First-principles study of structural, hydrogen storage, mechanical, electronic, and optical properties of K2NaXH6 (X = Al, As, Bi, Ga, In) double perovskite hydrides

Density functional theory was employed to investigate the structural, hydrogen storage, mechanical, electronic, and optical properties of K2NaXH6 (X = Al, As, Bi, Ga, In) double perovskite hydrides. The structural optimization confirmed their cubic symmetry with lattice parameters ranging from 8.08 Å to 8.92 Å. The calculated formation energies indicate the thermodynamic stability of all compounds, with K2NaAlH6 being the most stable. Mechanical stability was assessed using elastic constants, which satisfy Born's criteria, confirming the robustness of the studied materials. The gravimetric hydrogen storage capacity varies between 1.913 wt% and 4.506 wt%, with K2NaAlH6 demonstrating the highest storage potential. Additionally, hydrogen desorption temperatures were determined, with K2NaAlH6 exhibiting the highest value of 416 K, suggesting its suitability for practical hydrogen storage applications. Electronic structure analysis revealed that all compounds exhibit semiconductor behavior, with band gaps ranging from 0.667 eV to 2.41 eV, indicating potential applications in optoelectronic devices. The density of states analysis highlights the significant role of hydrogen in bonding interactions, contributing to the stability of these hydrides. Optical properties, including the dielectric function, refractive index, and absorption spectra, were examined, revealing strong absorption in the ultraviolet region, making these materials promising candidates for optoelectronic and photovoltaic applications. © 2025 Elsevier B.V.