeated several times. The samples were sealed in evacuated silica tubes and annealed at 1073 K for more than 360 ks. Crystal structure and lattice constants were determined by X-ray diffraction with Fe Ka! radiation. The magnetization measurements were performed using a vibrating sample magnetometer in a magnetic field of 12.5 kOe. 2.2. Crystal structure and magnetization The crystal structure of SmCo, was ascertained by X-ray diffraction to be of Cl5 type in the composition range 1.74 < x < 2.04 and no other line was found in the X-ray pattern. The lattice constants are about 7.25 A, almost independent of the composition x, and the value is very close to the value of SmCo*, a = 7.248 A, reported by Harris et al. [4]. The magnetizations at 0 K, extrapolated from the thermomagnetic curves, and the Curie temperatures are plotted as a function of x in Fig. 1. The magnetizations change from 1.21 ~z(SmCol.,J1 to 1.33 pz(SmCo,,,)-‘, but the Curie temperatures are about 230 K, independent of the composition. Magnetization measurements on SmCoz have been carried out
-r--l -730
SmCox a
.u.
0
00
-
000
m
1.25
-250
TC
= 5”
.
0
E
.
0.. 000
“0 7
_'
I
O: 0
-;; . -
I-’
200
0
lJ----J 1.7 I.8 I.8x 2.0 Fig. 1. Composition atures for SmCo,.
2.1
dependence
of lattice
constants,
magnetizations
and Curie
temper-
11
earlier by Farrel and Wallace [ 51, and their thermomagnetic curve shows that their SmCoz is a mixture of two ferromagnets with Curie temperatures about 230 K and higher than 280 K respectively. The magnetization of the ferromagnet with 230 K is estimated to be about 1.1 I.cB(SmCoz)-‘. As a representative example of SmCo,, the thermomagnetic curve of SmCo, is shown in Fig. 3. 2.3. Explanation of the magnetic properties based on molecular field model In SmCoz, 8a and 16d sites of the Cl5 structure are occupied by 8Sm and 16Co atoms respectively. Excess samarium atoms in SmCo, of x < 2 are supposed to occupy the 16d sites with cobalt atoms, and excess cobalt atoms in SmCo, of 2 < x are supposed to occupy the 8a sites with samarium atoms. According to molecular field model, the magnetism of SmCo, of x < 2 is due to a magnetization and an exchange parameter, i.e. a molecular field coefficient of a sublattice consisting of atoms at 16d sites. The magnetizations of the 16d sublattice calculated from the experimental results are 7.3 X lo3 - 7.4 X lo3 e.m.u. (ABJ’, nearly independent of the composition x. Values of the molecular field coefficient are also considered to be nearly independent of the composition x because the lattice constants are independent of the composition. These sublattice magnetizations and molecular field coefficients lead to Curie temperatures independent of the compositions which agree with the experimental results. There is no explanation of the composition-independent sublattice magnetization and lattice constants, and the explanation may be given by an energy band calculation.
3. Hydrides 3.1. Hydrogen absorption Hydrogen absorption for SmCo, was carried out in hydrogen pressures of 0.1 - 0.3 MPa at several temperatures, after degassing in vacuum at about 670 K. The amount of hydrogen absorbed was determined by gravimetry. The absorption isotherms of the SmCo,-H system, just after degassing, were taken at several temperatures. A C14-type hydride was formed as a first step of the hydrogenation and was obtained when the hydrogenation was performed below 373 K for reaction times less than about 40 ks. A PuNi,-type hydride was observed to occur together with the C14-type hydride when the sample was hydrogenated for about 1 Ms. An amorphous hydride appeared in the sample hydrogenated above 413 K, which decomposed into SmHz and cobalt above 673 K. 3.2. Cl4
type hydride
All hydrides of SmCo, prepared at room temperature have the Cl4 type of structure, and the amount of the absorbed hydrogen is about four atoms per SmCo,, independent of the composition x. The absorption iso-
12
therms of the SmCo,-H system at 313 K are shown in Fig. 2, as a representative example of the Cl6type hydride. The positions of the diffraction lines of this sample are given in Table 1; all the diffraction lines in the corresponding X-ray pattern are broadened. Indexing was made on the basis of the Cl4 type of structure, but the atomic arrangement is considered to be quite different from the arrangement of C14-type inter-metallic compounds because the intermetallic compounds have many lines characteristic of the Cl4 type other than those of the Cl4 type hydride and they do not have P
(MPI\
’
t
Fig. 2. Absorption isotherms of the SmCos-H system at 313 K, 373 K and 453 K. Samples: (313 K), C14-type hydride; (373 K), mixture of C14-type and PUN&type hydrides; (453 K), amorphous hydride. TABLE 1 X-ray pattern of SmCos and SmCozH4.e with Cl4 t ype of structure (Fe Ko radiation) SmCoz H4.0 (C14)
SmCoz (Cl 5) hkl
28
intensity
111
26.60
m
220 311 222 422 333 440 620 533
44.24 52.44 54.94 81.50 87.64 97.84 114.90 121.84
vs ws S
hkl
28
Intensity
003 110 112 004
37.3 40.7 48.2 50.5
m-vw
006 220
79.6 88.4
S
vs S
s S S
W W
m S
The diffraction lines of hydride are very broad. The intensity of the (003) line varies and disappears in a few samples.
13
the (0 0 3) line. The relative intensity of the (0 0 3) line is different in the various samples and a few samples do not have this line. No relation between the relative intensity and the preparation process can be given at present. The lattice constants are a = 5.60 A, c = 9.12 A, independent of SmCo, composition. They decrease with increasing hydrogenation temperature, reaching the values a = 5.50 A, c = 9.08 A for the sample hydrogenated at 413 K. The magnetic field dependence of the C14-type hydride shows this hydride to be paramagnet, a weak ferromagnetic contribution possibly being due to small amounts of decomposed free cobalt, as described later. The thermomagnetic curve of the sample (313 K, 1.06 MS) after the isothermal experiment at 313 K (Fig. 2) is shown as a representative example of the Cl4 type hydride in Fig. 3. 3.3. PuNi3-type hydride The PuNis-type hydride always coexists with the C14-type hydride. As an example, Table 2 shows the diffraction lines of the sample (373 K, 0.274 MS) after the isothermal absorption experiment performed at 373 K for 0.274 MS (Fig. 2). The relative intensities of the PuNis-type hydride are similar to those of YXZrl-XCoZ~Hr [6] and Y,Zr,_,FezgH, [7]. They are different from those of the PuNis-type compounds YxZr,_,Co,,, [8] and YxZr,_,Fe,, [7]. The PuNis-type hydride is supposed to contain about four hydrogen atoms per SmCo,, because the hydrogen content of the sample (373 K, 0.274 MS) is four atoms per SmCoz. The thermomagnetic curve of this sample is nearly the same as that of the (313 K, 1.06 MS) sample, and the PuNi,-type hydride on the magnetic properties is supposed to be similar to the C14-type hydride. 3.4. Amorphous
hydrides
The possibility of preparing amorphous hydride was discussed by Buschow and Beekmans [9]. These authors argued that the disappearance
0
100
200
300 TJKl 400
Fig. 3. Temperature dependence of the magnetizations of the hydrogenated SmCoT samples. Samples: (313 K, 1.06 MS), C14-type hydride; (413 K, 1.03 MS), mixture of C14-type and PuNis-type hydrides; (423 K, 74 ks), mixture of C14-type, PuNis-type and amorphous hydrides; (523 K, 8 ks), mixture of C14-type and amorphous hydrides; (453 K, 0.46 MS); amorphous hydride; (673 K, 0.24 MS), mixture of SmHz and cobalt.
14 TABLE 2 X-ray pattern of SmCozH4_0 (373 K, 0.274 MS) (Fe KCYradiation) Cl4 type PuNia type
Intensity
28 37.3 38.00 40.7 41.46 42.42 48.2 49.82 50.5 52.7 57.92 64.56 77.2 81.4
W
m m s m s s m
h
k
I
0
0
3
h
k
00
1 9,
10
7
11 0, 0 0 10, 0 0 11, 20 3
10 11
8 2
20 5, 11 9, 1 1 11 2 0 13, 30 6
1 0 11 20
7
110
1 1 2 0 0 4
W
VW m W
VW
30 2 0 14
3
The diffraction lines of the Cl4 type are broad and those of PuNia type are sharp. Lattice constants of the PuNis type hydride are a = 5.454 A, c = 26.74 A.
of sharp diffraction lines after hydrogen absorption is due to the formation of microcrystalline decomposition products rather than to the formation of amorphous hydride. However, the sample (453 K, 0.457 MS) was concluded to be amorphous, because the magnetization measurements shown in Fig. 4 reveal that the formation of the amorphous hydride and the de-
It
0
I
H/L
3
Fig. 4. Temperature dependence of magnetization. Increasing magnetization between 430 K and 520 K appearing in the sample (313 K, 1.06 MS) is due to a change from the Cl4 type to the amorphous state. Increasing magnetizations between 570 K and 620 K appearing in the samples (313 K, 1.06 MS) and (453 K, 0.46 MS) are due to decomposition of the amorphous hydride to SmHa and cobalt.
15
composition to SmHz and cobalt occur over definite temperature ranges. The X-ray pattern taken with Fe KCYradiation exhibits only a diffuse peak at 28 = 30” - 45“. The isotherms show that the amorphous hydride contains less than three hydrogen atoms per SmCoz (Fig. 2). The magnetization measured in 12.5 kOe is slightly temperature dependent and the value is 2.2 pn(SmCo,)-’ (Fig. 3). The Curie temperature was not determined because the sample decomposed at about 600 K. The amorphous hydride appears in the samples hydrogenated at 413 K and 523 K. This temperature range is nearly the same as the range in which the transformation from the Cl4 type to the amorphous state was found in the magnetization measurements (Fig. 4). The formation of the amorphous hydride depends also on the hydrogenation time. When hydrogenated at 523 K the sample prepared with a reaction time of 8.1 ks is a mixture of the Cl4 type and the amorphous hydrides, and the sample prepared for 70.2 ks is amorphous. 3.5. SmH2 and cobalt In the samples hydrogenated above 573 K, the X-ray patterns were analysed as a mixture of the lines indexed as f.c.c. structure with a = 3.573 A and a broad line at 28 = 56.5” (Table 3). The former lines agree well with those of SmH, reported as a = 3.563 a [6]. The line at 26 = 56.5” agrees with the (1 1 1) line of f.c.c. cobalt. The isotherms at 673 K in Fig. 5 shows TABLE X-ray
3 pattern
of SmCozHz.
2
36.32 42.24 56.5 61.28 73.32 77.22 92.3 101.1 103.52 107.44
1hydrided
at 573
K (Fe Kcu radiation)
In te nsi ty
SmHz hkl
WS
111 200
vs m
co hkl
111 220 311 222 400
S S W ww
220
ww
m m
331 420
22
y
:
~!?y&t*,, ,
300
400
500
Fig. 5. Absorption isotherms mixture of SmHz and cobalt.
(
600 T(K) 700
of SmCoz-H
system
at 573
K and 673
K. Samples
are a
16
that the hydrogen content is two atoms per SmCo;? supporting the formation of SmH,. The magnetization is nearly independent of temperature and the value is 3.4 ~a(SmCoJ1, which is nearly the same as the value of free cobalt decomposed from SmH,.
4. Structural change and decomposition of hydrides 4.1. Structural change from Cl4 type to amorphous state The sample (523 K, 8.1 ks) is a mixture of the C14-type and the amorphous hydrides, and the sample (523 K, 1.03 MS) is amorphous. The amorphous hydride is also found in the sample (413 K, 1.03 MS). Therefore the transformation from the Cl4 type to the amorphous state progresses with time in the temperature range between 413 and 523 K. This transformation was also ascertained using magnetization measurements. The thermomagnetic curve of the sample (313 K, 1.06 MS) shows two steps of increasing magnetization. The first step which starts near 430 K and finishes near 520 K is ascribed to the transformation from the Cl4 type to the amorphous state, as was ascertained by X-ray examination. 4.2. Structural change from Cl4 type to PuNi, type The C14-type hydride is formed as a first step of the hydrogenation, and the PuNi,-type hydride is formed after a lapse of time from the C14type hydride below 373 K. This was found in the samples (413 K, 1.03 MS) and (423 K, 74 ks) consisting of the C14-type and the amorphous hydrides. The critical temperature to separate the two types of transformations could not be determined. It is noteworthy that the structures of SmCo* occuring after hydrogenation are analogous to those of YxZr,_,Co,,, i.e. the C15, the four-layered hexagonal and the PuNis type obtained in the composition ranges 3t= 0, 0.1 < x < 0.3 and 0.3 < x respectively. The structure of the C14-type hydride may be the same as the four-layered hexagonal structure because the X-ray diffraction patterns of both structures are similar except for a few weak lines indicative of the four layered structure. The structural changes from the Cl5 type to the PuNi, type in YxZr,_xCo2.9 are considered to relate to the decreasing number of 4d electrons caused by substitution of the zirconium for the yttrium. The similarity between the structures of both systems suggests that the structural changes induced by hydrogenation are due to a decrease in the numbers of electrons at the metal sites which is probably caused by a transfer of electrons (less than one per SmCo*) from the metal atoms to the hydrogen atoms. This inference suggests that the hydrogen atoms accepting the small number of electrons change from a neutral valence to an H--like state and the hydrogen atoms in the PuNi&ype hydride accept more electrons than those in the Cl4-type hydride.
17
4.3. Decomposition of amorphous hydride to SmH, and cobalt The sample (453 K, 0.457 MS), a typical amorphous hydride, exhibits an increasing magnetization which starts near 570 K and finishes near 620 K (Fig. 4). The cooling curve is nearly the same as the sample (673 K, 0.27 MS), a mixture of SmH2 and cobalt. After magnetization measurements, the sample was ascertained to be a mixture of SmH2 and cobalt by X-ray examination. The same decomposition was also observed in the thermomagnetic curve of the sample (313 K, 1.06 MS) as the second step which starts near 570 K and finishes near 620 K (Fig. 4). The sample (413 K, 1.03 MS), a mixture of the C14-type, the PuNis-type and the amorphous hydrides, shows increasing magnetization which starts near 430 K and finishes near 650 K. The curves exhibit no distinct steps to indicate transformations from the Cl4 type to the amorphous state and from the amorphous state to SmHz and cobalt. Furthermore, the transformations seem to be affected by the presence of the PuNis-type hydride. 4.4. Dehydrogenation of SmH,-type and PuNi,-type hydrides The sample (673 K, 0.27 MS) shows a remarkable decrease in magnetization near 875 K, which is ascribed to a decrease in the free cobalt due to the formation of Sm-Co compounds with samarium obtained from dehydrogenated SmH2. According to X-ray examination, the sample obtained after the measurement contains SmCo*, SmCo, and other unidentified compounds in addition to small amounts of SmH, and cobalt. The PuNi,-type hydride seems to release hydrogen above 640 K. The sample (423 K, 74 ks), a mixture of the PuNi,-type and the amorphous hydrides, shows increasing magnetization between 430 K and 640 K (Fig. 6). According to X-ray examination, the sample corresponding to maximum magnetization at 640 K contains the PuNis-type hydride together with SmH, and cobalt, and the sample obtained after the measurement up to 900 K is a mixture of SmH2, cobalt, SmCo*, SmCo, and other unidentified
I 0
I
I
200
I
,
400
I
I
,
1
600 TiKl800
Fig. 6. Temperature dependence of magnetization. Decreasing magnetization above 875 K appearing in the sample (673 K, 0.24 MS) is due to the formation of Sm-Co compounds from cobalt and samarium dehydrogenated SmHz. Increasing magnetization above 430 K in the sample (423 K, 74 ks) is due to the transformation from the C14-type hydride to SmHz and cobalt, and the decreasing magnetization above 640 K is due to the formation of Sm-Co compounds from cobalt and samarium formed by pure hydrogenation of SmHz and the dehydrogenation of the PuNis-type hydride.
18
compounds. In this sample, the simultaneous presence of the three hydrides seems to affect the transformation, the decomposition and the dehydrogenation. 4.5. Positions of metal atoms in the PuNi,-type hydride Generally intermetallic compounds have a wide homogeneous range of composition. For example, ZrCo, has a Clfi-type structure for 1.9 < 3c< 3.0 [lo], and the excess cobalt atoms are supposed to occupy the 8a site with the zirconium atoms. Similarly, excess atoms in the PUN&-type hydride, SmCo2H4, are supposed to occupy the cobalt site. In a previous were discussed on the basis of the paper, the structures of YxZrl_,Co,, model that the structures consist of stacks of Laves-type A,B4 slabs and CaCus-type ABs slabs [S]. The PuNi, type of structure AB, is considered to be the alternate stacking [A,B,] [AB,], and excess atoms in the PuNistype hydride, SmCo,H4, are considered to be in the AB, slab as [SmzCo41 [ Sm( SmCo4)]. We consider that the hydrogen atoms occupy the interstitial sites forming tetrahedra around the samarium atoms. However, there is no experimental evidence to support this suggestion at present, although currently an analysis of the X-ray results is progressing.
References 1 2 3 4 5 6 7 8 9 10
D. G. Westlake, J. Less-Common Met., 90 (1983) 251. D. Shaltiel, J. Less-Common Met., 62 (1978) 407. K. I. Kohayashi and K. Kanematsu, J. Phys. Sot. Jpn., 55 (1986) 1336. I. R. Harris, R. C. Mansey and G. V. Reynor, J. Less-Common Met., 9 (1965) 270. J. Farrel and W. E. Wallace, J. Znorg. Chem., 5 (1966) 105. K. I. Kohayashi and K. Kanematsu, unpublished results. K. Itoh, T. Okagaki and K. Kanematsu, unpublished results. K. I. Kohayashi and K. Kanematsu, J. Phys. Sot. Jpn., 55 (1986) 4435. K. H. J. Buschow and N. M. Beekmans, Phys. Status Solidi, a, 60 (1979) 193. H. Fujii, F. Pourarian and W. E. Wallace, J. Magn. Magn. Mater., 24 (1981) 93.