Electronic properties of lightweight intermetallic Li-Al hydrides in the γ-phase

An ab initio electronic structure study of the potential light-weight hydrogen storage materials LiAlH4 and Li3AlH6, in their γ-phase, is presented. Electronic band structures and total and partial densities of states for both compounds are calculated by using the WIEN2k simulation code. We estimate an energy bandgap of ca. 3.82 eV for the γ-LiAlH4 semiconductor and no gap in the γ-Li3AlH6 metal-like compound. In the latter, the valence band is formed by two distinct energy bands originating from hydrogen orbitals hybridization with metal atomic orbitals. Optical properties such as the refractive index n(ω), the reflectivity R(ω), the absorption coefficient I(ω), and energy-loss function L(ω) are determined from the complex dielectric function ε(ω) for incident photon energies in the 0-40 eV interval. It is found that these quantities are isotropic for cubic γ-Li3AlH6, while they are similar but distinguishable in the orthorhombic γ-LiAlH4 compound. The γ-Li3AlH6 hydride, as an anode in lithium ion cells, is more Li reversible than the γ-LiAlH4. Results are discussed and compared with available published data.