High-pressure polymorphism as a step towards high density structures of LiAlH₄

Two high density structures β- and γ-LiAlH₄ are detected in LiAlH₄, a promising hydrogen storage compound, upon compression in diamond anvil cells, investigated with synchrotron X-ray diffraction and first-principle calculations. The joint of the experimental and theoretical results has confirmed the sequence of the pressure-induced structural phase transitions from α-LiAlH₄ (space group P2₁/c) to β-LiAlH₄ (P2₁/c-6C symmetry), and then to γ-LiAlH₄ (space group Pnc2), which are not reported in previous literatures. At the α to β transition point for LiAlH₄, the estimated difference in cell volume is about 20%, while the transformation from β to γ phase is with a volume drop smaller than 1%. The α to β phase transition is accompanied by the local structure change from a AlH₄ tetrahedron into a AlH₆ octahedron, which contributes to a large volume collapse