Electronic Structure of HydrogenStorage Materials

M. GUPTAInstitut des Sciences des Matériaux, Laboratoire d’Etude des Matériaux Hors Equilibre, Bâtiment 415,Université Paris-Sud, 91405 Orsay, France

Received 30 May 1999; accepted 1 September 1999

ABSTRACT: The electronic structure of two types of hydrogen storage materials hasbeen investigated by ab initio band structure methods. We show that the helium producedby the radioactive decay of tritium in a palladium matrix remains stable at the octahedralsite of the fcc lattice for He/Pd ratio of at least 0.25. Such He production results in a latticeexpansion of 2%. The experimentally observed increased stability of aged tritium in apalladium matrix is not entirely due to lattice expansion, which amounts to only 28% ofthe effect, but also to modifications of the electronic structure in the presence of helium. Inaddition, we show that He leads to lattice decohesion.The effect of Ni substitution in LaNi5 by 3d and s-p elements on the electronic structure ofthe intermetallic and its hydrides has been also investigated. These substitutions are foundto affect drastically the properties at the Fermi level and the filling of the Ni-d states. TheFermi level, EF, of LaNi4M (M = Fe, Co, Mn) is found to lie in the narrow additional M-3dsubband above the Ni-d states, leading to an increase in the density of states (DOS) at EF.In contrast, the substitution of Ni by an s element of the 3d series, Cu, or by an s-pelement, Al or Sn, results in a progressive filling of the Ni-d bands and in a decrease of theDOS at EF. In all the substituted intermetallic compounds, we find that the latticeexpansion accounts for less than 50% of the observed decreased stability. This shows theimportance of chemical effects. We also discuss the factors that affect the electronicstructure and the stability of the hydrides, and compare our results with availableexperimental data. c© 2000 John Wiley & Sons, Inc. Int J Quant Chem 77: 982–990, 2000

Key words: electronic structure; intermetallic compounds; tritides; hydrides;hydrogen storage

Introduction

T he storage of hydrogen in the form of metalhydrides has been investigated for the last

30 years [1 – 5]. It is considered to be safe and allowseasy hydrogen recovery. Furthermore, much larger

quantities of hydrogen can be stored in the samevolume compared to liquified form. Understandingthe electronic properties of metal–hydrogen systemsfrom a fundamental point of view and in relation toseveral aspects of their potential technological ap-plications is crucial [6]. In this article, the results ofour electronic structure calculations are illustrated

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by two examples: (i) the storage of tritium, theheavy isotope of hydrogen, in a palladium matrixand (ii) the effect of substitutions at the nickel siteon the electronic structure of the hydrogen storageintermetallic compound LaNi5. The motivations ofthis work are exposed below.

Tritium is important in the nuclear industry andits storage in the form of metal tritide has been en-visioned [7]. Unlike hydrogen, however, tritium isradioactive, has a half-life of 12.3 years, and decaysinto 3He through the decay reaction

T→ 3He+ β− + ϑ , (1)

where ϑ is an antineutrino and β− is an electronemitted with an energy of 18.5 keV. The recoil en-ergy of 3He in this decay process is very small,∼1.03 eV, and not enough to create any damage tothe lattice. In this decay process very large quanti-ties of 3He can be accumulated initially in the latticethat are not possible by other means, including ionimplantation, without creating any damage. A crit-ical concentration of 3He has to be attained beforeany 3He starts to be released [7 – 10].

Due to the presence of 3He, the tritide is no longera binary alloy, but a ternary one. This results ina considerable modification of the electronic andphysical properties of the tritide. In particular, thereare three types of modifications that are of directrelevance to the storage of tritium in the form ofa tritide. First, there is an expansion of the latticeas the tritide ages [7]. This produces mechanicalstresses in the matrix. Second, the tritide becomesbrittle as the 3He accumulates in the lattice. Third,the pressure-composition isotherms of tritium aremodified, and there is a lowering of the plateaupressure in the presence of 3He [11, 12]. This is acause of concern for the recovery of tritium sincea lowering of the plateau pressure signifies an in-creased stability of tritium in the matrix.

The location of 3He in the matrix is a questionof vital interest. It has usually been assumed onthe basis of experiments performed on thin filmsthat 3He in the tritides agglomerates in the form ofbubbles [8]. On the basis of our electronic structurecalculation for PdT, we show that 3He is retained inthe lattice at the octahedral interstitial sites where itis formed. Our results also provide a consistent ex-planation of the changes in the physical propertiesthat occur in the tritide due to the presence of 3He.

As a second example, we present the results ofour attempt to a better understand the electronicstructure of hydrides of LaNi5 and ternary inter-metallic compounds obtained by substitution of

Ni by a 3d element, Mn, Fe, Co, Cu, and by s-pelements such as Al and Sn. Since the early 1970s,a large number of experimental investigations havebeen performed on LaNi5 and related compoundsin relation to their exceptional hydriding properties[13, 14]. Substitutions at the La or Ni sites areused to modify the thermodynamic propertiesand play an important role in the selection ofthese alloys for specific technological applications,since they affect the stability and the hydrogencontent of the hydride [15 – 18]. To optimize thechoice of the intermetallic compound for a selectedapplication, a better understanding of the role ofeach alloy constituent on the electronic propertiesof the material is crucial. Several semi-empiricalmodels [19, 20] have been proposed for the heat offormation and heat of solution of metal hydrides,and attempts have been made to justify the maxi-mum hydrogen absorption capacity of the metallicmatrices. These models show that the energeticsof the metal–hydrogen interaction depend onboth geometric and electronic factors. Up to now,however, theoretical ab initio studies of hydridesof complex systems have been rather limited andcannot yet provide an answer to these interestingquestions. In this article, we discuss the role of thesubstituting element at the Ni site and the role ofhydrogen absorption on the modifications of theelectronic properties. The factors that affect thestability of the compounds are analyzed.

We first outline the computational method beforepresenting the theoretical results and their analysisin light of available experimental data.

Computational Details

The band structure calculations have been per-formed using density functional theory in the localdensity approximation. The von Barth–Hedin [21]approach was used to determine the exchange andcorrelation term of the crystal potential. Since weare dealing with unit cells containing a rather largenumber of atoms, we have used the self-consistentlinear muffin-tin orbitals method within the atomicsphere approximation [22]. The so-called combinedcorrection terms were included to account for theoverlap of atomic spheres. The densities of stateswere calculated in a 1-mRy mesh with the linear en-ergy tetrahedron method [23].

We present the results for the case where 25% oftritium in PdT has decayed into 3He. This concentra-tions of 3He is smaller than the critical concentration

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Rc ∼ 0.50 that has been reported recently for an agedPdTx. Although PdT has an NaCl type of fcc crystalstructure, in our calculations we use a simple cubiccell containing four PdT formulas per cell. Pd atomsare located at the corners and faces of this cell, whileT atoms occupy the octahedral interstitial sites at themiddle of the edges and the center of the cube. Dur-ing the radioactive decay process, 3He is producedat random positions at the interstitial sites. For thesake of simplicity, we assume an ordered arrange-ment of 3He atoms in the spirit of the mean fieldapproximation. In this ordered arrangement, the Heatoms occupy the center of the cube and are thusseparated by the lattice parameter a of the cubic lat-tice. This results in two types of Pd atoms in thelattice: three Pd atoms located at the faces of thecube, denoted as Pd3, that are the nearest neighborsof the 3He atom, and one Pd atom located at the cor-ner of the cube, denoted Pd1, that is the next nearestneighbors. The local environment of a Pd1 atom isthe same as in a PdT lattice since it has six T nearestneighbors. In contrast, a Pd3 atom has four T atomsand two 3He atoms as nearest neighbors.

LaNi5 crystallizes in the hexagonal CaCu5 struc-ture [24], space group P6/mmm, in which the Laatoms occupy the (1a) sites. There are two types ofNi atoms occupying, respectively, the (2c) sites inthe basal lanthanum planes (z = 0) and the (3g) sitesin the middle plane (z = c/2). Neutron scatteringdata [17, 25] have shown that the Ni substitutionsconsidered here occur predominantly at the Ni (3g)sites. We thus ignored, for simplicity, the partial, andgenerally small, occupancy of the (2c) sites as well asthe disorder and the possible clustering of the sub-stituting elements by assuming an ordered array ofM atoms. In the case of LaNi4Co, we also consideredCo substitution at the (2c) site.

Results and Discussion

EFFECT OF He ON THE ELECTRONICSTRUCTURE OF PdT

To determine the equilibrium lattice parameter inthe presence of 3He, the electronic structure calcula-tions were performed for a series of lattice parame-ters, and a minimum in the total energy was foundthat determines the value of the lattice parameter.We obtain a value of a = 4.2989 Å for PdT0.75He0.25,which is 2% larger than the value of a = 4.2141 Åobtained in a similar manner for PdT. This yieldsa lattice expansion of 0.08% per atomic percent of3He generated in the lattice. Unlike PdT, no known

compound PdHe exists. The electronic structure cal-culations were also performed for this compoundfor a number of lattice parameters, and no minimumin the total energy could be found. This confirms thealready known result that the compound PdHe doesnot exist, but it also demonstrates that once a criti-cal concentration of 3He has been obtained in thelattice, any further 3He generated in the lattice canno longer be retained due to the inherent lattice in-stability. The release of 3He can then occur either atthe rate of generation or through a collective mo-tion in the form of He bursts as proposed some timeago. There is experimental evidence that at least ini-tially all the He produced is retained in the latticeand very little or almost no He is released from thetritide. Once a critical concentration is reached, Hestarts to be released [8, 9]. This release is not grad-ual and is often termed rapid or accelerated release[26] to distinguish it from the very slow release thatoccurs in some tritides from fractured surfaces andgrain boundaries at low He concentrations [8, 9].Values of the critical concentration depend upon thetritide, but generally lie between 0.25 and 0.4 [27].Recently, a value of 0.5 was reported [28] for agedPdTx. There is also experimental evidence [29] thatrelease occurs in bursts of 109 atoms, which indi-cates that it is not a continuous process. Concerningthe location of He in the tritide, two types of mod-els have been proposed. The first model is based onthe behavior of He implanted in metals [7]. In thismodel, the He produced in the tritide does not stayin the intertitial sites, but migrates to form bubbles,at a very early stage. This requires considerable Hemobility. In the second model, in contrast, we con-sider the fact that the process of He production ina metal tritide is radically different from He intro-duction by ion implantation. In the latter, vacanciesare created due to the implantation of He+ ions thatserve as traps for He and lead to bubble formation,whereas no such defects can be created by the Heproduced in the tritide because the recoil energy ofHe in the radioactive decay of tritium is too small.Thus the He produced is trapped at the interstitialsites and is immobile [9, 11, 29]. Experimental ev-idence in favor of the first model is quite indirectand comes essentially from transmission electronmicroscopy (TEM) observations on thin films of tri-tides [30] and from interpretations of the nuclearmagnetic resonance (NMR) line shapes [10]. Bub-bles of 10–20 Å in diameter have been observed byTEM. However, it should be realized that the filmsare strained during charging by tritium which couldlead to bubble formation in the surface region inves-

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tigated in the TEM experiments. NMR line shapedata has been interpreted as evidence of bubbleformation [10], whereas similar data have been dis-cussed by other authors [31] in terms of trapped Heat interstitial sites. Furthermore, in desorption ex-periments, Kass [32] found a single trap with an acti-vation energy of 17 kcal in the aged tritide, whereastwo traps with activation energies of 15 and 62 kcalare found in He+-implanted tritides. The differenceof the two activation energies, 45 kcal, is roughly theHe-vacancy binding energy. This shows that vacan-cies are not created in a naturally aged tritide. TheHe diffusivity is also found to be very low, 10−13–10−14 cm2/year, which indicates that a typical Heatom diffuses at most a few lattice parameters peryear [9]. Also, the release of He in the form of burstsof 109 atoms does not seem to support the model ofbubble formation, because a bubble of ∼4000 Å isrequired to contain so many He atoms [29]. Instead,a mechanism that involves the collective movementof the He atoms is suggested. The present calcula-tions show that in the absence of defects, he can beretained in the octahedral interstices of the fcc latticeat least up to a He concentration of 25%.

FIGURE 1. (a) Total densities of states of thecompound PdT obtained for the calculated value of theequilibrium lattice parameter. (b) Total densities of statesof the compound PdT0.75He0.25 for the unrelaxed valueof the lattice parameter (that is, the value obtained forPdT).

In Figure 1 we show the total densities of states(DOS) of PdT and PdT0.75 He0.25 calculated with thelattice constant obtained in our calculation for PdT.A comparison of the two curves in Figure 1 allowsus to separate the effect of the electronic structurealone in the modification of physical properties dueto 3He. In Figure 2 the total DOS of PdT0.75He0.25 andthe decomposed partial densities of states (PDOS) atvarious sites are presented with the lattice parame-ter calculated for this compound. In PdT, the statesbelow ∼−5 eV result from the bonding Pd-d–T-sinteractions while those above ∼−5 eV and in thevicinity of the Fermi level are mainly of Pd-d char-acter. The replacement of T by He results essentiallyin three modifications in the electronic structure.First, a new structure of very narrow width ∼1 eVappears at ∼15 eV below the Fermi level. This isprimarily of He-s character with a very small ad-mixture caused by 3He with the nearest neighborPd3-d states. Second, new states also appear at the

FIGURE 2. Total densities of states of the compoundPdT0.75He0.25 obtained with a lattice parameter thatincludes relaxation due to He. The lower panels showthe partial densities of states at Pd1, Pd3, T, and He sites.

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Fermi level that were absent in PdT. These are infact the Pd3-d states that have been pushed upwardthrough an interaction with He, indicating the re-pulsive character of the atom in the matrix. Third,there is a depletion of states from the Pd-d–T-sbonding band due to the absence of 25% T atoms inthe matrix. It is important to note from Figure 1 thatthis depletion occurs preferentially from the higherlying electronic states. The net result of such a de-pletion is to enhance the cohesion of the remaining Tatoms in the lattice. This increases the relative stabil-ity of the tritide, thus lowering the plateau pressure.In Figure 1, the lattice expansion due to He has notbeen included, as was stated before. Thus, the ob-served lowering in the plateau pressure results, due,at least in part, to purely electronic modificationfrom the presence of 3He, and is not at all relatedto the lattice expansion. This is in contrast to theusual assumption in the literature [33, 34] that theincreased stability of tritium in the presence of 3Heoccurs purely due to the lattice expansion causedby 3He. Our calculations yield a value of 0.13 eVper atom of tritium for this increase in the stabilityof the tritide due to purely chemical effects that donot include any lattice expansion. The lowering ofthe energy of PdT0.75He0.25 due to the lattice expan-sion of 2% due to He is much smaller, ∼0.05 eV pertritium atom. This results in a value of 0.18 eV pertritium atom in the stability of the tritide, of whichonly ∼28% is due to the lattice expansion.

A comparison of Figures 1 and 2 shows that thedensities of states at the Pd1 site, which are the nextnearest neighbors of the 3He atom, remain practi-cally unchanged from those in PdT. This reflects thecompact nature of the 3He wave functions in the lat-tice. On the other hand, the states at the Pd3 sites,which are the closest to 3He, are substantially af-fected and have been pushed to higher energies.Such a shift signals a loss of cohesion of these Pdatoms in the lattice for which we obtain a value of∼0.94 eV/Pd atom. This has to be compared to thecohesive energy of ∼3.70 eV/Pd atom in Pd metal.This large decrease in the value of cohesion at thenearest neighbor Pd sites of 3He clearly renders thetritide fragile at these large concentrations of 3He inthe matrix.

EFFECT OF Ni SUBSTITUTION ON THEELECTRONIC STRUCTURE OF LaNi5 ANDITS HYDRIDES

The total DOS of LaNi5 is plotted in Figure 3(a).Its partial wave analysis around different atomic

sites shows that the occupied part of the conduc-tion band is dominated by the Ni-3d states witha nonnegligible bonding contribution of the La-5d.The main part of the La-5d state is located abovethe Fermi energy, EF, chosen here as the origin ofenergies. The 4f La states are empty; they are verylocalized, and give rise to a peak in the DOS cen-tered around 3 eV above EF. In agreement withour previous results using the tight-binding ap-proximation in the recursion scheme [35], we findthat the Ni-d bands are not filled in LaNi5. TheDOS at EF is high and mostly composed of Ni-d states since the La-5d contribution is only 4%.This result is in general agreement with low tem-perature heat capacity measurements that lead tolarge values of the electronic specific heat coeffi-cient, γ = 34.3 mJ (mol LaNi5)−1 K−2 [15] andγ = 42 mJ (mol LaNi5)−1 K−2 [36], and with mag-netic susceptibility data, χ = 4.6×10−6 emu g−1 [37].In spite of its large DOS value at EF, LaNi5 is a Pauliparamagnet. The calculated total DOS of LaNi5shown in Figure 3(a) is in good agreement withthe X-ray photoelectron spectroscopy (XPS) data ofFuggle et al. [38], concerning the width of the oc-cupied states, and with the high emission observedat EF in LaNi5 as well as in pure Ni. Moreover, thesatellite observed at −6 eV below EF in the Ni pho-toemission spectrum does not disappear in LaNi5.Since this satellite is due to d8 final state effects, itspresence in LaNi5 indicates that the Ni-d bands arenot full.

Our calculation shows that the interaction ofthe La-5d with the Ni-3d states below EF is largeenough to lead to negligible charge transfer betweenLa and Ni. We notice that the valley in the totalDOS of LaNi5 which separates the bonding Ni-3d–La-5d from their antibonding counterparts wouldcorrespond to a further filling of the Ni-d bandsby almost three electrons. In a simple model withcharge transfer of three electrons from La to Ni,since there are about 0.6 holes per atom in the Ni-dbands we could expect a complete filling of the Ni-dbands. Our theoretical results show that LaNi5 isnot a charge transfer compound and agree with thecore level photoemission spectra of Schlapbach [35]which indicate no significant shift in the La-3d andNi-2p core states in LaNi5.

Substitutions of Ni neighbors, M, on the left ofthe periodic table (M =Mn, Fe, Co) do not just leadto a simple depopulation of the Ni-3d bands. In allsubstituted intermetallics a new narrow subband,associated to the 3d states of the element M, ap-pears slightly above the Ni-d bands. As shown in

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FIGURE 3. The total DOS (full line curve, left-hand side scale; units are states per electronvolt unit cell) and thenumber of electrons (dashed line, right-hand side scale), including the six La-5p electrons, as a function of energy (inelectronvolts): (a) LaNi5; (b) LaNi4Mn; (c) LaNi5Cu; (d) LaNi4.5Sn0.5. The Fermi level is chosen as the origin of energies.

Figure 3(b), in the case of M=Mn, this new subbandof width 1 eV is centered around 0.5 eV above themain Ni-d peak. The corresponding structure is nar-rower and closer in energy to the Ni-d peak for M=Fe and Co. The lattice expansion associated with theNi substitution results also in a narrowing of the Ni-d bands due to a decrease in the Ni–Ni interaction.The Fermi level of the substituted intermetallic fallsin all cases in the narrow antibonding subband ofthe M element, while the Ni-3d–M-3d main bondingstructure is filled. As a consequence, the DOS at theFermi level increases further from LaNi5 to LaNi4M.This indicates, in agreement with experimental ob-

servations, a tendency toward magnetic ordering asa function of increasing M/Ni concentration.

The substitution of Cu at the Ni site results inthe presence of a new distinct Cu-3d subband cen-tered at ∼2 eV below the main Ni-d peak. This isin agreement with photoemission data [39], whichclearly show the growth of a new structure belowthe Ni-d bands as a function of increasing Cu/Niratio. We also observe in Figure 3(c) that the Ni-dbands occupancy increases by about one electronfrom LaNi5 to LaNi5Cu and that the DOS at EF hasdecreased drastically. However, the Ni-d bands arenot yet filled since two more electrons can be accom-

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modated up to the valley, which marks the rise ofthe La-5d states.

The heats of formation of the substituted alloysLaNi4M (M =Mn, Fe, Co, Cu) have been measuredby calorimetric methods [18]. In all cases, the substi-tuted intermetallics are less stable than LaNi5. Sincethese substitutions are accompanied by a volume in-crease, we have examined the role played by latticeexpansion in the decrease of stability by perform-ing total energy calculations for LaNi5 at the latticeparameters of the substituted compound. Using thelattice expansion 5.3% observed between LaNi5 andLaNi4Mn, we found that this volume increase leadsto a destabilization of LaNi5 of only 7.86 kJ mol−1,which represents only 31% of the experimental dif-ference between the enthalpy of formation 1Hf =−158.9 kJ mol−1 LaNi5 and 1Hf = −133.9 kJ mol−1

LaNi4Mn. Thus the decohesion associated with thelattice expansion which leads to a narrowing of theNi-d bands is not the main source of the decreasedstability observed in the alloy. The remainder is dueto pure chemical effects associated with the pres-ence of a new 3d subband, the filling of the main Nid structure, and the shift in the Fermi level position.We have obtained similar conclusions for the other3d substituting elements.

To study the effect of the substitution of Ni by ans-p element, we selected two compounds: LaNi4Aland LaNi4.5Sn0.5, the composition of which corre-sponds closely to the maximum content of tin thatcan be obtained experimentally. For the calculationwe used an orthorhombic cell containing two for-mula units LaNi4.5Sn0.5. The low energy structurefound in the total DOS plotted in Figure 3(d) be-tween −9.5 and −8.5 eV is mostly due to the sstates of Sn, whereas the Sn-p states are found athigher energies with a maximum contribution be-tween −6 and −4 eV. We note that the Ni-d peak isalmost, though not fully, filled since one more elec-tron would be required to shift the Fermi energy inthe valley between the Ni-3d and La-5d states. TheDOS at EF decreases from LaNi5 to LaNi4.5Sn0.5. Inthe case of LaNi4Al, EF falls at the bottom of thevalley, and the Ni-d main peak is entirely filled asshown in Figure 4.

It is thus clear that substitutions of Ni by an ele-ment with external s electrons such as Cu or s andp electrons such as Al and Sn lead to a progres-sive filling of the Ni-d states. This behavior couldbe confirmed by spectroscopic data such as photoe-mission, which are not yet available.

We have also investigated the electronic struc-ture of the hydrides of the substituted intermetallic

FIGURE 4. The total DOS of LaNi4Al (full line, left-handside scale) and the number of electrons (dashed line,right-hand side scale). The Fermi energy is chosen asthe origin.

compounds. In Figure 5(b) we plot the total DOSof LaNi4CoH4. In the calculation, we used neutrondiffraction data [40], which show that the deuteridebelongs to the orthorhombic space group D19

2h-Cmmmin which D occupies the (4e) and (4h) sites of thestructure. The main differences between the elec-tronic structures of the intermetallic and its hydridecan be briefly summarized, as follows: (i) An ad-ditional structure in the DOS of the hydride isobserved between −5 and −10 eV that is due tometal–hydrogen bonding and H–H interactions. Inthis energy range, the La-d contribution to the La—H bonding is similar to that of the Ni-d–H-s inter-action of the nickel atom. The Co-d contribution tothe bonding is smaller than that of Ni and occursat higher energies, above −6 eV. These atoms in-teract mostly with the H atoms located at the 4hsites. (ii) The Fermi energy of the hydride falls inthe valley of the DOS, which separates the transi-tion metal 3d from the La-5d states, and the valueof N(EF) decreases from the intermetallic to the hy-dride. Clearly, the Fermi level of the hydride ishigher than in the intermetallic since it correspondsto a filling of the main Co-3d peak. However, theshift in EF does not correspond to filling of the bandsin a rigid band model since the transition metal

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FIGURE 5. The total DOS (full line, left-hand side scale; in states per electronvolt unit cell) and the number ofelectrons (dashed line, right-hand side scale): (a) LaNi4Co; (b) LaNi4CoH4. The Fermi level is chosen as the origin.

d states are substantially modified by the metal–hydrogen interactions.

In the hydride LaNi4CoH4, the maximum hydro-gen content appears to be associated with filling ofthe Co-d peak. Further shift of the Fermi energywould cost energy since the DOS is not too high.

Summary and Conclusions

We have shown, on the basis of our ab initio elec-tronic structure calculations, that the 3He producedin PdT from the radioactive decay of tritium can beretained in the lattice at the octahedral interstitialsites where it is born. Our calculations were per-formed for the case where 25% of T transformed into3He, which is indeed a very large concentration. Wehave shown that this transformation resulted in alattice expansion of ∼2% that amounts to ∼0.08%per atomic percent of 3He produced, assuming alinear relationship between 3He production and thelattice expansion. The lowering of the plateau pres-sure, meaning an increased stability of the tritide,due to 3He, has usually been assumed in the lit-erature to arise from the lattice expansion causedby 3He. We have shown that in PdT0.75He0.25, only∼28% of the increase in the stability of the tritidecan be ascribed to the lattice expansion due to 3He.The rest is due to intrinsic effects caused by changes

in the electronic structure in the presence of 3He. Wehave also shown that 3He in the lattice renders thetritide fragile.

The substitution of Ni in LaNi5 by a 3d or by ans-p element leads to important modifications of theelectronic structure of the intermetallic. In LaNi4M(M = Fe, Co, Mn) the main Ni-d structure of theDOS is filled and the Fermi energy falls in the nar-row additional M-3d subband located above theNi-d states in a region of high DOS. In contrast, forM = Cu, the Cu-3d subband is centered 2 eV belowthe main Ni-d peak, in agreement with photoemis-sion data.

The substitution of Ni by Cu or by an s-p el-ement (Al or Sn) leads to a decrease of the DOSat EF associated with the progressive filling of theNi-d bands. We show that the effect of chemical sub-stitution plays an important role in the observeddecreased stability of LaNi4M compared to LaNi5

since the lattice expansion alone accounts for lessthan 50% of the decohesion of the alloy. The elec-tronic structure of the hydrides is characterized bythe presence at low energy of metal-hydrogen bond-ing states dominated by the Ni contribution. Thesebonding interactions play a major role in the sta-bility of the hydride. However, the Fermi energy isalways found to rise, a factor that affects the stabilityadversely.

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ACKNOWLEDGMENT

We would like to thank Institut de Développe-ment en Informatique Scientifique of Centre Na-tional de la Recherche Scientifique for providing thecomputing facilities used in the present work.

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