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A study on the correlation between electronic structures of RENi5 and their hydrides and hydrogen absorption properties
Journal of Physics and Chemistry of Solids 62 (2001) 2055±2058
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A study on the correlation between electronic structures of RENi5 and their hydrides and hydrogen absorption properties Guo Jin a,*, Huang Li b b
a Department of Physics, Guangxi University, Nanning 530004, China Department of Chemistry and Chemical Engineering, Guangxi Normal University, Guilin 541004, China
Received 24 November 2000; revised 6 January 2001; accepted 13 February 2001
Abstract The electronic structures of RENi5 (RE La, Ce, Pr, Nd) and their hydrides were investigated by the SCC-DV-Xa (SelfConsistent-Charge Discrete Variational Xa ) method. The results show that the stabilities of RENi5 hydrides are related to the charge transferred to the hydrogen atom, and decrease with increase of the transferred charge. There is a strong orbital interaction between the 3d orbit in the Ni atom and the 4f orbit in the RE atom, which is weakened when the hydride is formed. The equilibrium plateau pressure of RENi5 rises with lowering of the Fermi energy of the hydride. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: D. Electronic structure
1. Introduction In recent years, Ni/MH batteries have been widely used [1±3] and called the `the green battery' for their high energy density, long cycle life, low memory effect and the absence of cadmium or other toxic heavy metals. The negative electrode materials of Ni/MH batteries are hydrogen storage alloys. The hydrogen absorption properties of these alloys are very much dependent on the constituent metals involved even for isostructural compounds. For example, ternary alloys LaNi52xMx (M Al, Co, Cr, Cu, Mn) where some of the 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 structure, exhibit very different behavior in absorbing hydrogen plateau pressure [5]. In order to elucidate theoretically why there is much difference of hydrogen absorption properties among these alloys, Yukawa et al. [6] have investigated the alloying effect on the electronic structure of hydrogenated LaNi5, and have shown that the hydride properties are well understood in terms of the nature of the * Corresponding author. E-mail address: [email protected] (Guo Jin).
chemical bond between atoms in a small octahedron in which the absorbed hydrogen atom is located. Lin et al. [7] have also studied the relation between equilibrium plateau pressure and Fermi energy of RENi5 (RE La, Ce, Pr, Nd), and explained why these alloys exhibit very different equilibrium plateau pressure. In spite of these investigations interest, theoretical studies of the electronic structure of these alloys and stability of these hydrides are still lacking. Therefore, it is necessary to investigate the changes of electronic structure after hydrogen is absorbed and their effects on hydrogen absorption properties. In the present study, the electronic structures of RENi5 (RE La, Ce, Pr, Nd) and their hydrides are calculated by the Self-Consistent-Charge Discrete Variational Xa (SCCDV-Xa ) molecular orbital method, and the 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 compounds, RENi5 (RE La, Ce, Pr, Nd), using cluster model, Re4Ni12. The cluster model used, which is shown in Fig. 1 [6], is constructed on the basis of the crystal structure of LaNi5Hx
0022-3697/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0022- 369 7( 01) 00073-7
2056
G. Jin, H. Li / Journal of Physics and Chemistry of Solids 62 (2001) 2055±2058
Here, the Coulomb potential is the sum of nuclear and electronic contributions Vcoul r 2
X v
Z dr 0 r r 0 Zv 1 ; ur 2 Rv u ur 2 r 0 u
the statistical exchange potential in these calculations is de®ned as 3r r 3=2 ; Vxc r 23a 8p Fig. 1. Cluster model used in the calculation.
(x , 0.4). A hydrogen atom occupies the 3f site in the crystal with P6/mmm space group. That is a center site of the octahedron with the frame of four Ni atoms and two rare earth atoms. The hydrogenated clusters of RENi5 were expanded slightly to be about the same size as the 3f size of a phase RENi5 hydrides, maintaining the shape of the octahedron. The coef®cient of volume expansion of the cluster adopted for the calculations was about 4% [8]. In order to calculate the electronic states of RENi5 and their hydrides, we adopted the SCC-DV-Xa method which is a molecular orbital computing method [9]. The SCC-DVXa method is based on the nonrelativistic one-electron Hamiltonian equation h^ 2 12 7 2 1 V r 2 12 7 2 1 Vcoul r 1 Vxc r
where a is an exchange constant, which is normally chosen as 2/3 # a # 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 orbitals,
C i r
n X k1
Cki wk r
the coef®cient Cki can be obtained by the secular equation X Hkl 2 ESkl Cli 0 l
In the present calculation, the frozen shell model is used and outer shell electronic con®gurations are chosen as: La-4s24p64d104f05s25p65d16s26p0; Ce-4s24p64d104f25s25p65d06s26p0; Pr-4s24p64d104f35s25p65d06s26p0; Nd-4s24p64d104f45s25p65d06s26p0; Ni-3s 23p 63d 84s 24p 0; and H-1s 1. We have used the single-site
Table 1 The charge population in atomic orbits of RE2Ni12 and RE2Ni12H Atomic orbits H 1s 4s 4p 4d 4f 5s 5p 5d 6s 6p Ni(1) 3d 4s 4p Ni(2) 3d 4s 4p Ni(3) 3d 4s 4p
La2Ni12
La 1.9927 5.9749 9.9548 0.47 1.9638 5.6561 1.0668 1.5935 0.2475
La2Ni12H 1.3464
Ce2Ni12
1.8963 5.9544 9.9431 1.5225 1.9005 5.2579 1.1884 1.0601 0.3597
Ce 1.9782 4.8751 10.043 3.4143 2.3135 4.3908 2.1448 0.462 1.9203
8.2871 1.5543 0.4256
8.3272 0.9534 1.263
8.2433 1.395 0.5709 8.2994 1.5116 0.0831
Ce2Ni12H 1.4231
Pr2Ni12
1.8821 5.955 9.9018 2.7047 1.9165 5.2922 1.2279 1.2562 0.4431
Pr 1.9858 5.957 9.9117 3.2929 1.9057 5.3482 0.7858 1.4403 0.5914
8.313 0.8423 1.4178
8.3149 0.7924 1.4643
8.2135 0.6538 1.216
8.2794 0.428 1.4615
8.3791 0.9903 0.3809
8.39 0.8541 0.3824
Pr2Ni12H 1.4045
Nd2Ni12
Nd2Ni12H 1.3892
1.904 5.9626 9.8687 3.806 1.8397 5.3244 1.1132 1.2939 0.4017
Nd 1.9911 5.9728 9.9383 4.1189 1.9403 5.582 0.4507 1.6451 0.3445
1.9189 5.9683 9.8433 4.4634 1.8609 5.5027 1.1753 1.3757 0.4599
8.3122 1.2355 0.8566
8.316 0.797 1.3778
8.2823 1.5224 0.493
8.3169 0.8385 1.2933
8.2839 0.356 1.1725
8.3028 0.8763 1.0415
8.2835 0.417 1.2089
8.2503 1.3042 0.6159
8.2766 0.4658 1.2118
8.4094 0.8326 0.4268
8.3776 1.2053 0.1909
8.4038 0.8397 0.4433
8.3049 1.4873 0.0867
8.4156 0.8587 0.4028
G. Jin, H. Li / Journal of Physics and Chemistry of Solids 62 (2001) 2055±2058
orbital and chosen the Self-Consistent-Charge and Discrete Variational method for calculations. 3. Results and discussion Based on self-consistent-charge calculating, the results of a Mulliken population analysis of the molecular eigenfunctions are used in each iteration to produce `atomic' occupation numbers. The charge populations in atomic orbits of RENi5 and their hydrides are shown in Table 1. The table illustrates that more transferred atomic charges on the 4p, 4f, 5p, 5d, 5s, 6p orbits in the Ce atom have occurred than those in La, Pr and Nd atom. The change in the equilibrium hydrogen pressure at 298 K with RE atom is shown in Fig. 2 [5]. It can be seen from these experimental data that CeNi5 has equilibrium pressure for absorption much higher than LaNi5, PrNi5 and NdNi5 have. When the equilibrium pressure shown in Fig. 2 is compared with the transferred atomic charges on the 4p, 4f, 5d, 5s and 6p orbits in RE atom listed in Table 1, it is found that the large range of transferred orbital charges in the Ce atom may be the reason that causes the equilibrium pressure of CeNi5 alloy to increase greatly. Furthermore, it can be seen from Table 1 that little change occurs on the charge population of the 3d orbit in the Ni atom, which illustrates that electrons in the 3d orbit in the Ni atom do not react with the H atomic orbit, although these electrons take part in other orbital bonding. Fig. 2 illustrates the relation between equilibrium pressure of RENi5 and hydride Fermi energy. It is clear that the equilibrium pressure of the alloy rises with hydride Fermi energy lowering. Fig. 3 illustrates density of states (DOS) of hydrogen atom, and it is clear that the DOS is composed of both
Fig. 2. The relation between Ef and equilibrium plateau pressure.
Fig. 3. The s-type DOS of H in La2Ni12H.
2057
bonding and non-bonding areas. The stability of RENi5 hydride is related to charge transfer of hydrogen atom closely. Based on charge population analysis of hydrogen atom in Table 1 and DOS analysis in Fig. 3, it can be found that charges entering into the H 1s orbit are contributed to the non-bonding area and, with the charges increasing, the stability of RENi5 hydride will be decreased. From the above argument and the charge population analysis of the H 1s orbit in Table 1, the order describing stability of RENi5 (RE La. Ce, Pr, Nd) hydride can be obtained: LaNi5H is the most stable, CeNi5H is the least, and the stability of PrNi5H and NdNi5H are intermediate. The order is in agreement with experimental evidence [2]. Since each Ni in RENi5, whether Ni(1) or Ni(2), has the same distribution of 3d-type DOS, only the 3d-type DOS of Ni in LaNi5 is shown in Fig. 4. It can be seen from Fig. 4 that little change occurs in the 3d-type DOS of Ni atom with hydride forming, and illustrates that there is no action between the Ni 3d and H 1s orbits. Figs. 5a±8a show the 4f-type DOS of rare earth atoms in RENi5 alloys. It can be seen from these ®gures that the shape, only composed of a peak stepping over Fermi energy, of the 4f-type DOS of RE (RE La, Pr, Nd) atom in RENi5 alloy is almost the same shown in Figs. 5a±7a. The peak, however, is divided into two parts by the hydrogenation, one forming the bonding area and the other forming the nonbonding area shown in Figs. 5b±8b. Compared with that of La, Pr and Nd, the 4f-type DOS of Ce atom in Ce2Ni12 cell shown in Fig. 8a exhibits a completely different distribution, there is a bonding state at low energy and a non-bonding peak. It is believed that a non-bonding peak at the 4f-type DOS of rare earth is the major character exhibiting high equilibrium pressure in the alloy.
Fig. 4. The 3d-type DOS of Ni in La2Ni12 (a) and La2Ni12H (b).
Fig. 5. The 4f-type DOS of La in La2Ni12 (a) and La2Ni12H (b).
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G. Jin, H. Li / Journal of Physics and Chemistry of Solids 62 (2001) 2055±2058
storage hydrogen alloy powdered after absorbing and desorbing hydrogen repeatedly. 4. Conclusion
Fig. 6. The 4f-type DOS of Pr in Pr2Ni12 (a) and Pr2Ni12H (b).
Fig. 7. The 4f-type DOS of Nd in Nd2Ni12 (a) and Nd2Ni12H (b).
1. The stability of RENi5 hydride is closely related to the charge transfer of the hydrogen atom and stability will be decreased with the H 1s orbital charge increasing. 2. A non-bonding peak at the 4f-type DOS of the rare earth is the major character exhibiting high equilibrium pressure in the alloy. There is a strong reaction between the Ni 3d orbit and the RE 4f orbit, but no action between the Ni 3d orbit and the H 1s orbit. The reaction between the Ni 3d orbit and the RE 4f orbit forms a non-localized chemical bond, and the bond is broken by hydrogenation. 3. The equilibrium pressure of the alloy rises with hydride Fermi energy lowering.
Acknowledgements This work was supported by the National Nature Science Foundation of China (59861001), the Nature Science Foundation of Guangxi (9912005), the Young Teacher's Foundation of the National Education Department (1999-5). References
Fig. 8. The 4f-type DOS of Ce in Ce2Ni12 (a) and Ce2Ni12H (b).
Before hydrogen enters into the center of the octahedron, DOS peak, including Ni 3d-type and RE 4f-type, arises at Fermi energy. The overlap DOS between the Ni 3d orbit and the RE 4f orbit in the cluster illustrates that there is a strong reaction between these orbits, and they form a non-localized chemical bond. The non-localized bond, however, is broken by hydrogenation since the 4f-type DOS of rare earth is divided into a bonding and a non-bonding part and departs from Fermi energy. That may be one of reasons which cause
[1] J.J.G. Willems, K.H. Buschow, J. Less-Common Met. 13 (1987) 129. [2] H.F. Bittner, C.C. Badcook, J. Electrochem. Soc. 193 (1983) 130. [3] J.J.G. Willems, Philips J. Res. 1 (Suppl. 1) (1983) 39. [4] T. Sakai, K. Oguro, H. Miyamura, N. Kuriyama, J. LessCommon Met. 161 (1990) 193±202. [5] G.C. Che, J.K. Jiang, Y.D. Yu, (in Chinese), Acta Metallurgica Sinica 22 (5) (1986) B206±B210. [6] H. Yukawa, M. Moringa, Y. Takanashi, J. Alloys Comp. 253± 254 (1997) 322±325. [7] N. Chen, Q. Lin, W. Ye, Chinese Science Bulletin 40 (24) (1995) 2234±2236. [8] T. Suenobu, I. Tanaka, H. Adachi, G. Adachi, J. Alloys Comp. 221 (1995) 200±206. [9] A. Rosen, D.E. Ellis, H. Adachi, F.W. Averill, J. Chem. Phys. 65 (1976) 3629±3634.
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A study on the correlation between electronic structures of RENi5 and their hydrides and hydrogen absorption properties Guo Jin a,*, Huang Li b b
a Department of Physics, Guangxi University, Nanning 530004, China Department of Chemistry and Chemical Engineering, Guangxi Normal University, Guilin 541004, China
Received 24 November 2000; revised 6 January 2001; accepted 13 February 2001
Abstract The electronic structures of RENi5 (RE La, Ce, Pr, Nd) and their hydrides were investigated by the SCC-DV-Xa (SelfConsistent-Charge Discrete Variational Xa ) method. The results show that the stabilities of RENi5 hydrides are related to the charge transferred to the hydrogen atom, and decrease with increase of the transferred charge. There is a strong orbital interaction between the 3d orbit in the Ni atom and the 4f orbit in the RE atom, which is weakened when the hydride is formed. The equilibrium plateau pressure of RENi5 rises with lowering of the Fermi energy of the hydride. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: D. Electronic structure
1. Introduction In recent years, Ni/MH batteries have been widely used [1±3] and called the `the green battery' for their high energy density, long cycle life, low memory effect and the absence of cadmium or other toxic heavy metals. The negative electrode materials of Ni/MH batteries are hydrogen storage alloys. The hydrogen absorption properties of these alloys are very much dependent on the constituent metals involved even for isostructural compounds. For example, ternary alloys LaNi52xMx (M Al, Co, Cr, Cu, Mn) where some of the 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 structure, exhibit very different behavior in absorbing hydrogen plateau pressure [5]. In order to elucidate theoretically why there is much difference of hydrogen absorption properties among these alloys, Yukawa et al. [6] have investigated the alloying effect on the electronic structure of hydrogenated LaNi5, and have shown that the hydride properties are well understood in terms of the nature of the * Corresponding author. E-mail address: [email protected] (Guo Jin).
chemical bond between atoms in a small octahedron in which the absorbed hydrogen atom is located. Lin et al. [7] have also studied the relation between equilibrium plateau pressure and Fermi energy of RENi5 (RE La, Ce, Pr, Nd), and explained why these alloys exhibit very different equilibrium plateau pressure. In spite of these investigations interest, theoretical studies of the electronic structure of these alloys and stability of these hydrides are still lacking. Therefore, it is necessary to investigate the changes of electronic structure after hydrogen is absorbed and their effects on hydrogen absorption properties. In the present study, the electronic structures of RENi5 (RE La, Ce, Pr, Nd) and their hydrides are calculated by the Self-Consistent-Charge Discrete Variational Xa (SCCDV-Xa ) molecular orbital method, and the 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 compounds, RENi5 (RE La, Ce, Pr, Nd), using cluster model, Re4Ni12. The cluster model used, which is shown in Fig. 1 [6], is constructed on the basis of the crystal structure of LaNi5Hx
0022-3697/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0022- 369 7( 01) 00073-7
2056
G. Jin, H. Li / Journal of Physics and Chemistry of Solids 62 (2001) 2055±2058
Here, the Coulomb potential is the sum of nuclear and electronic contributions Vcoul r 2
X v
Z dr 0 r r 0 Zv 1 ; ur 2 Rv u ur 2 r 0 u
the statistical exchange potential in these calculations is de®ned as 3r r 3=2 ; Vxc r 23a 8p Fig. 1. Cluster model used in the calculation.
(x , 0.4). A hydrogen atom occupies the 3f site in the crystal with P6/mmm space group. That is a center site of the octahedron with the frame of four Ni atoms and two rare earth atoms. The hydrogenated clusters of RENi5 were expanded slightly to be about the same size as the 3f size of a phase RENi5 hydrides, maintaining the shape of the octahedron. The coef®cient of volume expansion of the cluster adopted for the calculations was about 4% [8]. In order to calculate the electronic states of RENi5 and their hydrides, we adopted the SCC-DV-Xa method which is a molecular orbital computing method [9]. The SCC-DVXa method is based on the nonrelativistic one-electron Hamiltonian equation h^ 2 12 7 2 1 V r 2 12 7 2 1 Vcoul r 1 Vxc r
where a is an exchange constant, which is normally chosen as 2/3 # a # 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 orbitals,
C i r
n X k1
Cki wk r
the coef®cient Cki can be obtained by the secular equation X Hkl 2 ESkl Cli 0 l
In the present calculation, the frozen shell model is used and outer shell electronic con®gurations are chosen as: La-4s24p64d104f05s25p65d16s26p0; Ce-4s24p64d104f25s25p65d06s26p0; Pr-4s24p64d104f35s25p65d06s26p0; Nd-4s24p64d104f45s25p65d06s26p0; Ni-3s 23p 63d 84s 24p 0; and H-1s 1. We have used the single-site
Table 1 The charge population in atomic orbits of RE2Ni12 and RE2Ni12H Atomic orbits H 1s 4s 4p 4d 4f 5s 5p 5d 6s 6p Ni(1) 3d 4s 4p Ni(2) 3d 4s 4p Ni(3) 3d 4s 4p
La2Ni12
La 1.9927 5.9749 9.9548 0.47 1.9638 5.6561 1.0668 1.5935 0.2475
La2Ni12H 1.3464
Ce2Ni12
1.8963 5.9544 9.9431 1.5225 1.9005 5.2579 1.1884 1.0601 0.3597
Ce 1.9782 4.8751 10.043 3.4143 2.3135 4.3908 2.1448 0.462 1.9203
8.2871 1.5543 0.4256
8.3272 0.9534 1.263
8.2433 1.395 0.5709 8.2994 1.5116 0.0831
Ce2Ni12H 1.4231
Pr2Ni12
1.8821 5.955 9.9018 2.7047 1.9165 5.2922 1.2279 1.2562 0.4431
Pr 1.9858 5.957 9.9117 3.2929 1.9057 5.3482 0.7858 1.4403 0.5914
8.313 0.8423 1.4178
8.3149 0.7924 1.4643
8.2135 0.6538 1.216
8.2794 0.428 1.4615
8.3791 0.9903 0.3809
8.39 0.8541 0.3824
Pr2Ni12H 1.4045
Nd2Ni12
Nd2Ni12H 1.3892
1.904 5.9626 9.8687 3.806 1.8397 5.3244 1.1132 1.2939 0.4017
Nd 1.9911 5.9728 9.9383 4.1189 1.9403 5.582 0.4507 1.6451 0.3445
1.9189 5.9683 9.8433 4.4634 1.8609 5.5027 1.1753 1.3757 0.4599
8.3122 1.2355 0.8566
8.316 0.797 1.3778
8.2823 1.5224 0.493
8.3169 0.8385 1.2933
8.2839 0.356 1.1725
8.3028 0.8763 1.0415
8.2835 0.417 1.2089
8.2503 1.3042 0.6159
8.2766 0.4658 1.2118
8.4094 0.8326 0.4268
8.3776 1.2053 0.1909
8.4038 0.8397 0.4433
8.3049 1.4873 0.0867
8.4156 0.8587 0.4028
G. Jin, H. Li / Journal of Physics and Chemistry of Solids 62 (2001) 2055±2058
orbital and chosen the Self-Consistent-Charge and Discrete Variational method for calculations. 3. Results and discussion Based on self-consistent-charge calculating, the results of a Mulliken population analysis of the molecular eigenfunctions are used in each iteration to produce `atomic' occupation numbers. The charge populations in atomic orbits of RENi5 and their hydrides are shown in Table 1. The table illustrates that more transferred atomic charges on the 4p, 4f, 5p, 5d, 5s, 6p orbits in the Ce atom have occurred than those in La, Pr and Nd atom. The change in the equilibrium hydrogen pressure at 298 K with RE atom is shown in Fig. 2 [5]. It can be seen from these experimental data that CeNi5 has equilibrium pressure for absorption much higher than LaNi5, PrNi5 and NdNi5 have. When the equilibrium pressure shown in Fig. 2 is compared with the transferred atomic charges on the 4p, 4f, 5d, 5s and 6p orbits in RE atom listed in Table 1, it is found that the large range of transferred orbital charges in the Ce atom may be the reason that causes the equilibrium pressure of CeNi5 alloy to increase greatly. Furthermore, it can be seen from Table 1 that little change occurs on the charge population of the 3d orbit in the Ni atom, which illustrates that electrons in the 3d orbit in the Ni atom do not react with the H atomic orbit, although these electrons take part in other orbital bonding. Fig. 2 illustrates the relation between equilibrium pressure of RENi5 and hydride Fermi energy. It is clear that the equilibrium pressure of the alloy rises with hydride Fermi energy lowering. Fig. 3 illustrates density of states (DOS) of hydrogen atom, and it is clear that the DOS is composed of both
Fig. 2. The relation between Ef and equilibrium plateau pressure.
Fig. 3. The s-type DOS of H in La2Ni12H.
2057
bonding and non-bonding areas. The stability of RENi5 hydride is related to charge transfer of hydrogen atom closely. Based on charge population analysis of hydrogen atom in Table 1 and DOS analysis in Fig. 3, it can be found that charges entering into the H 1s orbit are contributed to the non-bonding area and, with the charges increasing, the stability of RENi5 hydride will be decreased. From the above argument and the charge population analysis of the H 1s orbit in Table 1, the order describing stability of RENi5 (RE La. Ce, Pr, Nd) hydride can be obtained: LaNi5H is the most stable, CeNi5H is the least, and the stability of PrNi5H and NdNi5H are intermediate. The order is in agreement with experimental evidence [2]. Since each Ni in RENi5, whether Ni(1) or Ni(2), has the same distribution of 3d-type DOS, only the 3d-type DOS of Ni in LaNi5 is shown in Fig. 4. It can be seen from Fig. 4 that little change occurs in the 3d-type DOS of Ni atom with hydride forming, and illustrates that there is no action between the Ni 3d and H 1s orbits. Figs. 5a±8a show the 4f-type DOS of rare earth atoms in RENi5 alloys. It can be seen from these ®gures that the shape, only composed of a peak stepping over Fermi energy, of the 4f-type DOS of RE (RE La, Pr, Nd) atom in RENi5 alloy is almost the same shown in Figs. 5a±7a. The peak, however, is divided into two parts by the hydrogenation, one forming the bonding area and the other forming the nonbonding area shown in Figs. 5b±8b. Compared with that of La, Pr and Nd, the 4f-type DOS of Ce atom in Ce2Ni12 cell shown in Fig. 8a exhibits a completely different distribution, there is a bonding state at low energy and a non-bonding peak. It is believed that a non-bonding peak at the 4f-type DOS of rare earth is the major character exhibiting high equilibrium pressure in the alloy.
Fig. 4. The 3d-type DOS of Ni in La2Ni12 (a) and La2Ni12H (b).
Fig. 5. The 4f-type DOS of La in La2Ni12 (a) and La2Ni12H (b).
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G. Jin, H. Li / Journal of Physics and Chemistry of Solids 62 (2001) 2055±2058
storage hydrogen alloy powdered after absorbing and desorbing hydrogen repeatedly. 4. Conclusion
Fig. 6. The 4f-type DOS of Pr in Pr2Ni12 (a) and Pr2Ni12H (b).
Fig. 7. The 4f-type DOS of Nd in Nd2Ni12 (a) and Nd2Ni12H (b).
1. The stability of RENi5 hydride is closely related to the charge transfer of the hydrogen atom and stability will be decreased with the H 1s orbital charge increasing. 2. A non-bonding peak at the 4f-type DOS of the rare earth is the major character exhibiting high equilibrium pressure in the alloy. There is a strong reaction between the Ni 3d orbit and the RE 4f orbit, but no action between the Ni 3d orbit and the H 1s orbit. The reaction between the Ni 3d orbit and the RE 4f orbit forms a non-localized chemical bond, and the bond is broken by hydrogenation. 3. The equilibrium pressure of the alloy rises with hydride Fermi energy lowering.
Acknowledgements This work was supported by the National Nature Science Foundation of China (59861001), the Nature Science Foundation of Guangxi (9912005), the Young Teacher's Foundation of the National Education Department (1999-5). References
Fig. 8. The 4f-type DOS of Ce in Ce2Ni12 (a) and Ce2Ni12H (b).
Before hydrogen enters into the center of the octahedron, DOS peak, including Ni 3d-type and RE 4f-type, arises at Fermi energy. The overlap DOS between the Ni 3d orbit and the RE 4f orbit in the cluster illustrates that there is a strong reaction between these orbits, and they form a non-localized chemical bond. The non-localized bond, however, is broken by hydrogenation since the 4f-type DOS of rare earth is divided into a bonding and a non-bonding part and departs from Fermi energy. That may be one of reasons which cause
[1] J.J.G. Willems, K.H. Buschow, J. Less-Common Met. 13 (1987) 129. [2] H.F. Bittner, C.C. Badcook, J. Electrochem. Soc. 193 (1983) 130. [3] J.J.G. Willems, Philips J. Res. 1 (Suppl. 1) (1983) 39. [4] T. Sakai, K. Oguro, H. Miyamura, N. Kuriyama, J. LessCommon Met. 161 (1990) 193±202. [5] G.C. Che, J.K. Jiang, Y.D. Yu, (in Chinese), Acta Metallurgica Sinica 22 (5) (1986) B206±B210. [6] H. Yukawa, M. Moringa, Y. Takanashi, J. Alloys Comp. 253± 254 (1997) 322±325. [7] N. Chen, Q. Lin, W. Ye, Chinese Science Bulletin 40 (24) (1995) 2234±2236. [8] T. Suenobu, I. Tanaka, H. Adachi, G. Adachi, J. Alloys Comp. 221 (1995) 200±206. [9] A. Rosen, D.E. Ellis, H. Adachi, F.W. Averill, J. Chem. Phys. 65 (1976) 3629±3634.