Physical properties of LiXH₄ (X= B, Al) hydrogen storage materials: ab-initio study

The mechanical and thermodynamical features of LiBH₄ and LiAlH₄ complex hydrides in the α, β and γ phases are evaluated using density functional theory. To our knowledge, thermal parameters such as, the Grüneisen parameter γ, the heat capacity, and thermal expansion coefficient of LiXH₄ (X = B, Al) complex hydrides in α, β and γ phases are computed for the first time. The parameters are determined in terms of pressure (0–6 GPa) and temperature (0–600) K using the quasi-harmonic Debye model. The computed stress–strain relationships effectively yield the elastic constants of a single crystal. The estimated elastic constants of α-LiBH₄ compound are noticeably higher than those of several closely similar ABH₄ (A = Na, K, Rb, and Cs) borohydrides. In the β-LiAlH₄ tetragonal structure, the [001] direction is readily compressible as compared to the [100] direction. The compressibility modulus of the LiBH₄ compound is greater than that of the LiAlH₄. However, the distance between B and H is less than the distance between Al and H. This might be explained by the presence of covalent bonds in BH₄ and AlH₄ in LiXH₄ (X = B, Al). The melting temperatures of LiXH₄ (X = B, Al) are used to predict the temperatures of hydrogen decomposition. The obtained melting temperature of α-LiBH₄ is higher than that of β-LiAlH₄. Thus, LiAlH₄'s decomposition temperature at which hydrogen is emitted from a fuel cell is anticipated to be lower than that of the LiBH₄ compound. The bonding behavior of α-LiBH₄ is more directed than that of β-LiAlH₄. All the LiXH₄ (X = B, Al) are found to be ductile except α-LiAlH₄ which is found to be fragile. LiBH₄ experiences greater volume change during uniaxial deformation than LiAlH₄ and both are centrally strong solids. Our PBE calculations yield the linear compressibility and Young's modulus orientation dependent. The orthorhombic LiBH₄ and tetragonal LiAlH₄ are isotropic in bulk and anisotropic in Young's modulus.