First-Principles Studies Of Mg₂Ni And Its Complex Hydrides

The first-principles PAW pseudopotential method within the generalized gradient approximation (GGA) based on the density functional theory (DFT) is employed to investigate mainly the following three questions.1: The geometry and electronic structure of hydrogen storage material Mg₂Ni have been studied by first principles calculations. The crystal structure, density of states and valence electron charge density contours has been shown. On one hand, we want to obtain the theory lattice constant and electronic structure; on the other hand, the PAW+ (GGA+U) method will be justifed by comparing with results attained by other theoretical or experimental methods. It is found that the results obtained by our calculations are in good agreement with other theoretical results and experimental data.2: First-principle calculations are carried out to investigate the energetics and electronic structures of Mg₂Ni alloy and its high/low temperature hydrides. Results show that strong bonding interaction between Mg and Ni atoms exists in the Mg₂Ni alloy. Upon H insertion, charge transfer occurs from Mg to Ni-3d, which gives rise to a band gap between Ni-3d and Ni-4p orbitals. The interaction between Mg and Ni in LT/HT-Mg₂NiH₄ hydrides is weakened due to the covalent bonding between Ni-4s and H-1s, indicating that Mg₂Ni alloy is good for de-hydrogenation reaction and hydrogenation reaction.3: The first-principle calculations based on the denstiy functional theory also employed to investigate crystal structure and electronic structure of Mg₂Ni alloy's high/low temperature hydrides. The results show that it costs more energy to remove H atoms from LT-Mg₂NiH₄ than from HT-Mg₂NiH₄, indicating that HT-Mg₂NiH₄ is more suitable for de-hydrogenation reaction while the LT-Mg₂NiH₄ is suitable for hydrogenation reaction.