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Electronic structure of the LaNi5-xGax intermetallic compounds
Acta Metall. Sin.(Engl. Lett.) Vol.21 No.3 pp157-162 June 2008
ELECTRONIC STRUCTURE OF THE LaNi5−x Gax INTERMETALLIC COMPOUNDS D. Chen1,3)∗ , G.X. Li2) , D.L. Zhang1) and T. Gao3) 1) Department of Physics and Electronic Engineering, Xinyang Normal University, Xinyang 464000, China 2) College of Science, Qingdao Agriculture University, Qingdao 266109, China 3) Institute of Atomic and Molecular Physics, Sichuan University, Chengdu 610065, China Manuscript received 6 April 2007; in revised form 24 October 2007
The equilibrium structures and electronic structure of LaNi5−x Gax (x=0, 0.5, 1.0) compounds have been investigated by all-electron calculations. Based on the full geometry optimization, the densities of states and electron densities of LaNi5−x Gax are plotted and analyzed. It is clear that the substitution of Ga at the Ni site leads to a progressive filling of the Ni-d bands, the ionic interaction between Ni and Ni, with Ga plays a dominant role in the stability of LaNi5−x Gax compounds. The smaller the shift of EF toward higher energy region, the more stable the compounds will be. The increased contribution of the Ni-d-Ga-d interactions near EF and the low energy metal-gallium bonding bands indicate that the compounds become more stable. The results are compared with experimental data and discussed in light of previous studies. KEY WORDS FLAPW and GGA; LaNi5−x Gax ; Intermetallic compound; Electronic structure
1. Introduction The storage of hydrogen in the form of metal hydrides has been investigated for the last three decades[1−4] . LaNi5 alloys are of great technological interest in their applications, such as compressors, heat pumps, rechargeable batteries, and energy conversion materials[5−7] , besides, LaNi5 -H system is accepted as a prototype of hydrogen absorbing materials for its excellent hydrogenation properties. Much more hydrogen can be stored in the same volume compared to liquefied form. Partial substitution by M (M =Al, Co, Sn, Ga, Ge) for Ni in the LaNi5 improves some of the practical properties of these hydrogen storage compounds[8−12] . LaNi5−x Gax compounds have been investigated by X-ray diffraction since they affect the stability, the cycling lifetimes and the hydrogen content of the hydrides[13,14] . In LaNi5−x Gax systems, the Gibbs free energy of formation increases with x much slowly compared with LaNi5−x M x (M =Al, Sn, Si); this indicates a greater stability at given compositions of LaNi5−x Gax systems if compared with the systems containing Al, Sn and Si. Besides, the addition of Ga improves the low-temperature magnetic properties of LaNi3 Ga2 . The aim of this article is to present the results of a study on the electronic ∗
Corresponding author. Tel.: +86 13253854009. E-mail address: [email protected] (D. Chen)
· 158 · properties of LaNi5−x Gax when Ga is partially substituted for nickel. Understanding the electronic structure of LaNi5−x Gax compounds from a fundamental point of view and in relation to several aspects of their technological applications is important[15] . In this article, the crystal structure, electronic structure and the Ga site occupancy of LaNi5−x Gax (x=0, 0.5, 1.0) have been investigated by all-electron calculations. More information on the electronic structure of LaNi5−x Gax is obtained. The calculated electronic properties have been compared with the experimental results. 2. Crystal Structures and Computational Details LaNi5 crystal has a hexagonal CaCu5 structure (space group P 6/mmm). The La atom occupies the 1a (0, 0, 0) Wyckoff site. The two nonequivalent Ni atoms occupy the 2c (2/3, 1/3, 0) and the 3g (0.5, 0, 0.5) sites. X-ray powder diffraction[13] shows that the LaNi5−x Gax alloys still retain the CaCu5 structure until x≤2.5. To find the Ga-occupied site, two single-unit cell models are built: (1) the Ga atom substitutes the Ni atom at the 2c site; (2) the Ga atom substitutes the Ni atom at the 3g site. The result indicates that the Ga atom prefers to occupy the 3g site, which is shown in Fig.1. To analyze LaNi4.5 Ga0.5 system, a double unit cell is built along the c axis of the LaNi5 unit cell. Ga atom substitutes for Ni atom in the 3g (0.5a, 0.5a, 0.75c) site as shown in Fig.2.
Fig.1 Unit cell used for LaNi4 Ga (3g).
Fig.2 Model unit cell used for LaNi4.5 Ga0.5 (two formula units).
The calculations have been performed using the density functional theory (DFT) in the generalized gradient approximation (GGA)[16] . The Full-Potential Linearized Augmented Plane Wave (FLAPW) method has been used[17] . The generalized gradient approximation of PBE96[18] was used for the exchange-correlation energy functional. Atomic geometries were obtained by minimizing the Hellman-Feynman forces. The plane wave cutoff energy is chosen as 300 eV and the RKmax is chosen as 8.0 to get a good plane wave basis set. As for the Brillouin zone sampling, the 12×12×5 Monkhorst-Pack mesh was used, which is sufficient to make a good self-consistent convergence of the total energy.
· 159 · 3. Results and Discussion 3.1 Optimized structures The calculated lattice constants and the total energy for LaNi5−x Gax , the total density of states (TDOS) at EF and the electronic specific heat coefficients are listed in Table 1. Table 1 Lattice constants, EF , total energy, DOSs at EF and the electronic specific heat γ for LaNi5−x Gax Calculated a/nm c/nm LaNi5
Experimental a/nm
c/nm
EF
Energy
n(EF )
γ
eV
0.5021 0.3964 0.5017[19] 0.3986[19] −0.7053 –438154.6893 12.29
29.01 mJ/(mol LaNi5 ·K2 )
LaNi4.5 Ga0.5 0.5038 0.3990 0.5049[11] 0.4028[11] −0.6981 –443912.5317
6.71
15.83 mJ/(mol LaNi4.5 Ga0.5 ·K2 )
LaNi4 Ga/3g 0.5059 0.4067 0.5073[11] 0.4069[11] −0.6919 –449674.0121
1.85
4.37 mJ/(mol LaNi4 Ga·K2 )
LaNi4 Ga/2c 0.5114 0.4033
−0.6958 –449673.7422
The calculated lattice constants are excellent in accord with the experimental values. 2 2 n(E )[20] , where k is Boltzmann s constant. It was The electronic specific heat γ= π3 kB F B found that the lattice expansion is mainly along the c axis. The total DOS at EF decreases dramatically from 12.29, 6.71 to 1.85 states/eV.f.u, and the corresponding coefficients of the electronic specific heat γ are 29.01, 15.83 and 4.37 mJ/(mol LaNi5−x Gax ·K2 ). Total energy of LaNi5−x Gax also decreases, which indicates that the LaNi5−x Gax system becomes more stable. However, the Fermi energy EF is found to rise; a factor that affects the stability adversely. 3.2 Densities of states Total and partial DOSs of the gallium-substituted compounds LaNi5−x Gax (x=0, 0.5, 1.0); obtained with a single and double unit cell, are plotted in Figs.3–5; the origin of the energy scale is located at the Fermi energy EF . In Fig.3, the occupied part of the conduction band is dominated by the Ni-3d states with a small bonding contribution of La-d bands. An additional structure in the TDOS of the LaNi4.5 Ga0.5 is observed between –5 and –9 eV. The low energy structure found in the TDOS between –9 and –6.5 eV is mostly due to the Ga-s states, whereas the Ga-p states are found at higher energies with a maximum contribution between –6.5 and –4 eV. The Ga-d states are smaller than that of Ni and occur at higher energies, above –4 eV. In Fig.3, a bonding interaction of the Ga-p with La and Ni bands, as well as a bigger contribution of Ga-s interaction with the Ni-d bands of its four neighbors were found. In Fig.4, a new structure located around –8 eV is observed; its value increases progressively with increasing values of x, from 0.5 to 1.0. EF falls at the bottom of the valley, and it is clear that the substitution of Ga at the Ni site leads to a progressive filling of the Ni-d bands. This phenomenon could be confirmed by photoemission, which is not yet available. In Fig.5, there is a small DOS located at –6.5 eV below EF, and the Ni-d bands are not filled and the Fermi energy falls below the valley separating the bonding La-Ni high DOS
· 160 ·
Fig.3 Spin-averaged total DOS for LaNi4.5 Ga0.5 (a), partial DOSs projected onto La (b), partial DOSs projected onto Ni (c) and Ga (d) for LaNi4.5 Ga0.5 .
Fig.4 Spin-averaged total DOS for LaNi4 Ga (a) and partial DOSs projected onto Ga (b).
Fig.5 Spin-averaged total DOS for LaNi5 (a), La and Ni (b).
· 161 · peak from the empty antibonding region. Fig.5a is in good agreement with the X-ray photoelectron spectroscopy (XPS) data of Fuggle et al.[21] . In LaNi5−x Gax , the contribution of the Ni-d bands at EF remains higher than that of the La-d bands; TDOS at EF is dominated by the contribution of the Ni-d bands. The substitution of Ga at the Ni site results in a new distinct Ga-d subband centered at about 2 eV below the main Ni-d peak. Due to the lattice expansion, the Ni-d bands are narrower and the valley separating the bonding and anti-bonding states is smeared out. Two factors were found to affect the stability of LaNi5−x Gax compounds: (1) The increased contribution of the Ni-d-Ga-d interactions near EF and the low energy metalgallium bonding bands. (2) The location of EF , if EF is located at the bottom of the valley; the compounds will be stable.
Fig.6 Electron density of LaNi4.5 Ga0.5 for the (11¯ 20) plane (inner-most contour 10.0 a.u.−3 , outer-most 0.1 a.u.−3 ).
3.3 Charge distributions The electron densities for LaNi5−x Gax are shown in Figs.6 and 7. Bond charges between the gallium atom and the metal Fig.7 Electron density of LaNi4 Ga for the atoms are seen. It is clear that the highest (0002) plane (inner-most contour 5.0 charge density resides in the immediate vicina.u.−3 , outer-most 0.2 a.u.−3 ). ity of the atomic nuclei. In Fig.6, it is seen that the next-nearest neighbors of gallium atom are lanthanum atoms, and there is no directional La-Ga bonding detected. Slightly increased charge density appears between Ni-Ni and Ni-Ga; however, no covalent bond charge can be seen, suggesting that the NiGa interactions are ionic. The Ni-Ga interaction increases with x in LaNi5−x Gax . Inspection of the density contours in Fig.6 clearly shows the loss of charge from the La and Ni cores and redistribution of this charge into the interstitial regions. Finally, it was found that the decreasing strength of bonding interactions while going from Ni-Ni (strongest), to Ni-Ga (intermediate) to La-Ga (weakest) in Figs.6 and 7. 4. Conclusions By all-electron calculations, the crystal structure and the electronic structure of LaNi5−x Gax (x=0, 0.5, 1.0) are investigated and analyzed. Based on the calculation, it was found that the Ga atom prefers to occupy the 3g site, in accordance with Ref.[13]. In LaNi5−x Gax , the total DOS at EF is dominated by Ni-d bands. A new structure located around –8 eV below EF is observed; its value increases progressively with increasing values of x, from 0.5 to 1.0. It is found that the greater stability of the LaNi5−x Gax is
· 162 · mainly due to Ni-Ni, Ni-Ga bonding and to a lower value of n(EF ). The total DOS at EF decreases dramatically with increasing values of x, and it is clear that the substitution of Ga at the Ni site leads to a progressive filling of the Ni-d bands. It was found that the decreasing strength of bonding interactions while going from Ni-Ni (strongest), to Ni-Ga (intermediate) to La-Ga (weakest), the Ni-Ga interactions are ionic. There are two factors that affect the stability of LaNi5−x Gax compounds: (1) the increased contribution of the Ni-d-Ga-d interactions near EF and the low energy metalgallium bonding bands. (2) The location of EF , if EF is located at the bottom of the valley, the compounds will be stable. Acknowledgements—This work was financially supported by the National Natural Science Foundation of China (No.60777012). REFERENCES [1] K. Giza, W. Iwasieczko, V.V. Pavlyuk, H. Bala, H. Drulis and L. Adamczyk, J. Alloy. Compd. 429 (2007) 352. [2] J. Liu, Y. F. Yang and H.X. Shao, J. Alloy. Compd. 429 (2007) 285. [3] A. Anani, A. Visintin, K. Petrov, S. Srinivasan, J.J. Reilly, J.R. Johnson, R.B. Schwarz and P.B. Desch, J. Power Sources 47 (1994) 261. [4] J.H.N. van Vucht, F.A. Kuijpers and H.C.A.M. Bruning, Philips Res. Rep. 25 (1970) 133. [5] R. Wiswall, In: G. Alefeld and J. Volkl eds., Hydrogen in Metals II, Topies in Applied Physics (Springer, Berlin, 1978) p.201. [6] K.G. Zhu, J.Z. Shi and L.D. Zhang, Chin. Phys. 7 (1998) 504. [7] S.J. Lin and H.P. Zheng, Acta Phys. Sin. 10 (2005) 4680 (in Chinese). [8] M. Sluiter, M. Takahashi and Y. Kawazoe, J. Alloy. Compd. 248 (1997) 90. [9] M. Gupta, J. Alloy. Compd. 293-295 (1999) 190. [10] T. Haraki, N. Inomata and H. Uchida, J. Alloy. Compd. 293-295 (1999) 407. [11] M.H. Mendelsohn, D.M. Gruen and A.E. Dwight, Nature 269 (1977) 45. [12] M.H. Mendelsohn, D.M. Gruen and A.E. Dwight, Adv. Chem. Series 173 (1979) 279. ˘ Bla˘sina, J. Alloy. Compd. 359 (2003) 180. [13] A. Dra˘sner and Z. ˘ [14] Z. Bla˘sina and A. Dra˘sner, J. Phys. Condens. Matter 10 (1998) 4777. [15] P. Dantzer, Hydrogen in Metals III. Topics in Applied Physics (Springer-Verlag, Heidelberg, 1997) p.73. [16] J.P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865. [17] E.E. Krasovskii, Phys. Rev. 56B (1997) 12866. [18] J.P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865. [19] J.W. Oh, C.Y. Kim, K.S. Nahm and K.S. Sim, J. Alloy. Compd. 278 (1998) 270. [20] L.G. Hector Jr, J.F. Herbst and T.W. Capehart, J. Alloy. Compd. 353 (2003) 74. [21] J.C. Fuggle, F.U. Hillebrecht, R. Zeller, Z. Zolnierek and P.A. Bennett, Phys. Rev. 27B (1983) 2145.
ELECTRONIC STRUCTURE OF THE LaNi5−x Gax INTERMETALLIC COMPOUNDS D. Chen1,3)∗ , G.X. Li2) , D.L. Zhang1) and T. Gao3) 1) Department of Physics and Electronic Engineering, Xinyang Normal University, Xinyang 464000, China 2) College of Science, Qingdao Agriculture University, Qingdao 266109, China 3) Institute of Atomic and Molecular Physics, Sichuan University, Chengdu 610065, China Manuscript received 6 April 2007; in revised form 24 October 2007
The equilibrium structures and electronic structure of LaNi5−x Gax (x=0, 0.5, 1.0) compounds have been investigated by all-electron calculations. Based on the full geometry optimization, the densities of states and electron densities of LaNi5−x Gax are plotted and analyzed. It is clear that the substitution of Ga at the Ni site leads to a progressive filling of the Ni-d bands, the ionic interaction between Ni and Ni, with Ga plays a dominant role in the stability of LaNi5−x Gax compounds. The smaller the shift of EF toward higher energy region, the more stable the compounds will be. The increased contribution of the Ni-d-Ga-d interactions near EF and the low energy metal-gallium bonding bands indicate that the compounds become more stable. The results are compared with experimental data and discussed in light of previous studies. KEY WORDS FLAPW and GGA; LaNi5−x Gax ; Intermetallic compound; Electronic structure
1. Introduction The storage of hydrogen in the form of metal hydrides has been investigated for the last three decades[1−4] . LaNi5 alloys are of great technological interest in their applications, such as compressors, heat pumps, rechargeable batteries, and energy conversion materials[5−7] , besides, LaNi5 -H system is accepted as a prototype of hydrogen absorbing materials for its excellent hydrogenation properties. Much more hydrogen can be stored in the same volume compared to liquefied form. Partial substitution by M (M =Al, Co, Sn, Ga, Ge) for Ni in the LaNi5 improves some of the practical properties of these hydrogen storage compounds[8−12] . LaNi5−x Gax compounds have been investigated by X-ray diffraction since they affect the stability, the cycling lifetimes and the hydrogen content of the hydrides[13,14] . In LaNi5−x Gax systems, the Gibbs free energy of formation increases with x much slowly compared with LaNi5−x M x (M =Al, Sn, Si); this indicates a greater stability at given compositions of LaNi5−x Gax systems if compared with the systems containing Al, Sn and Si. Besides, the addition of Ga improves the low-temperature magnetic properties of LaNi3 Ga2 . The aim of this article is to present the results of a study on the electronic ∗
Corresponding author. Tel.: +86 13253854009. E-mail address: [email protected] (D. Chen)
· 158 · properties of LaNi5−x Gax when Ga is partially substituted for nickel. Understanding the electronic structure of LaNi5−x Gax compounds from a fundamental point of view and in relation to several aspects of their technological applications is important[15] . In this article, the crystal structure, electronic structure and the Ga site occupancy of LaNi5−x Gax (x=0, 0.5, 1.0) have been investigated by all-electron calculations. More information on the electronic structure of LaNi5−x Gax is obtained. The calculated electronic properties have been compared with the experimental results. 2. Crystal Structures and Computational Details LaNi5 crystal has a hexagonal CaCu5 structure (space group P 6/mmm). The La atom occupies the 1a (0, 0, 0) Wyckoff site. The two nonequivalent Ni atoms occupy the 2c (2/3, 1/3, 0) and the 3g (0.5, 0, 0.5) sites. X-ray powder diffraction[13] shows that the LaNi5−x Gax alloys still retain the CaCu5 structure until x≤2.5. To find the Ga-occupied site, two single-unit cell models are built: (1) the Ga atom substitutes the Ni atom at the 2c site; (2) the Ga atom substitutes the Ni atom at the 3g site. The result indicates that the Ga atom prefers to occupy the 3g site, which is shown in Fig.1. To analyze LaNi4.5 Ga0.5 system, a double unit cell is built along the c axis of the LaNi5 unit cell. Ga atom substitutes for Ni atom in the 3g (0.5a, 0.5a, 0.75c) site as shown in Fig.2.
Fig.1 Unit cell used for LaNi4 Ga (3g).
Fig.2 Model unit cell used for LaNi4.5 Ga0.5 (two formula units).
The calculations have been performed using the density functional theory (DFT) in the generalized gradient approximation (GGA)[16] . The Full-Potential Linearized Augmented Plane Wave (FLAPW) method has been used[17] . The generalized gradient approximation of PBE96[18] was used for the exchange-correlation energy functional. Atomic geometries were obtained by minimizing the Hellman-Feynman forces. The plane wave cutoff energy is chosen as 300 eV and the RKmax is chosen as 8.0 to get a good plane wave basis set. As for the Brillouin zone sampling, the 12×12×5 Monkhorst-Pack mesh was used, which is sufficient to make a good self-consistent convergence of the total energy.
· 159 · 3. Results and Discussion 3.1 Optimized structures The calculated lattice constants and the total energy for LaNi5−x Gax , the total density of states (TDOS) at EF and the electronic specific heat coefficients are listed in Table 1. Table 1 Lattice constants, EF , total energy, DOSs at EF and the electronic specific heat γ for LaNi5−x Gax Calculated a/nm c/nm LaNi5
Experimental a/nm
c/nm
EF
Energy
n(EF )
γ
eV
0.5021 0.3964 0.5017[19] 0.3986[19] −0.7053 –438154.6893 12.29
29.01 mJ/(mol LaNi5 ·K2 )
LaNi4.5 Ga0.5 0.5038 0.3990 0.5049[11] 0.4028[11] −0.6981 –443912.5317
6.71
15.83 mJ/(mol LaNi4.5 Ga0.5 ·K2 )
LaNi4 Ga/3g 0.5059 0.4067 0.5073[11] 0.4069[11] −0.6919 –449674.0121
1.85
4.37 mJ/(mol LaNi4 Ga·K2 )
LaNi4 Ga/2c 0.5114 0.4033
−0.6958 –449673.7422
The calculated lattice constants are excellent in accord with the experimental values. 2 2 n(E )[20] , where k is Boltzmann s constant. It was The electronic specific heat γ= π3 kB F B found that the lattice expansion is mainly along the c axis. The total DOS at EF decreases dramatically from 12.29, 6.71 to 1.85 states/eV.f.u, and the corresponding coefficients of the electronic specific heat γ are 29.01, 15.83 and 4.37 mJ/(mol LaNi5−x Gax ·K2 ). Total energy of LaNi5−x Gax also decreases, which indicates that the LaNi5−x Gax system becomes more stable. However, the Fermi energy EF is found to rise; a factor that affects the stability adversely. 3.2 Densities of states Total and partial DOSs of the gallium-substituted compounds LaNi5−x Gax (x=0, 0.5, 1.0); obtained with a single and double unit cell, are plotted in Figs.3–5; the origin of the energy scale is located at the Fermi energy EF . In Fig.3, the occupied part of the conduction band is dominated by the Ni-3d states with a small bonding contribution of La-d bands. An additional structure in the TDOS of the LaNi4.5 Ga0.5 is observed between –5 and –9 eV. The low energy structure found in the TDOS between –9 and –6.5 eV is mostly due to the Ga-s states, whereas the Ga-p states are found at higher energies with a maximum contribution between –6.5 and –4 eV. The Ga-d states are smaller than that of Ni and occur at higher energies, above –4 eV. In Fig.3, a bonding interaction of the Ga-p with La and Ni bands, as well as a bigger contribution of Ga-s interaction with the Ni-d bands of its four neighbors were found. In Fig.4, a new structure located around –8 eV is observed; its value increases progressively with increasing values of x, from 0.5 to 1.0. EF falls at the bottom of the valley, and it is clear that the substitution of Ga at the Ni site leads to a progressive filling of the Ni-d bands. This phenomenon could be confirmed by photoemission, which is not yet available. In Fig.5, there is a small DOS located at –6.5 eV below EF, and the Ni-d bands are not filled and the Fermi energy falls below the valley separating the bonding La-Ni high DOS
· 160 ·
Fig.3 Spin-averaged total DOS for LaNi4.5 Ga0.5 (a), partial DOSs projected onto La (b), partial DOSs projected onto Ni (c) and Ga (d) for LaNi4.5 Ga0.5 .
Fig.4 Spin-averaged total DOS for LaNi4 Ga (a) and partial DOSs projected onto Ga (b).
Fig.5 Spin-averaged total DOS for LaNi5 (a), La and Ni (b).
· 161 · peak from the empty antibonding region. Fig.5a is in good agreement with the X-ray photoelectron spectroscopy (XPS) data of Fuggle et al.[21] . In LaNi5−x Gax , the contribution of the Ni-d bands at EF remains higher than that of the La-d bands; TDOS at EF is dominated by the contribution of the Ni-d bands. The substitution of Ga at the Ni site results in a new distinct Ga-d subband centered at about 2 eV below the main Ni-d peak. Due to the lattice expansion, the Ni-d bands are narrower and the valley separating the bonding and anti-bonding states is smeared out. Two factors were found to affect the stability of LaNi5−x Gax compounds: (1) The increased contribution of the Ni-d-Ga-d interactions near EF and the low energy metalgallium bonding bands. (2) The location of EF , if EF is located at the bottom of the valley; the compounds will be stable.
Fig.6 Electron density of LaNi4.5 Ga0.5 for the (11¯ 20) plane (inner-most contour 10.0 a.u.−3 , outer-most 0.1 a.u.−3 ).
3.3 Charge distributions The electron densities for LaNi5−x Gax are shown in Figs.6 and 7. Bond charges between the gallium atom and the metal Fig.7 Electron density of LaNi4 Ga for the atoms are seen. It is clear that the highest (0002) plane (inner-most contour 5.0 charge density resides in the immediate vicina.u.−3 , outer-most 0.2 a.u.−3 ). ity of the atomic nuclei. In Fig.6, it is seen that the next-nearest neighbors of gallium atom are lanthanum atoms, and there is no directional La-Ga bonding detected. Slightly increased charge density appears between Ni-Ni and Ni-Ga; however, no covalent bond charge can be seen, suggesting that the NiGa interactions are ionic. The Ni-Ga interaction increases with x in LaNi5−x Gax . Inspection of the density contours in Fig.6 clearly shows the loss of charge from the La and Ni cores and redistribution of this charge into the interstitial regions. Finally, it was found that the decreasing strength of bonding interactions while going from Ni-Ni (strongest), to Ni-Ga (intermediate) to La-Ga (weakest) in Figs.6 and 7. 4. Conclusions By all-electron calculations, the crystal structure and the electronic structure of LaNi5−x Gax (x=0, 0.5, 1.0) are investigated and analyzed. Based on the calculation, it was found that the Ga atom prefers to occupy the 3g site, in accordance with Ref.[13]. In LaNi5−x Gax , the total DOS at EF is dominated by Ni-d bands. A new structure located around –8 eV below EF is observed; its value increases progressively with increasing values of x, from 0.5 to 1.0. It is found that the greater stability of the LaNi5−x Gax is
· 162 · mainly due to Ni-Ni, Ni-Ga bonding and to a lower value of n(EF ). The total DOS at EF decreases dramatically with increasing values of x, and it is clear that the substitution of Ga at the Ni site leads to a progressive filling of the Ni-d bands. It was found that the decreasing strength of bonding interactions while going from Ni-Ni (strongest), to Ni-Ga (intermediate) to La-Ga (weakest), the Ni-Ga interactions are ionic. There are two factors that affect the stability of LaNi5−x Gax compounds: (1) the increased contribution of the Ni-d-Ga-d interactions near EF and the low energy metalgallium bonding bands. (2) The location of EF , if EF is located at the bottom of the valley, the compounds will be stable. Acknowledgements—This work was financially supported by the National Natural Science Foundation of China (No.60777012). REFERENCES [1] K. Giza, W. Iwasieczko, V.V. Pavlyuk, H. Bala, H. Drulis and L. Adamczyk, J. Alloy. Compd. 429 (2007) 352. [2] J. Liu, Y. F. Yang and H.X. Shao, J. Alloy. Compd. 429 (2007) 285. [3] A. Anani, A. Visintin, K. Petrov, S. Srinivasan, J.J. Reilly, J.R. Johnson, R.B. Schwarz and P.B. Desch, J. Power Sources 47 (1994) 261. [4] J.H.N. van Vucht, F.A. Kuijpers and H.C.A.M. Bruning, Philips Res. Rep. 25 (1970) 133. [5] R. Wiswall, In: G. Alefeld and J. Volkl eds., Hydrogen in Metals II, Topies in Applied Physics (Springer, Berlin, 1978) p.201. [6] K.G. Zhu, J.Z. Shi and L.D. Zhang, Chin. Phys. 7 (1998) 504. [7] S.J. Lin and H.P. Zheng, Acta Phys. Sin. 10 (2005) 4680 (in Chinese). [8] M. Sluiter, M. Takahashi and Y. Kawazoe, J. Alloy. Compd. 248 (1997) 90. [9] M. Gupta, J. Alloy. Compd. 293-295 (1999) 190. [10] T. Haraki, N. Inomata and H. Uchida, J. Alloy. Compd. 293-295 (1999) 407. [11] M.H. Mendelsohn, D.M. Gruen and A.E. Dwight, Nature 269 (1977) 45. [12] M.H. Mendelsohn, D.M. Gruen and A.E. Dwight, Adv. Chem. Series 173 (1979) 279. ˘ Bla˘sina, J. Alloy. Compd. 359 (2003) 180. [13] A. Dra˘sner and Z. ˘ [14] Z. Bla˘sina and A. Dra˘sner, J. Phys. Condens. Matter 10 (1998) 4777. [15] P. Dantzer, Hydrogen in Metals III. Topics in Applied Physics (Springer-Verlag, Heidelberg, 1997) p.73. [16] J.P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865. [17] E.E. Krasovskii, Phys. Rev. 56B (1997) 12866. [18] J.P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865. [19] J.W. Oh, C.Y. Kim, K.S. Nahm and K.S. Sim, J. Alloy. Compd. 278 (1998) 270. [20] L.G. Hector Jr, J.F. Herbst and T.W. Capehart, J. Alloy. Compd. 353 (2003) 74. [21] J.C. Fuggle, F.U. Hillebrecht, R. Zeller, Z. Zolnierek and P.A. Bennett, Phys. Rev. 27B (1983) 2145.