First-principles insights into the structural, mechanical, optoelectronic, and thermodynamic properties of X2LiAlH6 (X = Na, K) for hydrogen storage application

The potential of X2LiAlH6 (X = Na, K) double hydride perovskites for hydrogen storage and optoelectronic applications has been investigated using first-principles simulations based on density functional theory (DFT). Current work systematically explores the structural, electronic, optical, mechanical, thermodynamic, and hydrogen storage properties of these compounds. Structural optimization confirmed the cubic Fm-3m phase for both compounds at 0 K. Phonon dispersion calculations revealed that K2LiAlH6 is dynamically stable, whereas Na2LiAlH6 exhibits soft acoustic phonon modes near the Γ, L, and X points, indicating that its cubic configuration is not a true energy minimum and would relax into a lower-symmetry phase. Despite this instability, Na2LiAlH6 retains a high theoretical gravimetric hydrogen capacity (7.04 wt%), exceeding the DOE (U.S. Department of Energy) 2025 target and making it a promising candidate for further exploration of its stable ground-state phase. Electronic structure analysis revealed indirect semiconducting band gaps of 2.63 eV (Na2LiAlH6) and 2.49 eV (K2LiAlH6) using the GGA-PBE functional. While Heyd-Scuseria-Ernzerhof (HSE06) functional corrections may increase the absolute bandgap values, the fundamental transition characteristics confirm their potential for UV-optoelectronic applications. Optical property calculations show strong absorption in the near-UV region, supporting their suitability for UV photodetectors and related devices. Mechanical stability was validated through the Born criteria, with Na2LiAlH6 exhibiting higher stiffness and rigidity (C11 ≈ 72.7 GPa, Y ≈ 72.9 GPa) compared to K2LiAlH6 (C11 ≈ 44.5 GPa, Y ≈ 55.4 GPa), attributed to stronger Al–H bonding. Thermodynamic analyses demonstrated that Na2LiAlH6 possesses a higher Debye temperature (θᴰ ≈ 699 K) and melting point, indicative of stronger interatomic bonding than many reported hydride perovskites. Our findings underscore that A-site substitution (Na → K) plays a decisive role in balancing structural stability, mechanical robustness, and hydrogen-storage capacity. K2LiAlH6 emerges as the experimentally viable candidate due to its stable cubic phase, while Na2LiAlH6 remains attractive for its superior hydrogen density and thermal robustness, pending further investigation of its true ground state.