Complex Rare-Earth Aluminum Hydrides: Mechanochemical Preparation, Crystal Structure and Potential for Hydrogen Storage

A novel type of complex rare-earth aluminum hydride was prepared by mechanochemical preparation. The crystal structure of the REAlH6 (with RE = La, Ce, Pr, Nd) compounds was calculated by DFT methods and confirmed by preliminary structure refinements. The trigonal crystal structure consists of isolated [AlH6](3-) octahedra bridged via [12] coordinated RE cations, The investigation of the rare-earth aluminum hydrides during thermolysis shows a decrease of thermal stability with increasing atomic number of the RE element. Rare-earth hydrides (REHx) are formed as primary dehydrogenation products; the final products are RE-aluminum alloys. The calculated decomposition enthalpies of the rare-earth aluminum hydrides are at the lower end for reversible hydrogenation under moderate conditions. Even though these materials may require somewhat higher pressures and/or lower temperatures for rehydrogenation, they are interesting examples of low-temperature metal hydrides for which reversibility might be reached.