Theoretical İnvestigations on Structural, Electronic, Elastic, Thermal, Optic and Hydrogen Storage Properties of Xalh6 (X= La, Ce, Pr) Aluminium Hydrides

Abstract

The structural, mechanical, electronic, thermodynamic, and hydrogen storage characteristics of trigonal XAlH6 (X = La, Ce, and Pr) has been studied by using density functional theory which are not previously explored experimentally/computationally. Firstly, the equilibrium crystal structures, formation enthalpies for XAlH6 (X = La, Ce, and Pr) are computed and assessed for synthesisability and thermodynamic stability of the materials. Subsequently, elastic constants of the materials were calculated and evaluated. The results indicated that XAlH6 (X = La, Ce, and Pr) is mechanically, thermodynamically stable, and feasible for synthesis. The examination of bulk modulus to shear modulus ratio, Cauchy pressure, and Poisson's ratio displayed that XAlH6 exhibits brittleness. The electronic band structure analyses demonstrate that there is a band gap of 2.05 eV and 0.95 eV between the maximum of the valence band and the minimum of the conduction band around the Fermi energy level for LaAlH6 and PrAlH6, respectively. Therefore, LaAlH6 and PrAlH6 have semiconducting properties. CeAlH6 is found to be metallic because the bands cross the Fermi energy level. The optical properties are also investigated in depth. In the visible spectrum, electromagnetic radiation with a polarization of [1 0 0] has a reflectivity of less than 40 %, hence XAlH6 can also be a good candidate as an anti-reflection material. The GDH (gravimetric hydrogen density) values are calculated to be 3.52 wt % for LaAlH6, 3.50 wt % for CeAlH6, and 3.48 wt % for PrAlH6. Additionally, the hydrogen desorption temperatures are determined to be 292.28 K for LaAlH6, 301.87 K for CeAlH6, and 361.01 K for PrAlH6. The findings suggest that these materials can be promising host for solid state hydrogen storage.