High-Pressure Raman and X-ray Diffraction Study of β- and γ-Polymorphs of Aluminum Hydride

Three polymorphs of alane, AlH3, (alpha, beta, and gamma) were synthesized and studied at high-pressure in diamond anvil cell by Raman spectroscopy and synchrotron X-ray diffraction. According to synchrotron X-ray diffraction study, beta-AlH3 is stable up to 6 GPa, followed by transformation into alpha phase at higher pressures. X-ray-induced decomposition gamma-AlH3 into constituent elements was found at 15 GPa. Raman scattering study at high pressure for both beta- and gamma-AlH3 reveals transition into the alpha phase with high concentration of structural defects. DFT calculations (VASP code) show that instability of cubic beta-alane crystal structure at high pressure is caused by the strong deformation of the [AlH6] polyhedra.