Low Pressure-induced Phase Transition Study of α-AlH3 and its Hydrogen Storage Evaluation for Energy Applications via Electronic, Structural, Thermodynamic and Optical Analysis

Metal hydrides are currently one of the most investigated materials for energy storage applications. In this study, the structural, electronic, and optical properties of α-AlH3 have been investigated from density functional theory (DFT) calculations. This work is aimed at studying the low pressure-induced phase transition of α-AlH3 hydride material in the range of 1 - 8 GPa pressure. Self-consistent field (scf) calculations based on the DFT approach were performed using exchange-correlation functionals from generalized gradient approximation (GGA). Structural properties such as lattice constant (4.6973 Å), formation enthalpy, bond distance of Al – H, Al – Al, and H – H, are observed to be consistent with experiments. The electronic density of state (DOS) revealed the band gap as 2.16 eV. Analysis of the gravimetric storage and volumetric storage capacities, as well as the desorption temperature of α-AlH3 have been performed. The electronic band structure analysis from increased applied pressures shows that the compound is an indirect band gap semiconductor, and retains this nature via the entire applied pressure. Also, a monoclinic distortion of the compound due to the octahedral [AlH6] tilting of the unit cell is sustained within the pressure range in this study, and this confirms reported experiments. However, the applied pressure evaluation of the electronic band structure revealed a transition state at 3 GPa. This observation was further confirmed by structural analysis owing to the formation of bonds, with no discontinuity found in the structure at increased pressures. Meanwhile, thermodynamic analysis depicts a transition state at 3 GPa with the free energy observed to be − 0.04 Ry, entropy S ≈ 2.1 x 10–4 Ry/K, and heat capacity CV is 1.457 x 10–4 Ry/K at Dulong–Petti limit. The negative free energy verifies the thermodynamic stability of the hydride. Accordingly, the optical study showed an abrupt frequency discontinuity in the spectra of electron energy loss (EEL) function at exactly 3 GPa. The EEL function profile reveals the hydride as a promising photo-absorber at energies below 33 eV.