First principles study on elastic and electronic properties of bialkali alanates M2M′AlH6

The elastic and electronic properties of a series of bialkali alanates M2M′AlH6 (Na2LiAlH6, K2LiAlH6, K2NaAlH6) have been investigated by first principles calculations. The calculated lattice parameters and decomposition enthalpies are in good agreement with the experimental and other available data. All M2M′AlH6 hydrides are found to be mechanically stable and brittle. The elastic moduli, Vickers hardness and the Debye temperature follow the order of Na2LiAlH6 > K2LiAlH6 > K2NaAlH6. The calculated elastic anisotropy indexes AZ, AU, AE indicate that the M2M′AlH6 hydrides exhibit obvious anisotropy and the sequence follows K2LiAlH6 > K2NaAlH6 > Na2LiAlH6. From the density of states, the crystal orbital Hamiltonian population, bonding charge density, electron localization function and the Bader charge analyses, it can be confirmed that the existence of polar covalent bonding between Al and H within the [AlH6]3− units and ionic bonding between [AlH6]3− units and M, M′.