Comparison of High Pressure-Induced Phases of Mg ( AlH 4 ) 2 as Hydrogen Storage Using Ab Initio Calculated NQCC Parameters

Nuclear quadrupole resonance (NQR) spectroscopy is proven to be a very sensitive technique for measuring distribution of electric charge around quadrupolar nuclei. Quadrupolar parameters of nuclei can be used as a tool to understand the electronic structure of compounds. The electronic structure of magnesium alanate, Mg(AlH4)2, as promising hydrogen storage materials for hydrogen fuel cell-powered automobile applications, has been studied in detail by ab initio calculated NQR parameters. Furthermore, using calculated nuclear quadrupole coupling constants (NQCCs) of hydrogen atoms (2H-NQCC), the electronic structure of α-Mg(AlH4)2 with its high-pressure forms, βand γMg(AlH4)2, was compared. The electric field gradient (EFG) at the site of 2H atoms was calculated to obtain NQCC parameters. The results show that in the γ-Mg(AlH4)2, 2H-NQCCs are smaller than that of other considered phases. In other words, Al–H bonds in γ-Mg(AlH4)2 nanocrystal is weaker than others and the charge transfer from Al to hydrogen atom is less than the others and therefore these hydrogens have weaker bonds with Al and easier condition for dehydrogenation is expected in γ-Mg(AlH4)2. Comparison of calculated dehydrogenation enthalpies of various Mg(AlH4)2 phases verifies this prediction. All calculations performed using Gaussian 03 at the HF/3-21G level of theory. The selected level and basis set give the rather acceptable qualitative NQCCs of hydrogen atoms.


INTRODUCTION
The alanates (complex aluminohydrides) possess a relatively high gravimetric hydrogen density.They are also considered among the most promising solidstate hydrogen-storage materials.][10][11] Mg(AlH 4 ) 2 was first synthesised in 1950 by Wiberg and Bauer, while its thermal decomposition behaviour was also studied. 6,9,10,13The crystal structure of Mg(AlH 4 ) 2 (hereafter, α-Mg(AlH 4 ) 2 ) has been determined by X-ray and neutron diffraction experiments. 14sing ab initio methods, the electronic structure and vibrational properties of some magnesium alanates were studied by Setten et al., and Spano and Bernasconi, respectively. 15,16It is known that Mg(AlH 4 ) 2 retains a higher capacity for hydrogen storage compared to sodium alanate. 17,18It was also reported that Mg(AlH 4 ) 2 readily decomposes at temperatures below 200°C. 10periments on X-ray powder-diffraction found that, under ambient conditions, the α-Mg(AlH 4 ) 2 structure (Figure 1) exhibits the space-group symmetry P -3m1 with a CdI 2 -layered structure.Except for Ca(BF 4 ) 2 -type structure, in which magnesium atom located at the centre of a distorted square anti-prism is coordinated by eight H atoms, in α-Mg(AlH 4 ) 2 and other proposed magnesium alanates, there almost exist two kinds of polyhedral.First is the AlH 4 tetrahedron in which the aluminium atom is tetrahedrally coordinated by one H1 and three H2 atoms.Second is the MgH 6 octahedron in which the magnesium atom is octahedrally coordinated by six H 2 atoms. 19lculations on the total energy indicate that under ambient pressure the structure of α-Mg(AlH 4 ) 2 found by experiments is more stable than the other proposed structures. 19Nevertheless, despite these detailed studies and the promise of magnesium aluminium hydride systems, there remain still considerable challenges, which are impeded by uncertainties about thermodynamic and kinetic parameters of removing hydrogen from them and adding hydrogen to them.Hu et al. showed that with pressure increasing the structural transition from α-to β-Mg(AlH 4 ) 2 (δ-Zr(MoO 4 ) 2 -type structure) and then β-to γ-Mg(AlH 4 ) 2 (Ca(BF 4 ) 2 -type structure) occur at 0.67 and 10.28 GPa respectively, accompanied by volume reductions of 6.6% and 8.7%. 19derstanding the bonding nature of aluminium and hydrogen is essential in order to improve its fundamental dehydrogenation performance.To further understand the nature of the bonding, charge density distribution is typically investigated by Nuclear Quadrupole Resonance (NQR) spectroscopy analysis. 20QR spectroscopy is a very sensitive technique for measurement of the electric charge distribution around quadrupolar nuclei (I > 1/2). 21The quantum mechanical approach is an effective method in determination of the charge distribution in molecules or complexes. 22In this method, the electric field gradient (EFG), resulting from whole molecular charges, can be estimated at any point in the molecular space. 23Verification of the EFG has become possible by the quadrupolar nuclei which possess a nuclear quadrupole moment and interact with molecular EFG tensor. 23This interaction is measured by the nuclear quadrupole coupling constant (NQCC).
][22][23] Thus, quantum chemistry calculation of the expectation values of the components of the EFG tensor allows calculation of the components of the NQCC tensor.The NQCC of a nucleus is a perfect criterion for determination of charge density on a nucleus.In the present paper, calculated NQCCs of 2 H nuclei in a unit cell of nanocrystal of Mg(AlH 4 ) 2 and some pressure-induced structural transitions of this compound were used to explore the electronic structure and steric factors controlling Al-H bond strength of these compounds.

COMPUTATIONAL DETAILS
Experimentally established nanocrystal structure data for α-Mg(AlH 4 ) 2 were used as input (Figure 1).Experimental structural data for β-and γ-Mg(AlH 4 ) 2 are not available, but the calculated findings from reference are included in Table 1. 13,19 unit cell of β-Mg(AlH 4 ) 2 and γ-Mg(AlH 4 ) 2 nanocrystals are shown in Figures 2  and 3.The HF/3-21G computational model as implemented in the Gaussian software package has been shown to be effective for efficient and accurate calculation of deuterium NQCC tensors.[24][25][26][27] We report here the results of calculations made for the NQCC tensors using this model as implemented in the Gaussian 03.24

Evaluations of NQCCs
NQR spectroscopy or zero Field NMR is a chemical analysis technique mediated by the interaction of the electric field gradient (EFG) with the quadrupole moment of the nuclear charge distribution. 22Briefly, the electric field gradient at the nucleus due to its external charges is conveniently described using spatial derivatives of the corresponding electrostatic potential, V, evaluated at the nucleus:   Thus, the EFG can be described by a real, symmetric, traceless 3×3 tensor that in the principal axes system the components satisfy that: q q q $ $ yy zz xx .A non-zero electric quadrupole moment arises for nuclei that are classically non spherical.Values of Q are conveniently expressed in units of 10 -24 cm 2 = 1 barn.Nuclear quadrupole coupling constant can be calculated using: where h is the Planck's constant, Q is nuclear electric quadrupole moment and q zz is the Z component of the EFG tensor in the principal axes system. 236][27] Among the wide range of published standard values of Q( 2 H), we selected Q( 2 H) = 2.86 mb reported by Pyykko. 28It is evident that since the bond properties depend on electrons, it is possible to replace hydrogen atoms by deuterium, assuming no structural changes will occur.

RESULTS AND DISCUSSION
Calculated NQCCs of nuclei seem to be an appropriate tool for better understanding of the electronic structure of compounds.NQCC is a proper criterion for the charge density of atoms.It is essential to calculate the electric field gradient (EFG) tensor at a nucleus to achieve theoretical calculation of NQCCs.According to NQCC expression, NQCC of nuclei is directly proportional to q zz .
Charge density on the nucleus and symmetry of EFG around the quadrupolar nucleus are effective factors in the values of q zz .It is evident that the importance of NQCCs is to be found in the different values of the field gradient for the same nucleus in different molecules.Nucleus with higher charge density has greater q zz and consequently larger χ.In this present work, the calculated NQCCs of hydrogens of pressure-induced phases of magnesium alanate were studied to find a possible relationship between their electronic structures and their hydrogen desorption ability.NQR parameters are highly sensitive to local charge distribution.These calculations worked and since there is no experimental data on NQCCs of considered compounds, the results of these calculations were applied in qualitative predictions.

Study of the Charge Density of Hydrogen Atoms in α-, β-and γ-Mg(AlH 4 ) 2
The α → β→ γ transition under high pressure is believed to be feasible for Mg(AlH 4 ) 2 .The mechanism of the high-pressure structural transition was described by analysing the variation in their structural and electronic structures by Hu et al., and it was found that all the α, β and γ phases exhibit a common nonmetallic feature and it's a great source for hydride ions. 19e results from the calculated density of states (DOS) for α-, β-and γ-Mg(AlH 4 ) 2 is consistent with the ionic bonding between Mg and the AlH 4 subunit. 19Mg(AlH 4 ) 2 is an ionic compound comprised of Mg 2+ and AlH 4

−
. It is a great source for hydride ions.Aluminium has a low electronegativity.Therefore, the Al-H bond is very polarised with Al being positive and H being negative.In order to form a binding, the electrical charge must be transferred from aluminium atom to hydrogen.For a strong binding the mentioned charge transfer must be significant and more complete.
6][27] In this work, Al-H bond strength in various modifications of Mg(AlH 4 ) 2 unit cell was studied using calculated NQCCs.The results are shown in Table 2.
From expression χ = e 2 Qq ZZ /h, it is obvious that NQCC of nuclei is directly proportional to q ZZ .Thus larger 2 H-NQCC is equal to stronger Al-H bond.
Table 2 shows that in γ-Mg(AlH 4 ) 2 , some calculated 2 H-NQCCs are smaller than 10 KHz while in α-and β-Mg(AlH 4 ) 2 some 2 H-NQCCs about 500 KHz and 1000 KHz are seen.Small 2 H-NQCCs in γ-Mg(AlH 4 ) 2 are related to hydrogen atoms with low charge density.In other words, Al-H bonds in γ-Mg(AlH 4 ) 2 nanocrystal is weaker than others and the charge transfer from Al to hydrogen atom is less than the others.Therefore, these hydrogens have weaker bonds with Al.It is expected that in the γ-Mg(AlH 4 ) 2 phase, hydrogen atoms can be removed easier and the γ phase stands out as a promising candidate for hydrogen storage and this highpressure induced phase has easier condition for dehydrogenation.

Comparison of Dehydrogenation Ability of Pressure-Induced Phases of Mg(AlH 4 ) 2 using Calculated Enthalpies
Dehydrogenation reaction enthalpies are based on the dehydrogenation down to the dihydride: In Table 3, calculated enthalpies of dehydrogenation reaction for one formula unit (f.u.) of α-Mg(AlH 4 ) 2 and other pressure-induced phases of Mg(AlH 4 ) 2 were compared.Inspection of Table 3 shows that ΔH value of γ-Mg(AlH 4 ) 2 is more negative than other considered phases and in this case the energy used for the bond breaking of γ-Mg(AlH 4 ) 2 is smaller than the energy released in the reaction, which corroborate the predicted results using calculated NQCCs.

CONCLUSION
According to the data obtained from charge distributions, the quadrupolar parameters of nuclei can be used as a useful tool to understand the electronic structure of the compounds.In γ-Mg(AlH 4 ) 2 , hydrogens have small NQCC and therefore these hydrogens have weaker bonds with Al and easier dehydrogenation is expected in γ-Mg(AlH 4 ) 2 .Comparison of calculated dehydrogenation enthalpies of Mg(AlH 4 ) 2 phases verifies this point.

Table 1 :
Structural parameters for Mg(AlH 4 ) 2 nanocrystals in ambient and high-pressure phases.

Table 3 :
The calculated enthalpies of dehydrogenation reaction for one formula unit, f.u. of α-Mg(AlH 4 ) 2 and other pressure induce phases of Mg(AlH 4 ) 2 .