Structural, Elastic, Optic, Electronic, Phonon, Thermodynamic, and Hydrogen Storage Properties of Bialkali Alanates M2lialh6 (M = Na, K)

Abstract

In this study, the structural, elastic, optical, electronic, phonon, thermodynamic, and hydrogen storage properties of bialkali alanates M2LiAlH6 (M = Na, K) were systematically investigated using density functional theory (DFT). The compounds crystallize in a cubic structure and exhibit mechanical and thermodynamic stability, as evidenced by their negative formation enthalpies and positive cohesive energies. Elastic constant calculations revealed that both materials are mechanically stable yet brittle. Optical analyses indicated strong absorption and reflectivity in the ultraviolet region, while electronic band structure results showed that the materials are wide-bandgap semiconductors with direct transitions. Phonon dispersion confirmed the dynamic stability of K2LiAlH6, while Na2LiAlH6 exhibited imaginary frequencies indicating instability. Thermodynamic properties, including Debye temperature, melting point, and minimum thermal conductivity, support the suitability of these compounds for thermal applications. Furthermore, Na2LiAlH6 demonstrated a higher hydrogen storage capacity, both gravimetric (3.42 wt%) and volumetric (100.63 gH2L−1), meeting the U.S. Department of Energy's volumetric target for 2025