32 K.H.J. BUSCHOW
T(°C) 400 300 200 150 100
100 I ' , I I
50
E
5
gl CI. 5
¢-
O
1.0
"1-
0.5
0.1
La NJ4.ITA[o3H~3 M Ni4.I5 A[o~~
Mg2Ni Hz, ~ "i
Gd Co3 H2 g---b.~
I I I I I I I I I q 215 ' ' ' '
1.5 2.0
50 25 0 -2C
, . . . . 10
,» .co..
s ~
o ~ p
c-
o 0.1 >-
I
~ N ~ FegMnlTi H 0.05
r'~ I I I I I I I I 0.01
3.0 315 4.0
1000IT (K -1 )
Fig. 19. Van 't Hoff type plots, describing the temperature dependence of the hydrogen desorption pressures in various materials. (Data are from Huston and Sandrock, 1980; Goudy et al., 1978; Kierstead,
1981a).
from the intercept with the vertical axis one can obtain the corresponding value of AS. Using the data of fig. 18 such a plot is shown for LaNi4.7A10.3H3 in the middle part of fig. 19. (The fact that the isotherms are not strictly horizontal in the plateau region was already discussed in section 3.3. The plot given in fig. 19 corresponds to the middle part of the plateau region.) The AH and AS values that can be derived for LaNi4.7A10.3H3 from this plot are 33.9 kJ/mole and 120.1 J / K m o l e H» In some cases it is not justified to identify the AH and AS values obtained by means of the Arrhenius plot as the formation enthalpy and formation entropy of the ternary hydride ABùH2m. In most systems there is not only a substantial solubility of hydrogen in the intermetallic compound, but also an appreciable deviation of the hydride phase from the stoichiometric composition. It is therefore more appropriate to look upon AH and AS as the enthalpy and entropy changes associated with the reaction « + x H 2 ~ / 3 (x < m). There is yet another complication. I t can be seen from fig. 9 (and to some extent also from fig. 18) that close to the critical temperature in particular, the composition both of the « phase and of the /3 phase varies considerably with temperature. This means that plateau pressures measured at different temperatures do not necessarily correspond to the same reactants and reaction products. Flanagan (1978b) has presented arguments showing that, in spite of this latter feature, conditions may be such that the linear relationship between lnpH ~ and 1IT can still be expected to hold. Values of AH and AS obtained by means of a linear lnpH2 versus 1/T plot at temperatures where the compositional changes of the c~ and /3 phases are quite severe therefore have to be interpreted with some caution. This is particularly true if the compositional dependence of the curve that
34 K.H.J. BUSCHOW
defines the two-phase region in fig. 9 is less symmetric about the critical composition (Flanagan 1978b).
Kierstead (1980a-d, i981 a, b) has had considerable success in fitting multiplateau hydrogen absorption isotherms in terms of the Lacher (1973) and Rees (1954) models. The phase separation corresponding to a given plateau pressure is taken to be governed by an attractive interaction between the hydrogen atoms. In order to describe more than one plateau orte has to postulate the presence of various groups of sites (i) having different heats (AHi) and entropies (ASi) of absorption. If there is a number n~ of sites of type i the corresponding hydrogen plateau pressure is determined by the four parameters n» AHg, ASi and T» where T~ represents the critical temperature (of heat of interaction H; = RT~) corresponding to these sites. In the modified Lacher model the various groups of hydrogen absorption sites are occupied independently and their presence persists after multiple phase changes and lattice expansion. In the modified Rees model there is a specific type of interdependence between the sites, since hydrogen absorption on a site of the type i produces new sites dissimilar from i owing to the presence of a H atom on the neighbouring type i site.
An example of the data analysis based on the Rees model is shown in fig. 20 for DyCo3Hx. Four different types of sites were assumed. Isotherms were calculated for five different temperatures (full lines in fig. 20). The total number of parameters used in this fitting procedure equals 16 (i = 4). Later Kierstead (1981c) was able to obtain even better fits by introducing entropies of interaction (T,. = H; - TS;) as well as temperature dependent enthalpies AH~(T) and entropies ASi(T) associated with the change in constant-pressure heat capacity (A C~). The number of parameters required for fitting the isotherm then equals 6m (n~, AH °, AS °, AC °, H~ and S~; i = 1 .. . . , m).
Here we wish to stress that the model used by Kierstead is useful for analysing measured data but it is not able to make any a priori predictions of the hydrogen absorption tendency in the various intermetallic compounds.
10 4
10 3
10 2 t o
~-101
1 0 °
T = 8 0 ° C ~ T = 60 % \ T =z,0°C\~ "~
B
:2oo%\ ~,~e--+-+--+-+ -+
, ~ /
1 0 110 I [ F
.v 2.0 3.0 4.0
H/Dy Co3
Fig. 20. Isotherms of DyCo3H « fitted by means of the Rees model (after Kierstead, 1981c).
To end this section it should be mentioned that the study of pressure-composition isotherms is not the only method that can be used to determine the values of A H and A S associated with the hydrogen sorption process. These values can also be derived from direct calorimetric measurements (see for instance Ohlendorf and Flotow, 1980a; Murray et al., 1981a, b; Wemple and Northrup, 1975; Mikheeva et al., 1978) and from magnetic measurements (Yamaguchi et al. 1982).
4.2. Experimental values o f A H and A S
In tables A1 to A5 in the appendix some experimental hydrogen sorption characteristics for a number of intermetallic compounds are listed. Inspection of the A H and A S values given in the tables shows that the former values vary considerably, whereas there are only relatively small differences between the latter. This feature is also apparent from the results shown in fig. 19. There is a considerable difference in the slope of the lines, whereas extrapolation of the lines leads to about the same intercept on the left-hand vertical axis. The corresponding values of A S and the various values of A S given in the tables are not far oft from the entropy of" hydrogen gas ( A S gas) (about 130J/KmoleH2 at 1 atm and room temperature). The near constancy of the A S values reflects the fact that in all the hydrogenation reactions the entropy of the hydrogen as a gas is lost upon entering the metal. Compared to this large entropy term other entropy effects, arising in the solid, can be shown to be relatively unimportant in most cases. If one neglects the solubility of hydrogen in the starting material the excess entropy A S ex in the ternary hydride can be expressed as
ASeX = AS~Ib + A K'vib ~ ~,host + Asel + ASconf • (14)
The contribution A S ~ b represents the vibrational entropy of the hydrogen atoms in the ternary hydride, z JA K~vib~.~host represents the entropy associated with the modifications of the the vibrational spectrum of the host compound. The contribution A S e~ is the entropy effect due to the changes in the electronic heat capacity upon charging, while A S c°nf represents the configurational entropy, originating from the statistical distri- bution of the hydrogen atoms over the more abundant interstitial sites in the hydride.
By means of the same arguments presented elsewhere (Buschow et al., 1982a) it can be shown that the first three terms of eq. (14) are relatively small: The vibrational spectrum of the H atoms in a metallic hydride can be treated approximately as an assembly of Einstein oscillators. Experimental results obtained on binary and ternary hydrides so far suggest that the characteristic Einstein temperature (0E) is relatively high. This follows for instance from neutron scattering results obtained by Rush et al. (1980) in the hydride of TiCu (fig. 21) using the relation kOE = hWH. It also follows from the specific heat data obtained by Wenzl and Pietz (1980) in various hydrides based on TiFe. A high value of 0 E is consistent with the fact that the mass of the hydrogen atoms is relatively low. With values of 0E in the fange 1000-1500 K one expects the vibrational heat capacity to rise steeply at temperatures between 300 K and 500 K, approaching the value 3R per mole of H atoms. Consequently, the heat capacity of H 2 gas and that of hydrogen in the hydrides will cross at a temperature
36 K.H.J. BUSCHOW
2
Ti CuH
0 40 200
i' I'
j~
/ \ , \
-7 ~''¢~
/
813 120 1;0
hw [meV)
Fig. 21. Neutron scattering spectra measured at 78 K for TiCuH~~. Data are from Rush et al. (1980).
not much exceeding room temperature (T/O E ~ 0.4), after which the change o f AS gas with temperature will be smaller than that of AS~I b. In any case the t e r m AS~I b will be relatively small compared to A S ga~ at room temperature. Even in the case of Pd hydride, where 0E = 800 K, the contribution is only of the order of 10 J/K mole H2 at 300 K (Boureau et al., 1979). In LaNi»H6.4 a value equal to 2 J/K mole H 2 w a s
estimated (Ohlendorf and Flotow, 1980b). The two t e r m s ASbest ~ and A S el are even smaller. In Pd hydride Boureau et al. obtained the values + 2 J / K mole H 2 and - 5 J/K mol H» The corresponding values estimated by Ohlendorf and Flotow (1980b) in LaNisH6.4 are 9.2 J/K mole H 2 and - 0.3 J/K mole H2, respectively.
The configurational entropy can be derived by means o f the relation ASt°C= k In W, where W is the number of complexions associated with the distribution of the number of hydrogen atoms over the available crystallographic sites. Application of Stirling's approximation leads to
z ~ S c ° n f ~ - - - k N ~ G~[Oi I n 0 i ~- ( 1 - Oi) ln(1 - Oi)],
i (15)
where 0i is the fractional occupancy of site i and Gi is the site multiplicity. If neutron diffraction data of the ternary deuteride of a given intermetallic compound are available the experimental values of 0i and G~ can be used to calculate A S c°nr by means of eq. (15). In LaNisH 6 values of AS °°nr equal to 20 J/(K g at) have been estimated.
However, we will return to this point later on, after having discussed experimental data on the crystal structure of ternary hydrides in more detail. In any case, even though A S c°nf is the largest contribution in eq. (14), the corresponding values are small compared with the value of AS gas.
4.3. Empirical relations for estimating AH of the hydrides of intermetallic compounds Given the fact that hydrogen absorption leads to rather similar values of the associated entropy change in the various intermetallics, it follows from the van 't Hoff relationship (eq. 13) that the equilibrium pressure would be predictable if AH were known. It is clear that such formation enthalpies can be derived from ab initio band structure calculations. For CsCl-type compounds like TiFeH 2 and TiPdH2 such calculations were made by Gelatt (1978), who obtained calculated AH values of the right order of magnitude. Such calculations are, however, rather cumbersome and lengthy in cases where the crystallographic unit cell is composed of many different inequivalent metal atom positions. For this reason the prediction of AH values and equilibrium pressures based on band structure calculations cannot be regarded as a universal method applicable to a large variety of intermetallics.
Methods in which the enthalpies were correlated with other physical quantities have proved to be more suitable for predicting the hydrogen absorption behaviour of intermetallics. In the empirical model described by van Mal et al. (1974) and Miedema et al. (1977) the enthalpy of formation of the ternary hydride RMH2m is expressed in terms of the formation enthalpy of the binary hydrides of the metals R and M and the formation enthalpy of the uncharged intermetallic compound RMn.
This model is an extension of Miedema's models to describe formation enthalpies of intermetallic compounds and alloys (Miedema et al., 1980). A brief description of this model was given in section 2. Hydrides of transition metals are treated as alloys composed of hydrogen metal atoms. The component R in Rmn that forms a relatively stable binary hydride is taken to be a minority element (n > 1). In the same way as in the heat of formation of the pure compound RMù (see section 2.1) it is assumed that the main energy contributions to the heat of formation of the ternary hydride RMùHx+y are due to energy effects at the atomic cell boundaries. In the ternary hydride the hydrogen atoms will completely surround the minority atoms.
The expression for AH is composed of three terms:
AH(RMnHx+y ) = AH(RHx) + AH(MnHy) - (1 - F)AH(RMn). (16) The first two terms on the right hand side of eq. (16) account for the energy effects associated with the R - H and M - H atomic cell boundaries. The third term accounts for the contacts between R and M being broken upon formation of the hydride. The factor (1 - F ) in front of the last term of eq. (16) accounts for the fact that the separation of R atoms from M atoms by means of the H atoms becomes less effective as n decreases. For more details we refer to the papers by Miedema et al. (1977), and Buschow et al. (1982a), where values for the different quantities appearing in eq. (16) are also listed. Some of the computational results are reproduced in table 4. The AH values of the corresponding Gd compounds are virtually the same as those listed for Y. In the case of the (trivalent) heavy lanthanides or light lanthanides one may extrapolate between the values of Y and Sc and between the values of La and Y, respectively. With a value of - 1 3 0 J/K mol H2 for the hydrogenation entropy a plateau pressure of 1 atm at room temperature corresponds to AH = - 39 kJ/mol H2 (eq. 13). By means of the results given in table 4 a quick indication can be obtained of the possible existence of a stable hydride (pp ~< 1 atm) for a given compound.
38 K.H.J. BUSCHOW TABLE 4
Calculated values of the formation enthalpies of ternary hydrides of several intermetallic compounds of La, Y and Sc with 3d, 4d and 5d transition metals. The formation enthalpies are expressed in kJ/mol H»
The hydride compositions were taken to be equal to RMsH6, RM3H» RM2H 4, and RMHz» for R=La or Y and RM»H» RM3H4, RM2H3 » and RMH 2 for R=Sc.
M S c M » ScM 3 ScM2 ScM YM» YM 3 YM 2 YM LaM 5 LaM 3 LaM 2 LaM V - 1 3 0 -142 -147 -185 -136 -144 -152 -181 -139 -146 -149 -173 Cr - 3 4 -101 -113 -162 - 9 3 -106 -122 -163 - 9 7 -108 -120 -156 Mn - 6 6 - 3 3 - 9 8 -151 - 7 2 - 5 5 - 1 0 4 -151 - 7 2 - 8 3 -100 -143 Fe - 4 7 - 6 5 - 5 4 -141 - 5 5 - 7 3 - 9 6 -147 -61 - 7 4 - 9 3 -140 Co - 2 5 - 4 0 - 6 4 -123 - 3 6 - 4 9 - 7 5 -131 - 3 5 - 4 7 - 7 0 -123 Ni - 1 4 - 2 6 - 5 3 - 1 1 4 - 2 4 - 3 6 - 6 4 -123 - 2 2 - 3 3 - 5 9 -115 Mo - 9 3 -113 -122 -172 -105 -119 -134 -174 -110 -123 -134 -169 Tc + 4 - 1 2 - 3 4 -103 - 2 - 2 4 - 5 4 -116 - 8 - 2 3 - 5 0 -110 Ru +15 + 0 - 3 0 - 9 5 +2 - 1 3 - 4 5 -110 +3 - 1 2 -41 -114 Rh +30 + 18 - 15 - 8 0 + 18 +6 - 2 7 - 9 5 +24 + 12 - 2 0 - 8 7 Pd + 31 + 29 - 6 - 63 + 24 + 15 - 12 - 77 + 34 + 29 - 2 - 68 W - 7 6 - 9 9 -111 -165 - 9 1 -107 -126 -170 - 9 7 -112 -126 -165 Re +10 - 9 - 3 6 -101 - 5 - 2 2 - 5 3 -117 - 6 - 2 2 - 5 0 -111 Os +11 - 6 - 3 4 - 1 0 0 - 3 - 1 9 -51 -115 - 3 - 1 9 - 4 7 -109 Ir +40 + 27 - 7 - 7 4 + 26 + 13 - 22 - 92 + 30 + 17 - 16 - 85 Pt +65 +58 + 19 - 4 5 +53 +43 + 7 - 6 6 +51 +51 + 16 +58
O w i n g to the s i m p l e f o r m u l a t i o n used in eq. (16) the m o d e l is s o m e t i m e s m i s i n t e r p r e t e d . A s a l r e a d y m e n t i o n e d , the t e r n a r y h y d r i d e s are r e g a r d e d as t e r n a r y i n t e r m e t a l l i c c o m p o u n d s , o n e o f the c o m p o n e n t s b e i n g h y d r o g e n . N i s h i m i y a et al.
(1982) a s s u m e i n c o r r e c t l y t h a t the m o d e l e n c o m p a s s e s the f o r m a t i o n o f RHz (ZrH2) clusters a f t e r a c o m p l e t e r u p t u r e o f the alloys. (Such clusters m a y arise i n c i d e n t a l l y a f t e r d e c o m p o s i t i o n o f the t e r n a r y h y d r i d e , w h i c h is a different m a t t e r , n o t u n d e r d i s c u s s i o n here.)
It s h o u l d be realized t h a t the c r u d e a p p r o x i m a t i o n s m a d e set a limit to the a p p l i c a b i l i t y o f the m o d e l . W e h ä v e o n l y c o n s i d e r e d e n e r g y effects a s s o c i a t e d w i t h the n e a r e s t n e i g h b o u r i n t e r a c t i o n a n d the d i s t r i b u t i o n o f H a t o m s b e t w e e n the R - H a n d M - H c o n t a c t surfaces (x a n d y in eq. 16) is a r b i t r a r y . C a l c u l a t i o n s m a d e b y m e a n s o f eq. (16) t h e r e f o r e c a n give o n l y a f i r s t - o r d e r e s t i m a t e o f the e n t h a l p y o f the t e r n a r y h y d r i d e s . F o r the p u r p o s e o f c o m p a r i s o n relative energies are r e q u i r e d , h o w e v e r , a n d d u e to the c a n c e l l a t i o n o f e r r o r s the a c c u r a c y o f eq. (16) is m u c h better.
T h e a p p l i c a b i l i t y o f eq. (16) is q u i t e useful, therefore, in p r e d i c t i n g trends. T h e s e c o m p r i s e :
(i) In a series o f c o m p o u n d s RMù w i t h fixed c o m p o s i t i o n n a n d the s a m e n o n - r a r e e a r t h c o m p o n e n t M the e q u i l i b r i u m p r e s s u r e s i n c r e a s e in the d i r e c t i o n f r o m L a to Lu.
(ii) I n a series o f c o m p o u n d s RMù w i t h fixed c o m p o s i t i o n n a n d r a r e e a r t h c o m p o n e n t R the e q u i l i b r i u m p r e s s u r e is l o w e r as the h e a t o f c o m p o u n d i n g o f R w i t h the c o m p o n e n t M into RMù is less n e g a t i v e (rule o f r e v e r s e d stability).
(iii) I f R a n d M are fixed the e q u i l i b r i u m p r e s s u r e increases w i t h i n c r e a s i n g n.
F o r s o m e series o f i n t e r m e t a l l i c h y d r i d e s listed in t a b l e s A1 to A 4 in the a p p e n d i x the Peq d a t a are f a i r l y c o m p l e t e . F o r these series we h a v e p l o t t e d the p l a t e a u p r e s s u r e s
102 / RNis(20"C)
/ / RCos(20"C)
10
g lo -~
R Co3 120"C1-
» o- ~ l ö 2
10 -3
l ö ~
lO-S Ce Nd Sm Gd Dy ~ Er ' Yb' La Pr Pm Eu Tb Ho Tm Lu
Fig. 22. Hydrogen plateau pressures in various series of rare earth compounds.
as a function of the rare earth component in fig. 22. These data reflect the trends (i) to (iii) mentioned above.
There are only a few exceptions where the model predictions were found to lead to the wrong answer. The most serious one is LaPts, which compound, according to the positive A H value in table A4, should not give rise to a stable ternary hydride.
Takeshita et al. (1981) reported the formation of a hydride phase LaP%H4 from LaPt»
at 1000 atm H a and room temperature. This discrepancy is as yet unexplained.
The wrong trend in the hydrogen sorption properties is predicted in the series TiM, where the ternary hydride stability should decrease in the sense M = Fe, Co, NJ.
Experimentally, however, an increase is observed (Yamanaka et al., 1975; Lundin et al., 1977). It should be noted that the three ternary hydrides observed differ considerably in hydrogen concentration, the composition at the end of the first plateau being approximately TiFeH2, TiCoH1.2 and TiNiH~.z. The model calculations are based on the same hydrogen content (TiMH2) and thus underestimate the (absolute) A H values of the ternary hydrides TiCoHL2 and TiNiHI. z when expressed per mole absorbed H 2 gas. Further complications which make the series TiFe, TiCo, TiNi less suitable for a comparison between model and experiment have been discussed in detail elsewhere (Buschow and Miedema, 1978).
40 K.H.J. B U S C H O W
50 0 I~" -50
N i Ho. 7 I
LaNisH6 /
~LaNi2i2'l* J
~: -100
2,_~5o I ,z4 .... I, ~~
7- q , ~ 3 ~'~" LO7NI3 H19
<~ -200, La H:
-2500 0.2 0.4 0.6 0.8 1.0
ot%Ni
Fig. 23. Formation enthalpy o f La-Ni hydrides plotted as a function of Ni concentration. The full line connects the experimental A H values o f NiH0. 7 and LaH 2 (after Busch et al., 1978a).
If one neglects the last term in eq. (16) the enthalpy of hydride formation takes the form of a sum of the weighted heat of formation of the binary hydrides of the two parent metals. Such an approach to estimating AH values has been advocated by Clinton et al. (1975) and by Busch et al. (1978b, c). Results of the latter authors are reproduced in fig. 23. Here the full line connects the A H data of NiH0.7 and LaH2.
It is clear that this phenomenological relationship between the heat of formation of the ternary and binary hydrides is quite useful for estimating A H values of ternary hydrides based on rare earth metals where the first term in eq. (16) is the dominant one. It is obvious that the procedure of weighted averages is adequate for dealing with trend (iii) mentioned above. Trend (i) is also predicted correctly. This can be seen by taking account of the fact that in a series RMn with M and n fixed the value of the binary hydride RH2, which determines the intercept of the straight line with the left-hand vertical axis in fig. 23, moves in an upward direction as one proceeds from La to Lu. Difficulties are met if orte considers very stable RM,, compounds with fixed R and n and a variable M component, such as RRu~, RRhn, RPtù (trend ii).
Since the heat of formation of the binary metal hydrides MH0.5 becomes more positive in the direction Pt, Rh, Ru (Bouten and Miedema, 1980), this means that the straight line corresponding to the weighted A H averages (such as in fig. 23) lies lowest for RPtù and highest for RRuù. Consequently, the stability of the ternary hydrides RMnHx is expected to become more negative in the sense RRuùH«I RRhnHx, RPt, Hx. This trend is opposite to that usually observed. For instance, the A H values of the ternary hydrides of GdRu2, GdRh2 and GdPt2 are reported (Jacob and Shaltiel, 1979) to become less negative in going from GdRu 2 to GdPt2 (AH = -60.3, 49.4 and > - 39 k J/mole H2, respectively). Difficulties will furthermore be met, even in reproducing trend (iii), in cases where the formation enthalpy of the binary hydrides of the parent metals is relatively low while the enthalpy of compound formation is relatively high (for instance in ternary hydrides of P d - N b compounds).
Jacob et al. (1977, 1980a) have proposed a model in which an important role is attributed to the local environment of the absorbed hydrogen atoms. These authors
suggest that the energy effects associated with the occupancy of a hydrogen atom of a given interstitial hole position (j) be determined by the weighted average of the heat of formation of the binary hydrides that surround this position (AHj). These latter quantities were estimated by means of the model of Miedema et al. (1980). The model given by Jacob et al. is suited, in particular, to predicting relative hole site occupancies Nj. According to Jacob et al. the Nj. values can be obtained by using Boltzmann statistics, leading to the expression
Nj = N exp(-AH~/kBT)
Ei e x p ( - AH;/kBT)" (17)
Note that this model leads to a quantitative description of the temperature dependence of the relative hole site occupancies. For predicting ternary hydride stabilities the model would seem less suited since, in principle, it would require a knowledge of crystallographic details of the ternary hydride. Fortunately most of the ternary hydrides do not give rise to crystal structures where the metal atoms are arranged in a completely different way from the arrangement in the crystal structure of the uncharged intermetallics. Model calculations can then be made based on structural details of these latter compounds (see for instance Shinar et al., 1978a, b).
Lundin et al. (1977) and Magee et al. (1981) have analysed the hydrogen absorption behaviour of intermetallic compounds in terms of hole sizes present in these compounds. In the Haucke compounds, in particular, a rather close correlation was found between the stability of the RM»H x hydrides and the tetrahedral hole sizes in RM». It is not quite clear in how far these correlations have a physical basis. In the first place the differences in lnp or AH are explained in terms of tetrahedral hole radii differences of only several thousandths of Ä, whereas it is known that hydrogen uptake gives rise to lattice expansions about two orders of magnitude larger.
Secondly, if the hole size in the uncharged Haucke compounds RCo» or RNi» were really important, one would expect the correlation to hold for compounds in which the R component has a valence different from 3. This seems not to be the case. It is also not clear why compounds in which the R elements have opposite valence deviations, as in CeCo» (Ce 4+) and CaNi» (Ca 2 +), often show deviations from the mentioned correlation in the same directions while CeC% and ThCo5 (Ce and Th both tetravalent) deviate in opposite directions. The correlation is further less well established if Ni in LaNi 5 is wholly or partly replaced by Pt. Further arguments against the importance of hole sizes are the fact that the linear relationship between In p and the hole size cannot be explained by elastic deformation of these interstitial holes. Also the volume expansion per hydrogen in a given series of compounds is not related to the associated plateau pressures (Busch et al., 1978a). It is conceivable that the variation in hole size across a series of compounds reftects the variation of other parameters determining the hydride stability. For instance, the lattice para- meters (and hence also the corresponding hole sizes) tend to be smaller as the uncharged compounds have higher stabilities or as the R components are less electropositive (Buschow et al., 1982). Nevertheless, the established correlation between tetrahedral hole size and ternary hydride stability can be used in tailoring the absorption properties to satisfy the conditions required in a given application of