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A comparative study of single- and co-substitution of Ti on dehydrogenation of Mg2NiH4 has been carried out from first-principles calculations based on density functional theory. In comparison with Ti single-substitution, the formation enthalpy in co-substituted Mg2NiH4 is higher, showing the lower thermodynamic stability. In Ti co-doped Mg2NiH4, the average Ni H bond length of entire unit cell is larger, implying weaker Ni H bonds and lower stability of complexes NiH44−. Ti co-substitution further shows a more favorable dehydrogenation effect on Mg2NiH4 due to lower hydrogen dehydrogenation energy, and significantly reduces the dehydrogenation reaction enthalpy of Mg2NiH4 by about 30% compared to the pure state. Further calculated electronic structure demonstrates that the co-substituted Mg2NiH4 displays a metallic behavior with the Fermi level locating at the doping band, and the underlying mechanism for improving dehydrogenation properties of co-substituted Mg2NiH4 can be attributed to the weakened Ni H interactions together with the narrowed energy gap.

The structural evolution and electronic mechanism for KBH 4 phase transition have been systematically investigated using first-principles calculations. The present results demonstrate experimentally observed stability order of α-KBH 4 > β-KBH 4 > γ-KBH 4 , and the predicted transition pressure at 0 K for cubic → tetragonal → orthorhombic phase transformation agrees reasonably with experimental measure. The atomic process indicates that KBH 4 structural evolution is closely related to rotation and deformation of [BH 4 ] − clusters as well as shift of [BH 4 ] − clusters and K atoms. Along with phase transformation, the band gap of KBH 4 narrows, the B H covalent bond weakens while K H interaction enhances, the K K anti-bonding and coulomb repulsion are also stronger.

The crystal structure of LiCa(AlH4)3 was investigated via first principle calculations, especially the positions of hydrogen atoms undetected in XRD experiments were predicted, then the thermodynamic favourability of the experimentally reported structure with respect to several candidates from the inorganic crystal structure database (ICSD) was confirmed. It is found that hexagonal packing of AlH4 layers along the c axis is present in LiCa(AlH4)3, and the detailed geometrical feature is further revealed. The electronic structures show that in LiCa(AlH4)3 the Li–AlH4 interaction is more covalent than in LiAlH4, while the Ca–AlH4 covalence is less than in Ca(AlH4)2. The overall stronger covalence in LiCa(AlH4)3 leads to weakened Al–H bonds. The Li–H interaction in LiCa(AlH4)3 dramatically turns out to be strong bonding, opposite to the Li–H anti-bonding in LiAlH4. The Ca–H bonds are more anti-bonding in LiCa(AlH4)3.

First-principles calculations based on density functional theory (DFT) were performed to study the destabilizing mechanism of co-doped MgH 2 with Al and Y. From the minimization of total electronic energy, the preferential positions of dopants are determined. The calculated formation enthalpy and substitution enthalpy show that incorporation of Al combined with Y atoms into MgH 2 is energetically favorable relative to Al doping alone. Due to strong interaction of the dopant Y with Mg and Al, the hydrogen dissociation energy and the dehydrogenation enthalpy are both reduced, indicating that the synergetic effect of Al and Y on destabilizing the MgH 2 is superior to that of Al doping. The electronic structures show that the breakage of Mg–H bond is much easier in co-doped case, because of the conduction band shift below the Fermi level and the hybridization of dopants with Mg atoms, which effectively decrease the hybridization between Mg and H.

The crystal and electronic structure of the title compound is investigated by first principle DFT calculations.