New High-pressure Phases of Lithium and Sodium Aluminum Tetrahydrides and their Implications to Hydrogen Storage

A systematic high-pressure study on LiAlH4 and NaAlH4 has been carried out using density-functional total-energy calculations. From the theoretical simulations, it is found that both LiAlH4 and NaAlH4 are subject to structural phase transitions at high pressure. α-LiAlH4 transforms to β-LiAlH4 (α-NaAlH4–type structure) at 2.6 GPa with a huge volume collapse of ca. 22 % (according to equilibrium volumes) higher than ever observed among hydrides. This unusual behaviour is associated with an electronic conversion from Al-s to -p states. At 33.8 GPa, LiAlH4 is predicted to undergo a β-to-γ transition from α-NaAlH4to KGaH4-type structure. α-NaAlH4 converts to β-NaAlH4 (SrMgH4-type structure) at 6.4 GPa with a 4% volume contraction. The high-pressure β phases of both compounds would store hydrogen more volume efficient than the corresponding α phases. The calculated electronic structures of these materials at ambient and elevated pressures indicate that all phases have a nonmetallic character. The calculated band gap for β-NaAlH4 is almost half of that for the α phase, whereas the band gaps for αand β-LiAlH4 as nearly same.