The Structures and Properties of Y-Substituted Mg 2 Ni Alloys and Their Hydrides : A First-Principles Study

The structures and properties of Y-substituted Mg2Ni alloys and the corresponding hydrides are investigated by a first-principles plane-wave pseudopotential method within density functional theory. Results show that Mg2Ni has the best structural stability when Y atom occupies the Mg(6f) lattice sites. The calculated enthalpies of formation for Mg2Ni, Mg2NiH4 and Mg15YNi8H32 are −51.612, −64.667 and −62.554 kJ/mol, respectively. It is implied that the substitution of Y alloying destabilizes the stability of the hydrides. Moreover, the dissociated energies of H atoms are decreased significantly, indicating that Y alloying benefits the improvement of the dehydrogenating properties of Mg2Ni hydrides. The calculation and analysis of the electronic structures suggest that there is a stronger interaction between H and Ni atoms than the interaction between H and Mg atoms in Mg2NiH4. However, the Ni-H bond is weakened by the substitution of Y. Therefore, the substitution is an effective technique to decrease the structural stability of the hydrides and benefit for hydrogen storage.


Introduction
Due to rich reserves in the earth's crust, high hydrogen capacity (3.6 wt%), light weight and low cost, Mg 2 Nitype alloy hydrides remain as attractive hydrogen storage materials [1] [2].However, the practical application of the alloy materials has not been achieved because of unfavorable thermodynamics, poor hydrogenation/dehydrogenation kinetics and releasing undesirable by-products [3].
Many researches have been devoted to overcoming these drawbacks and improving the properties of hydrogen storage via modifying microstructure by mechanical alloying [4], alloying with other elements [5] [6], adding catalysts [7] and composite structures [8].The effects of transition metals including Cu, Co, Mn, Y, Ti, N band Crelements [9]- [12] on the hydrogen storage properties of Mg-based metal hydrides are investigated and discovered that the properties of hydrogen storage are improved by alloying with a small amount of transition metals in different degrees.
It is believed that alloying of Mg 2 Ni with transition metals is beneficial to improve the hydrogenating and dehydrogenating kinetics.The electronic structure of element Y is 4d 1 5s 2 and it can be incorporated into the metal boride.In addition, its chemical properties and physical performance are similar to La which can be used as an alloy element for hydrogen storage.The density and cohesive energy of Y atom are also relatively small.Therefore, Y has great potential to improve the performance of Mg 2 Ni alloy and its hydride.Kalinichenka et al. [13] studied that Y can be solved in Mg 2 Ni and the Mg-Ni-Y alloy exhibits higher dehydrogenation rates comparing with that of the Mg-Ni alloy.Song et al. [14] reported the microstructure and the hydrogenation properties of melt-spun Mg 67 Ni 33−x Y x alloys and found that the hydrogen storage capacity and kinetics of Mg 2 Ni are improved with Y doping.Zhang et al. [15] investigated that the substitution of Y for Mg had an insignificant effect on the activation ability of the Mg 2 Ni-type alloys, but it dramatically improved the cycle stability of the asmilled alloys.These experiments proved that Y plays an important role in improving the properties of Mg 2 Ni alloy for hydrogen storage.Thus, my understanding is that, alloying of Mg 2 Ni with Y can be expected to improve some performances of hydrogen absorption/desorption capacity and kinetics significantly.
In recent years, a number of theoretical investigations about the doped/substituted complex hydrides using first-principles calculations have been reported [16]- [19].A first-principles study on the structures and properties of hydrogen storage alloy Mg 2 Ni, of aluminum and silver substituted alloys Mg 2−x M x Ni (M = Al and Ag), and of their hydrides Mg 2 NiH 4 , Mg 2−x M x NiH 4 was performed by Zeng et al. [20].Their results show that the hydrogen storage capacity is decreased by the substitution and the substitution destabilizes the hydrides.However, there are no available theoretical reports about the structures and properties of Y substituted Mg 2 Ni alloys and their respective hydrides to the authors' knowledge.The models are new for the materials to store hydrogen.
We focus primarily on the stable configuration of Mg 2 Ni alloys with Y substitution and determine the optimum position of Y. Furthermore, the energies, enthalpies of formation and electronic structures of Y alloying Mg 2 Ni and its hydrides are also calculated and analyzed using a first-principles plane-wave pseudo potential simulations based on the density functional theory in this paper.These simulations are beneficial to improve our understanding of the effects of substitution on the properties of Mg 2 Ni, and of the design about advanced magnesium-based hydrogen storage materials.

Computational Model
The crystal structure of Mg 2 Ni is hexagonal and its space group is P6 2 22 (No.180) [21], as shown in Figure 1(a

Computational Method
All the density-functional theory (DFT) calculations are performed using a plane-wave basis set with the projector augmented plane wave (PAW) method as implemented in the Vienna ab initio simulation package (VASP) [25]- [27].Projector Augmented Wave (PAW) potentials are used to treat the core-valence interaction [28].The PW91 [29] [30] generalized gradient approximation (GGA) is employed for the exchange-correlation functional.The electronic wave functions are expanded by plane waves with a kinetic energy cutoff of 350 eV to attain the required convergence.All of the self-consistent loops are iterated until the total energy difference of the systems between the adjacent iterating steps is less than 10 −7 eV.The Brillouin zone is sampled by 6 × 6 × 2 mesh points in k-space based on Monkhorst-Pack scheme [31] for all systems.The valence electrons of 1s for H, 2p and 3s for Mg, 3p, 3d and 4s for Ni, and 4d and 5s for Y are considered in the calculations.

The Structure of Substituted Mg 2 Ni by Y
In order to check the accuracy of the calculations, we first optimize the structure of Mg 2 Ni alloy and its hydride and compare the calculated lattice parameters with those determined experimentally.Then we consider the substitution of Mg and Ni by Y in independent spatial positions respectively.To single out a scenario that is most likely responsible for the stabilization of the crystal structure, the lattice parameters and enthalpies of formation ΔH for each case are calculated.The ΔH is calculated by taking the difference in total electronic energy of the products and the reactants [32]: In the case of the crystal structure Mg Y Ni x y z which including xMg, yY, zNi, the enthalpies of formation are calculated by the following equation: where ( )  [33].When Y atom is added into Mg 2 Ni, all the volumes of crystal structures will increase compared with the original structures.Moreover, it can be clearly observed that when the position of Mg (6f) is occupied by Y atom in Mg 2 Ni, the total energy and the enthalpy of formation are the minimum.It indicates that the structure of Mg 11 Y(6f)Ni 6 has the optimal stabilization among all the substituted structures.
In the same way, the reaction of formation and the enthalpy of formation of Mg 15 YNi 8 H 32 can be respectively written as Equations ( 5) and ( 6 where ( ) Mg Ni E , ( ) The results are shown in Table 2. From Table 2 we can see that the addition of Y clearly decreases the dehydrogenation energy of Mg 2 NiH 4 by about 47% to 0.983 eV.It suggests that although Y atom has poor effects on the destabilization of Mg 2 Ni, it breaks down the stability of Mg 2 NiH 4 positively and improve the dehydrogenation kinetics of Mg 2 NiH 4 which as one of the hydrogen storage materials.

Electronic Structure
In order to further understand the effects of Y atom on the dehydrogenation properties of Mg 2 NiH 4 alloy, the electronic properties of Mg 2 Ni and Mg 15 YNi 8 H 32 are studied by calculating total density of states (DOS) and partial density of states (PDOS).Compared to pure Mg 2 NiH 4 , the enthalpy of formation and dehydrogenation energy change markedly due to the substituted Mg 2 NiH 4 by Y. Figure 2(b) displays that below Fermi level Mg 15 YNi 8 H 32 has two main bonding peaks from −10.7 to −5.2 eV and −4.1 to −1.6 eV.It is not difficult to find that all the bonding peaks in total density of states move to the energy of deep potential well and the number of bonding electron reduces comparing to Mg 2 NiH 4 .It demonstrates that the substitution of Y alloying weakens the interaction of the atoms and destabilizes the structure of the hydride.The effects of Yd orbit on the bonding electron are significant especially for the energy region from −4.1 to −1.6 eV.What is more, Yp and d orbits contribute to the bonding electron and have mutual interaction with Nip and d orbits.It is also worth noting that the overlapping region between Nid and Hs orbits decreases obviously.It means that the interaction between Ni and H atoms become weak.

Conclusion
We have investigated the structure and properties of substituted Mg 2 Ni alloys by Y and the corresponding hydrides.The structure parameter, enthalpy of formation, dehydrogenation energy and electronic structure are calculated by the first-principles method based on density functional theory in this paper.Through analyzing the simulation results, we can draw the conclusions that when Y atom occupies the Mg(6f) lattice site, the structure of Mg 2 Ni is the optimal stable.The substitution of Y destabilizes the stability of Mg 2 NiH 4 and decreases the dissociated energies of H atoms due to the Ni-H bond weakened by Y. Therefore, the method of substitution is in favor of the dehydrogenation reaction for Mg-based hydrides as hydrogen storage materials.Moreover, we will continue to perfect this respect, for instance, whether the effect of Y elements in the case of different numbers of Y metals and different substituents will change.
total energy of substituted Mg 2 Ni by Y. of every atom in HCP Mg, HCP Y and FCC Ni crystals, respectively.x, y, z are the numbers of Mg, Y and Ni atoms, respectively.Through the calculation, the values of 595, −6.379 and −5.415 eV, respectively.Table 1 displays the volume, lattice constant, total energy and enthalpies of formation of all the structures including Mg 2 Ni, substituted Mg 2 Ni by Y and their hydrides.The lattice constants of Mg 2 Ni after geometry optimization are a = b = 5.180 Å, c = 13.232Å, which agree well with the experimental data a = b = 5.205 Å, c = 13.236Å [21].The enthalpy of formation of Mg 2 Ni is −3.211 eV, which means that the unit cell of Mg 2 Ni is −51.612kJ/mol.It is very close to the experimental values −51.9 kJ/mol

Figure 2
displays the DOS and PDOS of Mg 2 Ni and Mg 15 YNi 8 H 32 alloys.Form Figure 2(a) we can see that there are two main peaks in total density of states below Fermi level.The bonding electron of the energy region between −9.2 and −3.7 eV is mainly dominated by Hs, Nis and Nid orbits, partial Mgs orbit.It is implied that H atoms tend to bond with Ni rather than Mg atoms in the structure of Mg 2 NiH 4 .The result is in correspondence with the conclusion that the interaction Ni-H is stronger than that of Mg-H which studied by Jasen [34].There is a major contribution with Ni p, Ni d and Mg s orbits in the region from −2.4 eV to Fermi level.This indicates that Mg and Ni atoms have hybridization which keeps the structure of Mg 2 NiH 4 stable.In addition, Ni d orbit plays the dominating role in the bonding electron.

Figure 2 .
Figure 2. Total density of states and partial density of states of (a) Mg 2 NiH 4 , (b) Mg 15 YNi 8 H 32 .

Table 1 .
Volume, lattice constant, total energy, enthalpy of formation of Mg 2 Ni, Y-substituted Mg 2 Ni and their hydrides.

The Properties of Substituted Mg 2 NiH 4 by Y
[23]d on the stable structure of Mg 11 Y(6f)Ni 6 , we study the properties of substituted Mg 2 NiH 4 by Y. Firstly, We have proved that the theoretical lattice constants and internal atomic positions of Mg 2 NiH 4 are in good agreement with experimental results[23].The states are displayed in

Table 1 .
Various substitutive positions of Mg are considered.We find that the total energy of each new structure is very close.Thereby, a reasonable structure Mg 15 YNi 8 H 32 is selected to be investigated in detail, as shown in Figure1(c).In order to research the effects of Y on the properties of Mg 2 NiH 4 , We calculate the enthalpies of formation of Mg 2 NiH 4 and Mg 15 YNi 8 H 32 respectively.In general, the formation of Mg 2 NiH 4 can be expressed by the following reaction: )

Table 1 .
NiH 4 , Mg 2 Ni, Mg 15 YNi 8 H 32 and Mg 11 Y(6f)Ni 6 , respectively.For pure Mg 2 NiH 4 , the enthalpy of formation is −64.667kJ/mol which coincides closely with the experimental result −64.4 ± 4.2 kJ/mol reported by Reilly et al. [22].Furthermore, the enthalpy of formation of Mg 15 YNi 8 H 32 is −62.554kJ/mol which is higher than that of pure Mg 2 NiH 4 .It can be clearly seen that the introduction of Y atom has effects on the destabilization of Mg 2 NiH 4 in terms of energy.This is energetically favorable to perform the dehydrogenation reaction of substituted Mg 2 NiH 4 by Y.To make further investigation about the performance of dehydrogenation, we calculate the energies of Mg 2 NiH 4 and Mg 15 YNi 8 H 32 which dissociate the nearest 2 H atoms around Ni atoms.The dehydrogenation energy is calculated by Equation (7):