This section is concerned mainly with crystallographic and magnetic properties. In contrast to the vast amount of experimental work done on these properties, relatively little has been published on other properties such as transport properties. The few examples of specific heat measurements are not included in this section as they have already been discussed at more relevant places elsewhere in this review (see section 4.3). Since the N M R investigations performed on the ternary hydride dealt mainly with the motion of the H atoms, these properties were discussed in the section on diffusion (section 3.6).
The proliferation of research work on the magnetic properties of intermetallics and their ternary hydrides is primarily due to the interest in the fundamental physics aspects. Magnetism in metallic systems is not yet completely understood. Various theories exist, with often conflicting models. The possibility of changing the local conduction electron concentrations by the incorporation of hydrogen atoms is of relevance to many of these theories.
There is also interest from the technological side, since upon absorption of hydrogen a compound may change from ferromagnetic to paramagnetic or vice versa. This would imply an additional contribution to the sorption entropy. It follows from the discussion in section 4.2 that the entropy effect in a hydrogen sorption reaction is mainly determined by the hydrogenated material. In several applications involving a combination of two different metal-hydrogen systems (see section 6) it can be of advantage if not only reaction enthalpies but also reaction entropies can be freely chosen. The possibility to vary AS by using magnetic effects would therefore provide a welcome degree of freedom. More trivially, one could mention that a knowledge of the changes in magnetic properties upon charging would make it possible to obtain information on the hydride degradation by means of magnetic monitoring. It was mentioned earlier (section 4.4) that ternary hydrides are metas- table. Compounds RMùH2m tend to decompose into a hydride RHm, M metal and
48 K.H.J. BUSCHOW
hydrogen gas. This magnetic monitoring could rather easily be performed, in particular, if the uncharged and charged compounds were non-magnetic or only weakly magnetic and the free M metal were ferromagnetic.
To deal with the fairly complex aspect of magnetic property changes, the section on magnetism is organized as follows.
In order to distinguish between changes in 3d electron magnetism and changes brought about by a modified coupling involving the localized 4f moments, we will first discuss results obtained upon H2 absorption in intermetallics consisting of 3d elements and mainly those rare earth elements that are non-magnetic. These include the elements La, Y, Lu or Sc. In passing we will also discuss results obtained on compounds of 3d metals with Tl, Zr, H f or Th. In a separate section we will discuss the changes observed in compounds where the rare earth element also carries a magnetic moment. Finally we will give an evaluation of the experimental results and discuss the conclusions that can be derived from them regarding the bonding of the H atoms in the ternary hydrides.
5.1. Crystal structures
As far as the lattice of metal atoms is concerned one can say that the structural changes accompanying the hydrogen sorption are of minor importance. In most cases the symmetry of the crystal lattice is preserved and the effect of the hydrogen is merely to produce a lattice expansion. Incidentally the H2 absorption is accompanied by a small distortion which lowers the symmetry (Kuijpers, 1973; Viccaro et al., 1979b; Cho et al., 1982; Andreev et al., 1982). The X-ray diagrams of some of the compounds investigated did not contain diffraction peaks after H2 absorption. This shows that long-range lattice periodicity has been lost. The loss in long-range atomic ordering is closely connected with the fact that the ternary hydrides are metastable under certain conditions of H2 pressure and temperature and tend to decompose, a point already discussed in section 4. Further evidence in support of this decom- position follows from magnetic measurements (see section 5.2).
Owing to the small atomic scattering power of the H atoms no conclusive information on the crystallographic position of these atoms in the hydrides can be derived from results of X-ray and neutron diffraction. For a structure determination involving both the metal atoms and the H atoms one therefore has to rely on neutron diffraction studies made on the deuterium equivalents of the ternary hydrides. Until now only relatively few such studies have been undertaken. Kuijpers (1973) and Kuijpers and Loopstra (1974) investigated several hydrides of the RCo5 family and found that hydrogen absorption leads to a small orthorhombic distortion. The crystal structure of LaNi»D6 has been studied by several investigators (Bowman et al., 1973; Fischer et al., 1977; Andresen, 1978; Furrer et al., 1978; Burnasheva et al., 1978, Percheron-Guégan et al., 1980; Noréus et al., 1983). With respect to the original LaNi5 unit cell, hydriding has led to a volume increase of about 25~. In several investigations the symmetry was reported to have decreased from P6/mm to P31m (see table 5).
TABLE 5
Comparison of the refinement results of the LaNi» deuteride structure in the two-site model and the five-site model (after Achard et al., 1981). The values of [(sin 0)/2]mùx correspond to the last reflection used
in the refinement.
Deuteride Occupancy N Number of [(sin 0)/2]max Reliability Ref.
reflections (~ - 1) R-factor (~) 3c sites 6d sites
LaNisD6. 8 3 3.8 19 0.44 qualitative 1
LaNisD 6 2.86 3.14 21 0.39 5.4 2
LaNisD6. 4 3 3.4 - - 5.4 2
LaNisD». 8 3 2.8 - 0.52 6.4 3
LaNisD». 6 3 2.6 not known not known 4 4
LaNisD 6 3 3 23 0.44 12 5
LaNi»D6. s 3 3.5 - 0.72 16 6
LaNi»Ds. 5 3 2.5 92 - 8.3 7
3f 4h 6m 12n 12o
LaNisD6 » 0.64 0.52 1.91 2.14 1.29 43 0.55 7.3 6
1. Bowman et al. (1973). 3. Furrer et al. (1978). 5. Burnasheva et al. (1978). 7. Noréus et al. (1983).
2. Fischer et al. (1977). 4. Andresen (1978). 6. Percheron-Guégan (1980).
The results o f the refinements o f the L a N i s D 6 structure obtained in the various investigations are s u m m a r i z e d in table 5. It is seen that the n u m b e r o f reflections considered in the investigation by P e r c h e r o n - G u é g a n et al. a n d N o r é u s et al. (1983) is relatively large a n d adds weight to the low value o f the reliability factor R f o u n d in these studies. It can also be seen f r o m the table that the n u m b e r o f hole sites is five in the structure p r o p o s e d by P e r c h e r o n - G u é g a n et al., while it is only two in the other structures. This latter fact has led Wallace et al. (1981) to cast d o u b t on the validity o f this structure determination, arguing that the configurational e n t r o p y S c o r r e s p o n d i n g to this structure w o u l d be incompatible with the results o f t h e r m o d y n a m i c measurements. T h e value o f Sc calculated for the five-site structure by Wallace et al, on the basis o f eq. (15) given in section 4.2 equals 2 1 . 2 J K -1 ( g a t o m ) -1. This value is m u c h higher t h a n Sc = 4 . 3 J K -1 ( g a t o m ) -1 derived by O h l e n d o r f a n d F l o t o w (1980b) f r o m an analysis o f their specific heat d a t a based on eq, (15). It was s h o w n subsequently by A r c h a r d et al. (1981) that the value Sc = 21.2 J K ~ (g a t o m ) - ~ calculated by Wallace et al. is far f r o m realistic. Consid- erably lower values are obtained if a c c o u n t is taken o f the fact that at a given time the o c c u p a t i o n o f orte deuterium site prevents the o c c u p a t i o n o f sites, closer t h a n the m i n i m u m H - H separation (see also below).
The structure o f LaNi4.sA10.sD4.5 was investigated by C r o w d e r et al. (1982).
The distribution o f C o a t o m s in LaNi4Co a n d the structure o f the c o r r e s p o n d i n g deuteride was investigated by Gurewitz et al. (1983).
Closely related to the CaCu»-type structure o f L a N i 5 are the crystal structures o f L a N i 3 (CeNi3-type) and H o N i 3 (PuNi3), the deuterides o f which were investigated by
50 K.H.J. BUSCHOW TABLE 6
Interstitial hole sites in RöMn23 compounds. In the Th6Mn23 structure type there are four different Mn sites, indicated by Mn(i); i = 1, 2, 3, 4
corresponds to the sites 4b, 24d, 32f~ and 32f z.
Wyckoff Polyhedron Corner
symbol type atoms
4a octahedron 6 R
32f 3 tetrahedron 3 R + 1 Mn(4)
96k tetrahedron 2 R + 1 Mn(4) + 1 Mn(2)
48i octahedron 2 R + 1 Mn(l) + 1 Mn(2) + 2 Mn(3)
means of neutron diffraction by Andresen et al. (1978) and Solov'ev et al. (1981), respectively. The results of the structure determination of HoNi3D1.8 obtained by Solov'ev et al. are reproduced in fig. 27. The deuterium atoms are accommodated in two types of interstitial holes (indicated close to the basal plane of the unit cell).
The major part of the D atoms occupies the (deformed) tetrahedral hole sites of the type 18h (11.47 D atoms per unit cell
HogNi27 ).
These hole sites are surrounded by two Ho atoms and two Ni atoms. The remaining D atoms (4.72 D atoms perB
Z×
Fig. 27. Schematic illustration of the crystal structure of HoNi3DL8 as proposed by Sotov'ev et ak (1981).
The large filled circles represent the Ho positions, the medium-sized open circles the Ni positions, and the smaller filled ¢ircles the deuterium positions. The two types of tetrahedral holes corresponding to the deuterium positions are indicated in the lower part of the figure.
Ho9Ni27 ) are located at tetrahedral hole sites of the type 6c, surrounded by one Ho atom and three Ni atoms.
Neutron diffraction studies on R6Mn23Dx were made by Commandré et al. (1979) for R = Y and by Hardman et al. (1980) for R = Th. The energetically interstitial sites in the cubic ThöMn23 structure are summarized in table 6 (Jacob, 1981). Included in this table are the number and kinds of metal atoms occurring at the corners of the polyhedrons. It can be derived from the results of Commandré et al. (1979) on Y6Mn23D8.3 that the hole sites 4a and 32f3 are preferentially occupied (0.6 and 7.7 D atoms, respectively). Results obtained on Th6Mnz3DI6 (Hardman et al., 1980) show that further deuteration had resulted in a preferential occupation of the 48i site. Only at concentrations as high as Y6Mn23De3 is there a (partial) occupation of the 96k site.
Jacob (1981) analysed the interstitial hole occupancies in the above materials by means of the semi-empirical model proposed by Jacob et al. (1980a) and mentioned in section 4.3. From top to bottom the hole sites in table 6 correspond to lower absolute values of the hole site enthalpies. The preferential occupancy of the 48i site, occurring before accommodation of D atoms in the 96k site takes place, is rather unexpected. Jacob et al. explained this discrepancy by noting that there is only a rather short separation ( ~ 1.6 A) between the already filled 32t"3 site and the empty 96k site. The strong electrostatic repulsion between the H atoms raises the 96k hole site enthalpy and makes this site less attractive to occupation by D atoms, Jacob et al. also suggested that the electrostatic repulsion between the H atoms is sensitive to small changes in the relative H - H interatomic distance between 96k and 32t"3. This would mean that in the light rare earth compounds, which have relatively large lattice constants (see fig. 28), the repulsion is less than in the case ofcompounds of the heavy rare earths. Consequently, the former are expected to have a somewhat higher absorption capacity than the latter. This would be in agreement with the fact that the change in lattice constant upon charging decreases in going from left to right in fig. 28.
Apart from the results mentioned above, structural information regarding the H positions in ternary rare earth base hydrides is still lacking. Nevertheless it is interesting to mention some results obtained on other ternary hydrides. Neutron diffraction experiments on intermetallics charged with deuterium comprise TiFe (Fruchart et al., 1980a; Thompson et al., 1979; Fischer et al., 1978b), ZrV2 (Didisheim et al., 1979a, 1981; Fruchart et al., 1980b), ZrMn: (Didisheim et al., 1979b) and HfV2 (Irodova et al., 1981). These experiments, too, support the notion that once a hydrogen atom is absorbed, other similar positions are not allowed to become occupied when they are present at too short a distance. Didisheim et al.
(1981) investigated ZrNi2H3.6 and showed that the hydrogen atoms are distributed in a disordered way at high temperatures, although there is a minimum distance associated with the H - H interseparation. From the diffuse background peak in the neutron scattering patterns it was derived that this minimum distance is equal to about 2.2 ~. In the hydride of TiFe there are many more sites than H atoms and the results derived from neutron diffraction measurements are still under discussion.
Leaving this hydride out of consideration, one could say that the experimentally determined site occupancies agree quite weil with the concept that the driving force
52 K.H.J. BUSCHOW 13.5
• ~ 13.0 E O
c- O
8 12.5
"5
J
R 6 Nn23Hx
O O
R6 Mn23
0 O
O
12.0 I I I I I I I I I I I I l
Ce Nd Sm Gd Dy H Er Yb
La Pr Pm Eu Tb o Tm Lu
Fig. 28. Lattice constants as a function of the R component in various RöMn23 compounds (lower part) and the corresponding hydrides (upper part).
underlying ternary hydride formation is the creation of an optimal area of contact between the H atoms and the atoms of the strongly hydrogen-attracting component (van Mal et al., 1974; Jacob et al., 1980a).
We conclude this section on crystal structures by mentioning that there are also cases where the absorption of hydrogen has resulted in well-defined structural changes of the lattice of metal atoms as well. Some results have been collected in table 7. It can be seen that the structure changes can lead to a lowering as well as to an increase of the metal atom lattice symmetry.
TABLE 7
Changes in crystal structure observed in some intermetallic com- pounds.
Compound Structure Hydride Structure Ref.
ZrCo CsCl(c) ZrCoHz6 CrB(o.r.) 1,2
HfCo CsCl(c) HfCoH3. 2 CrB(o.r.) 2
EuPd CrB(o.r.) EuPdH 3 CsCl(c) 3
YbNi FeB(o.r.) YbNiHx CsCl(c) 4
Mg2Ni MgNi2(h ) Mg2NiD 4 fcc 5, 6
GdCu2 CeCu2(o.r.) GdCu2Hx MoSi2(tetr.) 7 1. Irvine and Harris (1978).
2. Van Essen and Buschow (1979).
3. Buschow et al. (1977).
4. Buschow (unpublished results).
5. Gavra et al. (1979).
6. Schefer et al. (1980).
7. De Graaf et al. (1982b).
5.2. Magnetic properties 5.2.1. Ni compounds
As mentioned in section 4.4, the ternary hydrides are actually metastable with respect to the binary rare earth hydrides and the pure 3d metals (or a more 3d-rich intermetallic compound). Partial or complete decomposition of the ternary hydrides during the sorption reaction can therefore lead to reaction products that are no longer single phase. This would seriously hamper the investigation of the magnetic properties of the ternary hydrides. Difficulties of this kind have been encountered, for instance, in charging the Pauli paramagnetic compound La7Ni3, which was found to decompose into LaH3 and LaNi» (Busch et al., 1978b). Magnetically more revealing information was obtained from charging experiments with the Pauli paramagnetic compound Th7Ni » isotypic with La7Ni3 . Malik et al. (1980) found a decrease of the susceptibility upon charging. They interpreted their results as indicating that charging is accompanied by a decrease of the density of states.
Evidently decomposition of the ternary hydride was avoided here or was of minor importance. This would be the case if the reaction products of the decomposition were of the same type as in La7Ni3 (i.e. the binary hydride of Th and ThNi»). Tiny amounts of the decomposition products are likely t o remain unnoticed in the magnetic measurements, since ThNi5 is Pauli paramagnetic and has a magnetic susceptibility equal to 1.9 x 10-3 emu/mole (Elemans et al., 1975), which is of the same order of magnitude as that of Th7Ni 3 and its hydride.
Charging of the paramagnetic compound LaNi» usually leads to the presence of increasing amounts of Ni as a ferromagnetic impurity phase after repeated cycling (Siegmann et al., 1978). Busch and Schlapbach (1978) were able to show, however, that the magnetic susceptibility had decreased after each charging operation, indicating a smaller susceptibility in LaNisH6. This was explained in terms of a decreasing density of states (Schlapbach, 1980). A decrease of the susceptibility was also reported by Palleau and Chouteau (1980). Experimental results of these authors are reproduced in fig. 29. Palleau and Chouteau furthermore studied the effect of Ni
i I I
LaNisH x fresh sample
b X
I I i
0 2.5 5 7.5
Fig. 29. Dependence of the magnetic susceptibility on x in LaNi»H x after continued charging of LaNi»
(after Palleau and Chouteau, 1980).
54 K.H.J. BUSCHOW
precipitation after cycling and question whether this precipitation arises as a consequence of the absorption reaction. According to their experimental results the Ni precipitation would occur during the dehydriding rather than during hydriding.
Finally a decreasing intrinsic magnetic susceptibility in LaNi» was also proposed by Walsh et al. (1976) on the basis of EPR measurements on Gd-doped LaNisH 6.
More obvious changes in rnagnetic properties upon charging with hydrogen gas occur in YNi3. As already mentioned in section 2, Gignoux et al. reported YNi3 to be a weak itinerant ferromagnet. Similar results were also obtained by Buschow and van Essen (1979). These latter authors furthermore showed that the ferromagnetism disappears upon absorption of hydrogen gas. This was interpreted as being the result of a decrease in the density of states or a weakening of the intra-atomic Coulomb repulsion. A different type of change in magnetic properties was found in CeNi3.
Owing to the tetravalent character of Ce in this compound, the Ce atoms do not have a magnetic moment and also the 3d electron magnetism resurgence is not present here. Hydrogen uptake in CeNi3 leads to a change in valence of Ce from 4 + to 3 +.
This follows from the observation of a Curie-Weiss-type temperature dependence of the reciprocal susceptibilitY in the hydride with an effective moment appropriate to Ce 3 + (Buschow, 1980b). Further results reflecting the changes in magnetic properties of Ni-base intermetallics after charging with hydrogen are given in table A6.
5.2.2. Co compounds
The compound Y C o 2 is a strongly exchange-enhanced Pauli paramagnet. The absorption of H2 has been reported to lead to an increase of the magnetic susceptibility (Buschow, 1977b), which was ascribed to atomic disordering. The occurrence of atomic disordering upon charging has already been discussed in section 4.4. It arises as a consequence of the metastable nature of the ternary hydrides.
Support for this interpretation was obtained from 57Fe Mössbauer spectroscopy performed on YCo 2 doped with a small amount of enriched Fe (Buschow and van der Kraan, 1983). Hydrogen uptake was found to lead to small clusters of Co atoms too small to be observable by standard X-ray diffraction. As can be seen from the results shown in fig. 30, a large hyperfine field is present on the Fe atoms owing to magnetic ordering within the Co clusters. The mutual magnetic coupling of the Co clusters is weak, so that their presence does not contribute much to the bulk magnetization. The formation of small clusters of free Co resulting from a partial decomposition of the ternary hydride after charging was also studied in detail in PrCo2Hx (de Jongh et al., 1981). The temperature dependence of the zero field a.c.
susceptibility gives rise to a maximum, reminiscent of a spin glass or of a magnetic glass. The shift of this maximum to higher temperatures when the a.c. frequency is increased (see fig. 31) was interpreted in terms of the Néel model for super- paramagnetic particles with randomly oriented local anisotropy axes. The amount of Co present as clusters was estimated to be as high as 70~, although it remains almost unnoticed in the temperature dependence of the magnetization (see also fig.
43 in section 5.2.5).
In this connection it is interesting to mention the results of Malik et al. (1980), who studied the effect of hydrogen absorption on the magnetic properties of Th7Co 3. This
1.56-
8 1.50-
o
Œ
I I FI l
Y Cot9 5 Feoo5 H x
I I I I I
-6 -4 -2
I I $ t I I I
T=3OOK
t
I
I I I [ I I
0 2 4 6
Doppler Veloeity (mm.s -I)
Fig. 30. 57Fe Mössbauer effect spectra o f YCo 2 (doped with enriched Fe) after charging with hydrogen gas (after Buschow and r a n der Kraan, 1983).
1.8 1.8
o 9=?..32 Hz
1.6 b V=117 Hz ~ " ~ ' ~ Pr Co2 H, 1.6
=
1.4 1.4
1.2 1.2
~1.o ~.o~
t °
× 0.6 0.6
0.4 0.4
0.2 0.2
i
00 10 2'0 30 40 50
T1K)
Fig. 31. Real (Z') and imaginary (Z") parts of the complex a.c. susceptibility Z = Z' - iz" as measured with various frequencies in PrCo2H 4 as a function of temperature (after de Jong et al., 1981).
compound is isotypic with Th7Ni 3 and LaTNi3 mentioned in the preceding section.
Their results indicate an increase in susceptibility upon H2 absorption. This was ascribed to a decreasing number of 3d etectrons owing to charge transfer from Co to H. In this case, too, one cannot exclude the possibility that this increase of the susceptibility is not an intrinsic effect but results from traces of decomposition products not detectable by standard X-ray diffraction. F r o m the results mentioned in the preceding section and from the results of Schlapbach et al. (1982) obtained on ThTFe 3 it follows that some decomposition is likely to occur, so that an increase