ZHANG Yilong, CUI Man'ai, LIU Yanhui. Crystal Structure and Stability of LiAlH4 from First Principles[J]. Chinese Journal of High Pressure Physics, 2018, 32(2): 021103. doi: 10.11858/gywlxb.20170561
Citation:
ZHANG Yilong, CUI Man'ai, LIU Yanhui. Crystal Structure and Stability of LiAlH4 from First Principles[J]. Chinese Journal of High Pressure Physics, 2018, 32(2): 021103. doi: 10.11858/gywlxb.20170561
ZHANG Yilong, CUI Man'ai, LIU Yanhui. Crystal Structure and Stability of LiAlH4 from First Principles[J]. Chinese Journal of High Pressure Physics, 2018, 32(2): 021103. doi: 10.11858/gywlxb.20170561
Citation:
ZHANG Yilong, CUI Man'ai, LIU Yanhui. Crystal Structure and Stability of LiAlH4 from First Principles[J]. Chinese Journal of High Pressure Physics, 2018, 32(2): 021103. doi: 10.11858/gywlxb.20170561
ZHANG Yilong(1998—), male, undergraduate, major in computational physics under high pressure.E-mail:122696148@qq.com
Corresponding author:
CUI Man'ai(1979—), female, master, lecturer, major in computational physics under high pressure.E-mail:macui@ybu.edu.cn; LIU Yanhui(1971—), female, Ph.D, professor, major in computational physics under high pressure.E-mail:yhliu@ybu.edu.cn
The structural stability of LiAlH4, a promising hydrogen storage material, under high pressure was researched using the ab initio pseudopotential plane wave method.It is found that the phase transition occurs at 1.6 GPa from the α-LiAlH4 phase to the β-LiAlH4 (space group I2/b) phase.This phase transition is identified as first-order in nature with volume contractions of 18%.Moreover, the analysis of the phonon dispersion curves suggests that phase transition is related to the phonon softening.Mulliken population analyses indicated that the ambient phase (α-LiAlH4) is expected to be the most promising candidate for hydrogen storage.
Light metal complex hydrides are promising materials for solid-state hydrogen due to their gravimetric hydrogen density[1-5].For example, lithium alanate (LiAlH4) and sodium alanate (NaAlH4) have theoretical hydrogen capacities of 10.6% and 7.5% (mass fraction), respectively.Hence, LiAlH4 and NaAlH4 could be viable candidates for practical usage as on-board hydrogen storage.However, a serious problem with these materials is their poor kinetics and lacking reversibility with respect to hydrogen absorption/desorption.Recent experimental evidences show that LiAlH4 and NaAlH4 release 7.9% and 5.6% (mass fraction) of H, respectively.This represents nearly 4 and 5 times more stored hydrogen than LaNi5-based alloys which are currently used in nickel-based hydride batteries.However, it is difficult to accurately identify the positions of the hydrogen atoms by the high-pressure X-ray and neutron diffraction studies.Therefore, exploration of the phase stability and structures has attracted a great deal of attention.
Under ambient conditions, LiAlH4 crystallizes at a monoclinic structure with a space group P21/c (denoted as α-LiAlH4).The structure consists of AlH4 units separated by Li+ ions, and the hydrogen atoms are arranged around the aluminum atoms in an almost regular tetrahedral configuration.On the theoretical side, the α-LiAlH4 structure transforms into a new tetragonal β-LiAlH4 structure (space group I41/a) at 2.6 GPa with a 17% volume contraction[6].However, the neutron diffraction confirms that the structure of the high pressure β-LiAlD4 has a monoclinic space group I2/b[7].It is determined that there is a reversible phase transition back to the α-LiAlD4 with a slow release of pressure and cooling.Consequently, the Raman measurement confirmed that the ambient structure transform to the β-LiAlH4 structure (space group I2/b) at 3 GPa[8], as well as the previously reported transition pressure between 2.2 and 3.5 GPa[9].
In this work, we report our results on the geometries and electronic structures of LiAlH4 with pressure using the ab initio calculations.The total-energy calculations for the pressure-induced phase transition in LiAlH4 were performed.The structural transition from α-to β-LiAlH4 happened at 1.6 GPa.The full phonon dispersion curves were calculated, for the first time, to provide the evidence of phonon stability for pressured-induced phase transition.We also discussed the density of states and bond overlap populations about the two phases.
1.
Computational Method
The calculations were performed within the density functional theory (DFT), using the Perdew-Burke-Ernzerh of generalized gradient approximation functional[10-12].A plane-wave norm-conserving pseudopotential method[13] as implemented in the CASTEP code[14] was employed.We used the plane-wave kinetic-energy cutoff of 850 eV which was shown to give excellent convergence of the total energies and structural parameters.According to the Monkhorst-Pack method, k-point spacing smaller than 0.3 nm-1 was individually adjusted in reciprocal space to the size of each computational cell[15].The optimization was performed, all forces on atoms were converged to be less than 0.1 eV/nm and all the stress components are less than 0.02 GPa.The tolerance in the self-consistent field (SCF) calculation was set to 10-6 eV/atom.The phonon frequencies were calculated by the direct approach, which is based on the first-principles calculations of the total energy, the Hellman-Feynman forces, and the dynamical matrix as implemented in the phonon packages[16-17].Convergence test gave the use of 2×1×1 and 2×2×1 supercell in the phonon calculation.
2.
Results and Discussions
We optimize the crystal structures allowing simultaneously variations of unit cell and atomic positions at selected pressures. The calculated lattice parameters of α-LiAlH4 (space group P21/c) and β-LiAlH4 (space group I2/b) are listed in Table 1, which also includes the experimental data for comparison[18].For the α-LiAlH4 phase, the deviations between our calculated results and the experimental values are less than 2%, which is sufficiently accurate.This strongly supports the choice of the pseudopotentials and GGA approximation for the current study.The theoretical generated pressure-volume curve (see Fig. 1) shows that the α-LiAlH4 transforms to a monoclinic phase of β-LiAlH4 with a volume contraction (ΔV/V) of 18%, which suggests that this is a first-order phase transition.The huge volume contraction is consistent with the Raman scatting measurement and the other theoretical results[6, 8].In order to give a clear picture of the structural transition, the difference of the enthalpy (per formula unit) between the α-LiAlH4 and β-LiAlH4 phases as a function of pressure is shown in the insert of Fig. 1.It can be clearly seen that the β-LiAlH4 phase becomes energetically more favourable than the α-LiAlH4 phase above 1.6 GPa, which is the transition pressure from α-LiAlH4 to β-LiAlH4.
Table
1.
Optimized structural parameters, atomic position parameters for the α-LiAlH4 and the β-LiAlH4 structures
Figure
1.
Volume vs. pressure curves of α-LiAlH4 and β-LiAlH4 phases (Enthalpy difference (per formula unit) between α-LiAlH4 and β-LiAlH4 as a function of pressure is shown in the insert.)
To investigate the dynamic stability, we calculated the phonon dispersion curves along some high-symmetry lines in the Brillouin zones (BZ) and the corresponding phonon density of states (DOS) for the LiAlH4 structure.No imaginary frequency is observed throughout the whole BZ, indicating that the two novel phases are dynamically stable in the pressure region from this study.We also calculated the phonon dispersion curves at the phase transition pressure for the α-LiAlH4 structure in Fig. 2.For comparison, the stability of phonon dispersion curves for the α-LiAlH4 at 0 GPa are shown in Fig. 2(a).In Fig. 2(b), it is indicated that the phase transition should not be related to the pressure-induced phonon softening.The total and partial densities of states for the two phases are shown in Fig. 3. Analysis of the calculated electronic density of state reveals that the two phases exhibit a common insulating feature with a finite energy gap.The valence band (VB) in α-LiAlH4 phase is split into two regions.Subsequently, the two peaks combine into one broader peak in the β-LiAlH4 phase producing the broadening of VB under high pressure.This is originated from the shortened interatomic distance upon squeezing, for the VB region is mainly dominated by H 1s, and Al 2s, 2p states.It shows that the H and Al atoms become the directional covalent bonds within the AlH4 tetrahedron or AlH4 octahedron layers. The bottom of the conduction band just above the Fermi energy is composed of the Al 2s, 2p, Li 1s, and H 1s states, which are consistent with the ionic bonding between the Li and the AlH4 unit.We found more mixing of the s and p states for Al atom in the β-LiAlH4 phase.Consequently, the electronic transition from the Al-s to -p state is related for the huge volume collapse during the α-LiAlH4 to β-LiAlH4 phase transition.
Figure
2.
Phonon dispersion relations and density of state in high-symmetry directions for the α-LiAlH4 structure at 0 and 1.6 GPa
Figure
3.
Calculated total and partial electronic densities of states for α-LiAlH4 structure at 0 GPa (a) and β-LiAlH4 structure at 1.6 GPa (b) respectively
In order to better understand the bonding interactions between H, Al and Li atoms, we studied the Mulliken charges and the bond overlap population (BOP) P on the basis of Mulliken Populations in Table 2[19-20].The scaled BOP (Ps) is defined in PAl(Li)─Hs=PAl(Li)─H/LAl(Li)─H with L representing the average bond length of Al─H or Li─H, respectively[21].It is found that the PAl─Hs value for the α-LiAlH4 phase (0.506) is smaller than that of the β-LiAlH4 phase (0.534) as the coordination number of Al (4) stays unchanged at the α-β transition.Previous studies have shown that the smaller the BOP, the lower the hydrogen desorption kinetic energy[22].From this point of view, the current study suggests that the activation energy of the α-LiAlH4 is lower than that of the β-LiAlH4. The ambient phase of α-LiAlH4 is expected to be the most promising candidate for hydrogen storage.
Table
2.
Average net charges, bond length (L) and scaled bond overlap population (Ps) between H, Al, and Li atoms in the α-LiAlH4 (at 0 GPa) and the β-LiAlH4 (at 2.0 GPa) structures
We investigated the pressure-induced phase transformations in LiAlH4 using the first-principles based on the density functional theory with the plane-wave basis.The structural transitions from α-LiAlH4 to β-LiAlH4 occurs at 1.6 GPa, accompanied with about 18% volume collapse, originating from the electronic transition of Al-s to -p states.The phase transition should be related to the pressure-induced phonon softening.The BOP analysis shows that the activation energy of the α-LiAlH4 is smaller than that of the β-LiAlH4. The α-LiAlH4 is expected to be the most promising candidate for hydrogen storage.
Acknowledgements:
This work was supported by the High Performance Computing Center of Yanbian University.
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ZHANG Yilong, CUI Man'ai, LIU Yanhui. Crystal Structure and Stability of LiAlH4 from First Principles[J]. Chinese Journal of High Pressure Physics, 2018, 32(2): 021103. doi: 10.11858/gywlxb.20170561
ZHANG Yilong, CUI Man'ai, LIU Yanhui. Crystal Structure and Stability of LiAlH4 from First Principles[J]. Chinese Journal of High Pressure Physics, 2018, 32(2): 021103. doi: 10.11858/gywlxb.20170561
Table
2.
Average net charges, bond length (L) and scaled bond overlap population (Ps) between H, Al, and Li atoms in the α-LiAlH4 (at 0 GPa) and the β-LiAlH4 (at 2.0 GPa) structures
Figure 1. Volume vs. pressure curves of α-LiAlH4 and β-LiAlH4 phases (Enthalpy difference (per formula unit) between α-LiAlH4 and β-LiAlH4 as a function of pressure is shown in the insert.)
Figure 2. Phonon dispersion relations and density of state in high-symmetry directions for the α-LiAlH4 structure at 0 and 1.6 GPa
Figure 3. Calculated total and partial electronic densities of states for α-LiAlH4 structure at 0 GPa (a) and β-LiAlH4 structure at 1.6 GPa (b) respectively
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