Computational Investigation of Hydrogen Storage and Photocatalytic Potential of Double‐Hydride Perovskites R b ₂ L i

ABSTRACT

In this study, we have investigated the structural and physical properties of Rb ₂ LiXH ₆ (where X = Al, B) by using density functional theory, with particular focus on its photocatalytic, optoelectronic, phonon, and thermodynamic features, highlighting its potential for hydrogen storage applications. The results reveal that both substances, Rb ₂ LiAlH ₆ and Rb ₂ LiBH ₆ , exhibit a cubic crystal structure, with computed lattice parameters of 5.98 and 5.74 Å, respectively. Moreover, the calculations of formation and cohesive energies for the studied structures confirm that both materials are feasible to synthesize and exhibit thermodynamic stability. Hydrogen storage properties, including gravimetric and volumetric capacities and desorption temperatures, are evaluated. The gravimetric capacities are reported as 2.86% for Rb ₂ LiAlH ₆ and 3.11% for Rb ₂ LiBH ₆ . In this research, the photocatalytic activity was also assessed, indicating that the Rb ₂ LiAlH ₆ compound exhibits improved photocatalytic performance, supporting its suitability for photocatalytic water splitting. The electronic band structure shows that Rb ₂ LiAlH ₆ and Rb ₂ LiBH ₆ compounds exhibit indirect band‐gap semiconducting behavior, with GGA band gaps of 1.809 and 0.096 eV and HSE‐03 band gaps of 2.603 and 0.383 eV, respectively. Both double perovskite hydrides exhibit  mechanical stability and demonstrate brittle properties, as revealed by Poisson's and Pugh's ratios. Thermodynamic parameters, including heat capacity , enthalpy, entropy, Debye temperature , melting temperature, lattice parameters, and minimum thermal conductivity, are analyzed to assess stability at elevated temperatures and suitability for industrial applications. The optical properties indicate that Rb ₂ LiAlH ₆ and Rb ₂ LiBH ₆ exhibit enhanced absorption coefficients in the order of (10 ⁴  cm ⁻¹ ) and hold potential for optoelectronic applications in the UV and visible regions. The overall analysis reveals Rb ₂ LiXH ₆ (where X = Al, B) as a promising applicant for future H ₂ retention due to its favorable gravimetric capacity and remarkable resilience.