Hydrogen Bridging Characterization in the Polynuclear La2

Aug 1, 2007 - of extended H bridging in complex hydrides deserves a more detailed .... Ni2H7 (Figure 3e and f), Ni4H12 (Figure 3g and h) substructures...
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J. Phys. Chem. C 2007, 111, 12391-12396

12391

Hydrogen Bridging Characterization in the Polynuclear La2MgNi2H8 Hydride Emilio Orgaz† Departamento de Fı´sica y Quı´mica Teo´ rica, Facultad de Quı´mica, UniVersidad Nacional Auto´ noma de Me´ xico, CP 04510, Me´ xico, D.F. Me´ xico ReceiVed: June 19, 2007

We investigated the chemical bonding properties of hydrogen bridging in extended polynuclear hydride La2MgNi2H8. We applied a mixed molecular and solid-state theoretical approach in order to characterize the covalent interaction in the Ni-H units. This approach modifies the full ionic picture issued from the standard electron-counting rules. We reproduced the experimentally observed magnetic as well as electric properties of this compound as a confidence test of our method. A systematic study of the H site energy permitted us to energetically differentiate the 16 inequivalent hydrogen atoms in this hydride. We found that the H bridging bond in La2MgNi2H8 is particularly stable, compared to standard Ni-H bonds.

1. Introduction Polynuclear hydrides appear essentially in complex metalorganic molecules and have been largely investigated during the last few decades.1 In the solid state, extended polynuclear hydrides are rare2,3 and represent an interesting opportunity field to design new materials having the possibility of weak hydrogen bonding and reversible hydrogen absorption-decomposition reactions. From a more fundamental view, hydrogen in complex hydrides behaves completely different from standard interstitial hydrides because of a loss of H mobility and the nature of the stronger metal-hydrogen interactions. Recently, Chotard, Filinchuk, Revaz, and Yvon discovered a new hydride having extended polynuclear structures, La2MgNi2H8.3 In this research, the authors characterized the crystal structure of La2MgNi2H8 and established that it behaves as a diamagnetic semiconductor. This compound exhibits two subunits involving a variety of oriented nickel-hydrogen bonds. The first structure, Ni2H7, shows a H bridging between the Ni atoms as sketched in Figure 1a and fully described in ref 3. A second, more complex, structure (Figure 1b) exhibits two independent H bridging bonds, having the formula Ni4H12. In addition, this complex compound shows three inequivalent H atoms not bound to Ni. The existence of extended H bridging in complex hydrides deserves a more detailed investigation of the bonding properties. We start with the description of the electronic structure of the reference intermetallic compound, La2MgNi2. Then, we present the modifications in the electronic structure introduced by H absorption. We support these results by molecular calculations in representative clusters of both Ni-H substructures. Finally, we characterized the H energy site of all of the inequivalent H atoms in this polynuclear hydride. 2. Methodology: Computational Details Ab initio calculations of the electronic structure of the intermetallic compound La2MgNi2 and the corresponding hydride La2MgNi2H8 were carried out by means of the all-electron full potential-linear augmented plane waves (LAPW) method.5 The muffin-tin radii were set to 2.20, 1.80, 2.0, and 1.0 au for the La, Ni, Mg, and H atoms, respectively. The RKmax parameter, †

E-mail: [email protected].

Figure 1. Substructures (a) Ni2H7 and (b) Ni4H12 showing hydrogen bridging in the La2MgNi2H8 hydride.

which controls the plane wave expansion, was selected in order to obtain converged eigenvalues up to 10-4 eV, (RKmax ) 5). Geometry optimizations of the crystal structures were obtained by means of the projected augmented plane wave method (PAW).6 To compute the H energy site, we construct a 2 × 2 × 2 supercell and remove a H atom from the structure. We repeat this procedure for the 16 inequivalent H atoms and compare the total electronic energy of these relaxed structures with that obtained for the fully stoichiometric hydride. Total (DOS) and partial density of states (PDOS) were computed from the energy bands obtained with the LAPW method. In all of the calculations, we employed the generalized gradient approximation7 to the density functional theory. Molecular cluster calculations8 were carried out on the anionic subunits Ni2H7 and Ni4H12 by means of a density functional approach (DFT) and using the Gaussian basis set 3-21Gff. 3. Results and Discussion La2MgNi2 intermetallic compound crystallizes in the Mo2FeB2-like structure and belongs to the P4/mbm space group.4 In Figure 2, we plot the total and site-projected DOS computed for the La2MgNi2 intermetallic compound. The spin-polarized results indicate that this system is metallic and nonmagnetic, as has been observed experimentally. Previous investigations

10.1021/jp0747611 CCC: $37.00 © 2007 American Chemical Society Published on Web 08/01/2007

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Figure 2. (a) Total density of states (in states/eV cell) for the La2MgNi2 intermetallic compound obtained with the LAPW method. Partial density of states (in states/eV atom) for the (b) La atom, in inset La 4f contribution, (c) Ni atom, in inset Ni 3d contribution and (d) Mg atom. The Fermi energy is set to zero.

TABLE 1: Total Density of States at the Fermi Energy (states/eV cell) and Density of States Projected at Each Atomic Site at the Fermi Energy (states/eV atom) total

La s

La p

La d

La f

Ni s

Ni p

Ni d

Mg s Mg p

7.612 0.010 0.037 0.175 0.056 0.010 0.043 0.365 0.015 0.065

of the electronic structure of this compound have been reported.4 Within the extended Hu¨ckel approximation, the authors found that the main contribution to the Fermi energy arises from the La d states. The angular momentum-resolved atomic contributions to the DOS are summarized in Table 1. The most important contributions of the DOS at Fermi energy are provided by the Ni d and La d states. Our results also indicate that the main contributions arising from La f appear, as expected, 2 eV above the Fermi energy (Figure 2b). To characterize the orbital contributions to the DOS band-by-band, we inspected the wave function coefficients at some of the high symmetry points of the Brillouin zone. In Tables 2 and 3, we summarize these wave function coefficients resolved by atom type and angular momentum. With this information and the site-projected DOS, it is possible to characterize the bonding interactions in the valence band. We then characterize the first structure of the total DOS, appearing at the bottom of the energy scale, from ca. -5.5 eV below the Fermi energy up to -2.6 eV, as Mg s/Ni s/La s bonding orbital interactions concerning bands 1-5 in Tables 2 and 3 (see Figure 2b-d). It is important to note the marginal Ni d contributions in this energy range. The second

broad structure in the total DOS concerns mainly the Ni d states (Figure 2b). However, La sd, Mg s,p and Ni s orbital contributions are present in this energy range. This is a well-known effect of the dispersion of the energy bands in metallic systems. We will observe a strong energy localization an a drastic reduction of the band dispersion after hydrogenation, conferring a more molecular-like behavior to the resulting hydride. In fact, the energy localization of the bands opens an energy gap; the system exhibits a semiconducting behavior,3 as can be appreciated in the ab initio total DOS plot (Figure 3a). The computed energy gap is 0.78 eV, which is usually underestimated by the DFT methods. After hydrogen absorption, this system exhibits a symmetry reduction (P 21/c) and a cell expansion of 20.5%. The Ni-Ni distances increase from ∼3.08 Å in La2MgNi2 to ∼3.76 Å in the hydride, in order to allocate the Ni-H bonds at short distances (∼1.54 to 1.68 Å). In Figure 3b-i, we plot the site-projected DOS for a representative element (La1, Ni1, and Mg1, after the nomenclature of Chotard et al.3) in the structure. We also include plots of the H site-projected DOS (Figure 3ei). The first remarkable observation of the total DOS (Figure 3a) compared to that of the parent intermetallic (Figure 2a) is the extension of the DOS structure down to ca. -8.5 eV and the more-localized character of the energy bands. Using the wave function coefficients at the high symmetry points of the associated Brillouin zone (not shown), we can label the different peaks appearing in the DOS plots. The structure of the total

Hydrogen Bridging Characterization

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TABLE 2: Wave Function Coefficients (Arbitrary Units) at the Γ point of the La2MgNi2 Intermetallic Compound band E (eV) La s La p La d La f Ni s Ni p 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

-5.39 -2.87 -2.86 -2.86 -2.73 -2.60 -2.46 -2.26 -2.26 -2.26 -2.11 -1.73 -1.69 -1.69 -1.68 -1.68 -1.66 -1.66 -1.50 -1.42 -1.42 -1.21 -1.17 -1.17 -0.83 -0.59 -0.24 0.09 0.09 0.63 0.83 1.11

1.27 0.05 0.77 0.77 0.00 0.02 0.33 0.06 0.06 0.00 0.24 0.00 0.06 0.00 0.07 0.07 0.00 0.00 0.00 0.07 0.07 0.00 0.00 0.00 0.00 0.00 0.31 0.06 0.06 0.00 0.00 0.01

0.00 0.34 0.59 0.59 1.60 0.05 1.07 0.11 0.11 0.00 0.01 0.00 1.33 0.27 0.07 0.07 0.04 0.04 0.00 0.29 0.29 0.02 0.17 0.17 0.25 0.38 0.54 1.15 1.15 0.13 0.03 0.22

0.02 1.41 0.92 0.92 0.26 1.23 0.31 1.19 1.19 0.96 0.74 0.77 0.05 0.04 0.34 0.34 0.64 0.64 0.20 0.17 0.17 0.24 0.09 0.09 1.06 0.00 2.47 1.11 1.11 4.40 5.49 3.50

0.00 0.04 0.04 0.04 0.05 0.01 0.36 0.08 0.08 0.03 0.06 0.04 0.08 0.46 0.33 0.33 0.09 0.09 0.43 0.27 0.27 0.25 0.16 0.16 0.33 0.06 0.70 1.13 1.13 1.20 0.30 1.15

2.00 0.44 1.60 1.60 0.00 0.18 2.40 0.05 0.05 0.00 0.06 0.00 0.69 0.00 0.01 0.01 0.00 0.00 0.00 0.10 0.10 0.00 0.00 0.00 0.42 0.00 0.78 0.86 0.86 0.00 0.00 0.02

0.00 0.08 0.26 0.26 0.15 0.09 0.45 0.05 0.05 0.02 0.02 0.00 0.01 0.02 0.08 0.08 0.02 0.02 0.00 0.09 0.09 0.05 0.01 0.01 0.02 0.03 0.77 0.47 0.47 0.55 1.30 0.26

Ni d

Mg s Mg p

0.06 8.60 1.65 1.65 3.37 12.31 4.99 14.17 14.17 17.44 15.08 18.09 15.42 19.87 15.88 15.88 19.18 19.18 20.21 17.12 17.12 19.49 21.43 21.43 12.39 18.84 1.17 4.67 4.67 2.14 0.33 8.05

2.38 1.48 0.00 0.00 5.96 0.46 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.16 1.40 0.00 0.00 0.00 0.00 0.00 0.20

0.00 0.00 0.73 0.73 0.00 0.00 0.00 0.08 0.08 0.02 0.00 0.16 0.00 0.00 0.59 0.59 0.00 0.00 0.00 0.40 0.40 0.00 0.00 0.00 0.00 0.00 0.00 1.23 1.23 0.00 0.00 0.00

TABLE 3: Wave Function Coefficients (Arbitrary Units) at the X point of the La2MgNi2 Intermetallic Compound band E (eV) La s La p La d La f Ni s Ni p Ni d Mg s Mg p 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

-3.84 -3.14 -2.92 -2.60 -2.60 -2.25 -2.24 -2.08 -1.99 -1.99 -1.80 -1.78 -1.78 -1.73 -1.54 -1.40 -1.36 -1.23 -1.22 -1.21 -1.21 -1.20 -1.20 -0.96 -0.96 -0.87 -0.78 -0.28 0.02 0.02 0.27 0.83

0.00 0.00 0.29 0.01 0.01 0.86 0.20 0.13 0.01 0.01 0.00 0.28 0.28 0.00 0.00 0.00 0.00 0.03 0.00 0.00 0.00 0.03 0.03 0.00 0.00 0.01 0.00 0.05 0.17 0.17 0.00 0.00

0.15 0.02 2.07 0.15 0.15 0.31 0.03 0.03 0.01 0.01 0.08 0.01 0.01 0.65 0.17 0.44 0.09 0.38 0.00 0.03 0.03 0.19 0.19 0.36 0.36 0.07 0.00 0.15 0.70 0.70 1.38 1.57

1.94 1.59 0.09 1.84 1.84 0.79 0.68 0.42 0.75 0.75 0.91 0.48 0.48 0.33 0.24 0.03 0.02 0.46 0.36 0.08 0.08 0.17 0.17 0.59 0.59 2.28 0.44 3.29 2.97 2.97 1.12 0.69

0.01 0.06 0.03 0.12 0.12 0.04 0.05 0.43 0.12 0.12 0.04 0.15 0.15 0.10 0.09 0.10 0.05 0.59 0.01 0.14 0.14 0.09 0.09 0.15 0.15 0.12 0.15 0.83 0.43 0.43 1.03 2.71

3.14 0.00 1.92 1.70 1.70 0.31 0.00 0.10 0.14 0.14 0.00 0.02 0.02 0.00 0.33 0.00 0.01 0.90 0.00 0.10 0.10 0.00 0.00 0.22 0.22 0.06 0.00 0.02 0.02 0.02 0.00 0.25

0.07 0.37 0.09 0.08 0.08 0.43 0.01 0.38 0.05 0.05 0.00 0.07 0.07 0.04 0.02 0.00 0.00 0.07 0.00 0.01 0.01 0.02 0.02 0.17 0.17 0.70 0.06 0.44 0.64 0.64 0.00 0.78

1.81 1.74 1.97 3.35 3.35 4.18 17.01 11.87 16.65 16.65 17.54 16.41 16.41 18.18 18.63 20.27 21.16 14.27 20.28 21.03 21.03 20.98 20.98 17.49 17.49 8.55 20.17 2.42 4.16 4.16 0.58 0.86

0.00 5.11 2.63 0.00 0.00 0.01 0.00 1.57 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.01 0.00 0.08 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.42 0.36 0.00 0.00 0.00 0.01 0.00

0.00 0.00 0.17 1.08 1.08 0.86 0.00 0.07 0.18 0.18 0.00 0.01 0.01 0.12 0.00 0.10 0.00 0.18 0.00 0.03 0.03 0.00 0.00 0.11 0.11 0.00 0.00 1.05 0.12 0.12 3.17 2.20

DOS is complex because of the large number of electronic states per cell. However, it is possible to identify three structures below the Fermi energy at the bottom of the energy scale. A broad

structure extends over 4.5 eV and concerns mainly the Ni s/La s/Mg s interactions. Between ca. -5.5 and -4 eV, the peaked structure in the total DOS arises from the Ni d/H s bonding interactions. This is clear in the site-projected PDOS in Figure 3b and e-h. Although the contribution of the H atoms to the DOS is small, the role of each H atom among the 16 inequivalent ones can be identified by means of the siteprojected DOS. The DOS are plotted for the H atoms in the Ni2H7 (Figure 3e and f), Ni4H12 (Figure 3g and h) substructures and for the three H not bound to Ni atoms (Figure 3i). In these Figures, the first neighbors to each H atom are indicated. As can be appreciated, the H atoms labeled H12, H13, and H23 exhibit peaks in the PDOS at energies 2 eV below the majority of the terminal H atoms (Figure 3f and h). This is indicative of a clear bonding situation, slightly stronger than that observed for the remaining H atoms linked to Ni (Figure 3e and g). The PDOS of the H atoms not bound to any Ni atom (H1, H2, and H3) are plotted in Figure 3d. The peaks of the PDOS for this kind of hydrogen appear at ca. -3.5 eV below the Fermi energy and ca. 1.5 eV above the main contribution of the terminal H atoms. In conclusion, the H atoms can be qualitatively differentiated. The H atoms forming bridges between Ni atoms (H12, H13, and H23) appear to be more stable than the remaining H atoms, except for the three H1, H2, and H3, which seemto be less bound than the others. The corresponding localized (narrow) H s structures appear at energies between ca. -3.6 and -2.6 eV; above the energies at which the other H atoms bonded to Ni appear. To support the precedent observations, we computed the electronic structure of two clusters representing the Ni-H environments found in the La2MgNi2H8 hydride. Ab initio DFT partial geometry optimizations of the anions [Ni2H7]7- and [Ni4H12]12- substructures were carried out. We fixed dihedral angles and bond distances in order to mimic the geometry of these subunits in the solid. Our results should then be taken as a semiquantitative approach. However, some properties can be obtained and will be helpful to sustain the results of the electronic structure computations in the solid state. The molecular electrostatic potential for both substructures (not shown) indicate that there are not significant differences in the charge distribution among the H atoms. The natural bond orbital (NBO) analysis allows us to assign the electron occupation numbers to the atomic orbitals and suggest an electronic configuration for the atoms in the clusters. The [Ni2H7]7- anion has been described as a zerovalent Ni atom and H atom in a full hydrido state. Our results indicate a different situation, which can be summarized in the formula [Ni0.95H0.73]7-. In this formula, 2 7 the extra seven electrons are localized partially in the nickel atoms suggesting the electronic configuration, after the NBO analysis, 28Ni[Ar]3d8.214s1.104p1.95. A charge depletion toward 4p orbitals keeps a d8-like configuration. The situation for the [Ni4H12]12- is similar. We obtain a charge distribution consistent H0.77]12-. The suggested that the with the formula [Ni0.704 12 electronic structure for nickel is essentially the same than that obtained for the [Ni2H7]7- substructure. The only difference arises from the smaller occupation of the 4p natural orbital. In spite of the NBO proposal, a symmetry analysis suggests that the natural hybridization of the experimentally observed fourfold coordinated Ni atom should be sd.3 In Td symmetry it involves A1 + T2 irreps concerning the 4s and the 3dxy, 3dxz, and 3dyz orbitals. In this situation, the 4p orbitals also belong to the T2 symmetry and can participate in directed bonding. The situation of Ni in La2MgNi2H8 is clearly less-symmetric than the Td case. However, the same arguments can be used in less-symmetric

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Figure 3. (a) Total density of states (in states/eV cell) for the La2MgNi2H8 hydride obtained with the LAPW method. Partial density of states (in states/eV atom) for the (b) La1 atom, in inset La 4f contribution, (c) Ni1 atom, in inset Ni-3d contribution, (d) Mg1 atom, (e) H atoms in the Ni2H7 substructure, (f) bridging H atoms in the Ni2H7 substructure, (g) H atoms in the Ni4H12 substructure, (h) bridging H atoms in the Ni4H12 substructure, and (i) interstitial H atoms, not directly bounded to Ni. For the DOS plots at the H atom sites, the first two neighbors are indicated. The Fermi energy is set to zero.

Hydrogen Bridging Characterization point groups, where 4p orbitals can be involved in the orbital’s hybridization yielding a s(dp)3 proposal. It is important to recall that in transition metals, where the ligands do not have p orbitals, the π back-donation stabilizing mechanism is not possible. A symmetry reduction from a hypothetic Td point group implies a loss of degeneracy of the p- and d-like electronic states of nickel and the possibility of a d8 configuration. It is known that Ni compounds usually exhibit a square-planar geometry9 and are paramagnetic. However, in the present case, the distorted fourfold coordination of nickel (neither Td nor D4h) is exhibited by a large number of complex hydrides.2 The experimental observation of diamagnetism in this and other complex hydrides containing low-symmetry fourfold-coordinated Ni atoms could result in agreement with a d8-like picture where the extra charge in the anion cluster is distributed among the more-electronegative H atoms. So, the experimentally observed diamagnetism is not in contradiction with a d8 nickel electronic configuration because the Ni local crystal field involves a complete break of the d-state’s degeneracy. Alternatively, space partition methods employed to assign charges to atoms in molecules or solids are not always straightforward and frequently yield assignments hard to justify from a chemical viewpoint. This is particularly true in extended systems. We make use of several, and intrinsically limited, methods; muffin-tin charges (LAPW), atomic spheres charges (PAW), Mulliken population analysis, and NBO (molecular dft computations). For example, the Mu¨lliken population analysis provides occupation numbers indicating a strong charge accumulation on the nickel atom, indicating a very unlike 2.8charge state. This is far from the Ni0 state that is obtained when La and Mg are in the 3+ and 2+ cationic state, respectively, and the H atom in the 1- full hydrido form. At this point, it is important to note that in the intermetallic compound La2MgNi2 the Ni charge state is consistent with a partially empty d band. The charge analysis indicates only eight electrons in this band (PAW and LAPW computations) while this solid is diamagnetic. In the investigated clusters, NBO provides a more reasonable charge redistribution, close to the a priori expected atomic charge assignments. To clarify and give more evidence of the differentiated behavior of the H atoms in La2MgNi2H8, we computed the H site energy for each H atom. The H site energy, H, was computed10,11 according to the fictitious reaction La16Mg8Ni16H64 f La16Mg8Ni16H63 + 1/2H2. In Table 4, we summarize the results for this decomposition enthalpy. Although the true equilibria criteria at finite temperatures is the Gibbs free energy, we assume that the entropy contributions,12 including the zeropoint one, are small and essentially the same for each hypothetical nonstoichiometric product under investigation. It is interesting to note that the experimental dissociation energies of the gas-phase diatomic molecules NiH+ and NiH are 1.7 and 2.7 eV, respectively.13 As can be appreciated in Table 4, this molecular property is twice as large as that found in the solid state. Experimental solid solution enthalpies for interstitial transition-metal hydrides range from -0.1 to -1.0 eV/H and remain almost constant along the solid solution region of the phase diagram.14 The values reported in Table 4 are consistent with those typically found for interstitial hydrides. Table 4 has been organized by putting together H atoms having the same kind of neighbors. Substructure I(II) makes reference to Ni2H7(Ni4H12). The H atoms bound to Ni in substructure I exhibit H values ranging from 0.92 to 1.02 eV. These figures show a small dispersion, reflecting the common features of these atoms in the substructure. The H atoms in substructure II, having essentially the same environment and also showing a single bond

J. Phys. Chem. C, Vol. 111, No. 33, 2007 12395 TABLE 4: H Energy Site EH (in eV/H) for Each Inequivalent H Sitea subH H site structure (eV/H) H21 H24 H41 H43 H22 H42

I I I I I I

0.92 1.02 0.99 0.95 1.19 1.21

Ni(1.65) Ni(1.54) Ni(1.53) Ni(1.59) Ni(1.62) Ni(1.75)

Mg(1.84) Mg(2.17) Mg(1.92) Mg(1.97) La(2.37) H(2.23)

H(2.25) H(2.36) H(2.27) H(2.39) La(2.37) La(2.29)

H(2.32) H(2.55) H(2.41) H(2.47) La(2.41) La(2.30)

H11 H14 H32 H31

II II II II

0.89 0.90 0.85 1.01

Ni(1.60) Ni(1.54) Ni(1.51) Ni(1.51)

Mg(2.22) Mg(2.00) Mg(1.88) H(2.35)

La(2.47) H(2.43) La(2.32) H(2.35)

H(2.48) H(2.44) H(2.35) H(2.41)

H12 H13 H23 H1 H2 H3

II bridge 1.11 II bridge 1.10 I bridge 1.02 0.69 0.62 1.08

Ni(1.51) Ni(1.66) Ni(1.53) Mg(2.11) H(2.23) Mg(1.93)

Ni(1.68) Ni(1.74) Ni(1.58) H(2.43) H(2.25) Mg(1.97)

La(2.49) La(2.43) H(2.27) H(2.50) La(2.26) H(2.36)

H(2.51) H(2.44) H(2.32) H(2.56) Mg(2.28) La(2.43)

a Atomic sites are identified accordingly with the experimental results from Chotard et al.3 First neighbors to each H site are indicated along with the interatomic distances (Å).

to Ni atoms, have H values in the 0.85 to 1.01 eV region. Again, these results are close and the mean values are comparable in both substructures. H22 and H42 exhibit different local environments and show slightly different but significantly larger H values. The H12, H13, and H23 atoms forming bridges between Ni atoms exhibit H values ranging from 1.02 to 1.11 eV. It is interesting to note that the mean value for these three H atoms (1.08 eV) is slightly larger than those obtained for the H atoms linked to one Ni atom in substructures I and II (0.97 and 0.91 eV mean values, respectively) but smaller than those observed for the H22 and H42 sites (1.20 eV). The H values obtained for the H1, H2, and H3 atoms depend on the interactions with the first and second neighbors, located at significantly larger distances (see Table 4). This evidence is in reasonable agreement with the qualitative appreciation of the PDOS plots. However, it is surprising that the bridging H atoms do not show a strong difference in the H values. 4. Conclusions At this point, we can conclude that, from an energetic view point, the La2MgNi2H8 hydride essentially shows four kinds of H atoms. A set of H atoms bound to Ni exhibiting an H value of around 1 eV, a second set of H atoms forming bridges between Ni atoms exhibiting energies slightly higher than those of the first kind, H atoms with strong binding energies (∼1.2 eV) and, finally, almost interstitial H atoms, weakly bound, with H values down to 0.62 eV. It is remarkable the correlation between the positions of the H peaks in the projected density of states and the values of the hydrogen site energy. Moreover, the molecular investigation indicates that the oriented covalent bonding of nickel with hydrogen involves the 4p orbitals and preserves a diamagnetic d8 configuration. In a thermal decomposition of this hydride, we expect that the less-bound H atoms will be desorbed first (H1 and H2). In a pure energetic basis, neglecting the configurational entropy effect on the free energy, we suggest that only 2 of the 16 possible H sites will be concerned in a low-energy thermal decomposition. The next H atoms that can be desorbed should arise from substructure II. Thus, in light of our findings, it is reasonable to expect a small effective H content for this compound during a reversible charging-decomposition cycle. We suggest that thermal desorption experiments and the measure of the absorption iso-

12396 J. Phys. Chem. C, Vol. 111, No. 33, 2007 therms could help to corroborate some of the findings of this investigation. Acknowledgment. Financial support was provided by DGAPA-UNAM under grant no. IN102202. We thank DGSCAUNAM for providing the supercomputing facilities. References and Notes (1) Transition Metal Hydrides, Dedieu, A. ed.;Wiley-VCH: New York, 1992. (2) Yvon, K.; Renaudin, G. In Encyclopedia of Inorganic Chemistry, 2nd ed.; King, R. B., Ed.; Wiley: New York, 2005; pp 1814-1846. (3) Chotard, J. N.; Filinchuk, Y.; Revaz, B.; Yvon, K. Angew. Chem., Int. Ed. 2006, 45, 7770-7773. In the present work, we will strictly follow the atomic site nomenclature employed in this report. (4) Hoffmann, R. D.; Fugmann, A.; Rodewald, U. Ch.; Po¨ttgen, R. Z. Anorg. Allg. Chem. 2000, 626, 1733. (5) Blaha, P.; Schwarz, K.; Madsen, G. K. H.; Kvasnicka, D.; Luitz, J. WIEN2k, An Augmented Plane WaVe + Local Orbitals Program for Calculating Crystal Properties; Technische Universita¨t: Wien, Austria, 2001; ISBN 3-9501031-1-2. (6) (a) Kresse, G.; Hafner, J. Phys. ReV. B 1993, 47, R6726. (b) Kresse, G.; Furthmuller, J. Comput. Mater Sci. 1996, 6, 15. (c) Blo¨chl, P. E. Phys. ReV. B 1994, 50, 17953. (d) Kresse, G. J. Joubert, ibid. 1999, 59, 1758. (7) Perdew, J. P.; Burke, S.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3865.

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