Cubic and orthorhombic structures of aluminum hydride Al H3 predicted by a first-principles study

The most stable structure of aluminum hydride Al H3 is believed to be a hexagonal symmetry. However, using the density functional theory, we have identified two more stable structures for the Al H3 with the cubic and orthorhombic symmetries. Based on the quasiharmonic approximation, the cubic and orthorhombic Al H3 are almost degenerate when the zero-point energies are included. The geometric and electronic structures, the phonon, and the thermodynamic properties for the hexagonal, cubic, and orthorhombic Al H3 have been studied by means of density functional theory and direct ab initio force constant approach. The calculated electronic structures, phonon density of states, and thermodynamic functions [including S(T) and H(T) - H(0)] for the three hydrides are similar. The results show that these three hydrides have negative enthalpies of formation, but positive free energies of formation. This conclusion is the same as that made by Wolverton for the hexagonal Al H3 [Phys. Rev. B 69, 144109 (2004)]. The thermodynamic properties indicate that the orthorhombic and cubic Al H3 should be more difficult to dissociate than the hexagonal Al H3.