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

Abstract

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.