Ab initio analysis of the structural, hydrogen storage, mechanical, electronic, and optical characteristics of Cs2XAlH6 (X = K, Na, Rb) double perovskite hydrides

We present a detailed computational investigation of the structural, optical, mechanical, and hydrogen storage properties of Cs2XAlH6 (X = K, Na and Rb) double perovskite hydrides. All compounds crystallize in a cubic Fm3m structure, with lattice parameters influenced by the ionic radius of the X-site cation. Electronic structure calculations reveal an indirect band gap, supporting the materials stability. Hydrogen storage analysis indicates thermodynamic stability, efficient desorption temperatures, and high storage capacities, particularly for lighter cations like Na. Optical properties show high static dielectric constants, strong absorption in the visible-UV range, and light-dispersive behavior, suggesting suitability for energy storage and photonic applications. Mechanical studies confirm good ductility and resilience under repeated loading, while thermal stability is verified through ab initio molecular dynamics and phonon dispersion analyses. Collectively, these findings highlight Cs2XAlH6 compounds as versatile candidates for hydrogen storage, energy, and optical technologies.