An investigation of correlation between electronic structure ofLaNi4M (M�/Ni, Cu, Mn, Al) and hydrogen absorption properties

Jin Guo *, Wen-lou Wei, Shu-yuan Ma, Ying-jun Gao

Department of Physics, Guangxi University, Nanning 530004, PR China

Received 11 October 2002

Abstract

The electronic structure of LaNi4M (M�/Ni, Cu, Mn, Al) and their hydrides were investigated by the self-consistent-charge

discrete variational Xa method. The results show that the stabilities of LaNi4M hydrides are related to charge transferred to

hydrogen atom, which is found to be strengthen by decrease of the transferred charge; the cycle lifetime of LaNi4M is effected by

strength of bond between 4f orbit in La atom and 4p orbit in Ni(1) atom, the stronger the bond is, the longer the cycle lifetime is;

there is also a orbital action between 3d orbit in Ni(3) atom and 4f orbit in La atom, the orbital action, however, is weaken when

hydride is formed.

# 2002 Elsevier Science B.V. All rights reserved.

Keywords: Electronic structure; Hydrogen absorption properties; LaNi4M hydride

1. Introduction

Owing to their excellent electrochemical properties,

LaNi5-base alloys have been widely used as negative

electrode materials of Ni/MH batteries [1�/3]. However,

the hydrogen absorption properties of LaNi5-base alloys

are very much dependent on the constituent metal,

involved even for isostructural compounds. For exam-

ple, ternary alloys LaNi5�xMx (M�/Al, Co, Cr, Cu,

Mn) where some of Ni atoms in the LaNi5 alloy are

substituted maintaining the CaCu5-type crystal structure

have been examined to improve the electrode properties

such as the cycle life or the charge and discharge

capacity [4], and RENi5(RE�/La, Ce, Pr, Nd) which

are also proved to maintain CaCu5-type crystal struc-

ture exhibit very different behavior in absorbing hydro-

gen plateau pressure [5]. In order to elucidated

theoretically why there are much difference of hydrogen

absorption properties among these alloys, Yukawa et al.

[6] have investigated the alloying effect on the electronic

structure of the hydrogenated LaNi5, and have shown

that the hydride properties are well understood in terms

of the nature of the chemical bond between atoms in a

small octahedron in which the absorbed hydrogen atom

is located. In spite of these investigations interest,

theoretical studies of the electronic structure of thesealloys and stability of these hydrides are still lacking.

Therefore, it is necessary to investigate the changes of

electronic structure after hydrogen absorbed and their

effects on hydrogen absorption properties.

In the present study, the electronic structures of

LaNi4M (M�/Ni, Cu, Mn, Al) and their hydrides are

calculated by the self-consistent-charge discrete varia-

tional Xa (SCC-DV-Xa) molecular orbital method, andthe effects of electronic structures on both hydrogen

absorption properties and the stabilities of hydrides are

also discussed.

2. Model and method

In this work, we have calculated the electronic

structures of four representative CaCu5-type com-

pounds, LaNi4M (M�/Ni, Cu, Mn, Al), using cluster

model, La4Ni10M2. The cluster model used is shown inFig. 1 [3], which is constructed on the basis of the crystal

structure of LaNi5Hx (x B/0.4). A hydrogen atom

occupies the 3f site in the crystal with P6/mmm space* Corresponding author.

E-mail address: guojin@gxu.edu.cn (J. Guo).

Materials Science and Engineering B98 (2003) 21�/24

www.elsevier.com/locate/mseb

0921-5107/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 9 2 1 - 5 1 0 7 ( 0 2 ) 0 0 5 8 4 - 6

group. That is a center site of the octahedron with the

frame of four Ni atoms and two La atoms. The

hydrogenated clusters of LaNi4M were expanded

slightly to be about the same size as 3f size of a phase

LaNi4M hydrides, maintaining the shape of the octahe-dron. The coefficient of volume expansion of the cluster

adopted for the calculations was about 4% [7].

In order to calculate the electronic states of LaNi4M

and their hydrides, we adopted SCC-DV-Xa method

which is a molecular orbital computing method [8]. The

SCC-DV-Xa method is based on the nonrelativistic one-

electron Hamiltonian equation

h��1

292�V (r)��

1

292�Vcoul(r)�Vxc(r)

Here the Coulomb potential is the sum of nuclear and

electronic contributions

Vcoul(r)��Xn

Zn

jr � Rnj�g

dr?r(r?)

jr � r?j;

the statistical exchange potential in these calculations is

defined as

Vxc(r)��3a

�3r(r)

8p

�3=2;

where a is exchange constant which is normally chosen

as 2/30/a0/1. In the calculations exchange constant a is

chosen as 0.7.

As in the usual LCAO-MO method the system orbital

wavefunctions are chosen as linear combinations of

atomic orbits,

Ci(r)�Xn

k�1

Cki8k(r)

the coefficient Cki can be obtained by secular equationXl

(Hkl �ESkl)Cli�0

In the present calculation, frozen shell model is used

and outer shell electronic configurations are chosen as:

La, 4s24p64d104f05s25p65d16s26p0; Ni, 3s23p63d84s24p0;

Mn, 3s23p63d54s24p0; Cu, 3s23p63d104s14p0; Al,

2s22p63s23p1; H, ls1. We have used the single-site orbital,

chosen self-consistent-charge and discrete variationalmethod for calculating.

3. Results and discussion

One of important properties for hydrogen storage

alloy is equilibrium plateau pressure. Sakai et al. [4] has

summarized in experimental that the replacement of Ni

by Cu, Mn, H1 elements lower equilibrium pressure and

the relation is: PH(LaNi5)�/PH(LaNi4Cu)�/

PH(LaNi4Mn)�/PH(LaNi4Al). Based on both self-con-

sistent-charge calculation and cluster model shown inFig. 1 the relationship between equilibrium plateau

pressure and number of transferred charge on H atom

entering octahedron center was obtained and shown in

Fig. 2. It can be seen from Fig. 2 that charge number on

H1s orbit increases with M�/Al, Mn, Cu, Ni for

LaNi4M in turn.

Fig. 3 illustrates density of state (DOS) of hydrogen

atom entering octahedron center of alloy, and it is clearthat the DOS is composed of both bonding and non-

bonding area. The stability of LaNi4M hydride, which

increasing causes equilibrium plateau pressure lowered,

is related to charge transfer of hydrogen atom closely.

Based on relationship between number of transferred

charge on hydrogen and equilibrium plateau pressure

shown in Fig. 2 and DOS analysis in Fig. 3, it can be

found that charge entering H1s orbit is contributed tonon-bonding area, and with the charge decreasing the

stability of LaNi4M hydride will be increased.

Another important property for hydrogen storage

alloy is cycle life which is defined as the ratio of initial

capacity to capacity after n cycle, Cn /C0. Fig. 4 and Fig.

5 illustrate the relations between cycle life of LaNi4M

and transferred charge on La and Ni(1) atom, respec-

tively. It can be seen from the two figures that the cyclelife of LaNi4M increases with transferred charge in-

creasing, which transfer from N(1) to La when hydrogen

Fig. 1. Cluster model used in the calculation.

Fig. 2. The relationship between the number of transferred charge on

H and equilibrium plateau pressure.

J. Guo et al. / Materials Science and Engineering B98 (2003) 21�/2422

is absorbed. Besides, cycle life effected by charge

number shown in Fig. 4 and Fig. 5 can be explained

by the distribution of DOS of Ni(1) and La atom in

LaNi4M (M�/Ni, Cu, Mn, Al). Since each La atom in

LaNi4M (M�/Ni, Cu, Mn, Al) has the same distribu-

tion of 4f-type DOS, only the 4f-type DOS of La atom

in LaNi4Al hydride is shown in Fig. 6. It can be seen

that 4f-type DOS of La atom in LaNi4M (M�/Mn, Ni,

Cu, Al) only composed of a peak stepping over Fermi

energy before hydrogen entering the center of octahe-

dron. The peak, however, is divided into parts by the

hydrogenation, one forming bonding area another

forming non-bondong area. Fig. 7 shows the 4p-type

two DOS of Ni(1) atom in LaNi4M (M�/Ni, Cu, Mn),

where the 4p-type DOS of Ni(1) atom in LaNi5 hydride

is chosen because the distribution of 4p-type DOS of

Ni(1) in LaNi4M (M�/Ni, Cu, Mn) is about the same,

and illustrates that there is the same bond between La 4f

orbit and Ni(1) 4p orbit in LaNi4M (M�/Ni, Cu, Mn).

Fig. 8 shows the 4p-type DOS of Ni(1) atom in LaNi4Al

alloy. After hydrogenation, bonding peak of 4p-type

DOS of Ni(1) atom in LaNi4Al moves nearer to Fermi

energy and is much higher than those in LaNi4M (M�/

Ni, Cu, Mn) shown in Fig. 7. Compared with bonding

peak shown in Fig. 7 and Fig. 8, the bonding peak of 4p-

type DOS of Ni(1) atom in LaNi4Al is much higher than

those in LaNi4M (M�/Ni, Cu, Mn). It illustrates that

the bond while LaNi4 hydride forming between Ni(1) 4p

orbit and La 4f orbit in LaNi4Al is stronger than those

in LaNi4M (M�/Ni, Cu, Mn). It may be the major

reason that the cycle life for LaNi4Al alloy is much

longer than those for LaNi4M (M�/Ni, Cu, Mn) and

the cycle life for LaNi4M (M�/Ni, Cu, Mn) is about the

same. The above argument is in agreement with experi-

mental evidence [2].Since Ni(3) atom in LaNi4M has the same distribution

of 3d-type DOS, only the 3d-type DOS of Ni(3) atom in

LaNi5 is shown in Fig. 9. It can be seen from Fig. 9 that

there is little change occurred in 3d-type DOS of Ni(3)

atom while hydride forming, and illustrates that there is

no action between Ni(3) 3d and H1s orbit. Before

Fig. 3. The 1s-type DOS of H in La2Ni12H.

Fig. 4. The relationship between the number of transferred charge on

La and capacity ratio.

Fig. 5. The relationship between the number of transferred charge on

Ni(1) and capacity ratio.

Fig. 6. The 4f-type DOS of La in La2Ni10Al2 (a) and La2Ni10Al2H (b).

Fig. 7. The 4p-type DOS of Ni(1) in La2Ni12 (a) and La2Ni12H (b).

J. Guo et al. / Materials Science and Engineering B98 (2003) 21�/24 23

hydrogen entering into the center of octahedron, DOS

peak, including Ni(3) 3d-type and La 4f-type, arises at

Fermi energy. The overlap DOS between Ni(3) 3d orbitand La 4f orbit in the cluster illustrates that there is a

strong reaction between these orbits, and form non-

localized chemical bond along the lines of c axis in unit

cell. The non-localized bond, however, is broken by

hydrogenation since 4f-type DOS of La atom divided

into bonding and non-bonding part and departed from

Fermi energy shown in Fig. 6. That may be one of

reasons which cause storage hydrogen alloy powderedafter absorbing and desorbing hydrogen repeatedly.

4. Conclusion

1) The stability of LaNi4M hydride is related to charge

transfer of hydrogen atom closely. Charge entering

H1s orbit is contributed to non-bonding area, and

with the charge decreasing the stability of LaNi4M

(M�/Ni, Cu, Mn) hydride will be increased.2) The bonding action between Ni(1) 4p and La 4f

orbit in LaNi4M alloy is major factor effecting cycle

life for alloy. The action strengthened is favorable to

cycle life.

3) Ni(3) 3d and La 4f orbit form non-localized bond

along the line of c axis, but the bond is broken by

hydrogenation, which is one of reasons causing

storage hydrogen alloy powdered after absorbingand desorbing hydrogen repeatedly.

Acknowledgements

This work was supported by the National Nature

Science Foundation of China (59861001, 50171023), the

Nature Science Foundation of Guangxi (0144033,

2000220).

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Fig. 8. The 4p-type DOS of Ni(1) in La2Ni10Al2 (a) and La2Ni10Al2H

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Fig. 9. The 3d-type DOS of Ni(3) in La2Ni12 (a) and La2Ni12H (b).

J. Guo et al. / Materials Science and Engineering B98 (2003) 21�/2424