First-principles study of physical properties of sodium-based metal hydrides NaXH₆ (X= P, As, Sb) for hydrogen...

The current study aims at exploring structural, electronic, and thermodynamic properties of complex metal hydrides NaXH₆ (X = P, As, Sb) with DFT (Density Functional Theory) as the main calculation software. The tolerance factor with volume optimization has been used to attain structural integrity. The lattice parameters of NaPH₆, NaAsH₆ and NaSbH₆ are 7.47, 8.03 and 8.89 Å, respectively. Among the three compounds, NaPH₆ has the highest values for both volumetric hydrogen capacity (96.3 g H₂/L) and gravimetric hydrogen storage (10.1%). The desorption temperature is observed to be 422 K, 199 K, and 347 K for NaPH₆, NaAsH₆ and NaSbH₆, respectively. The computed density of states and band structures (DOS/PDOS) indicate that the entire three hydrides are of semiconducting nature with definite energy separations among the valence bands and the conduction bands. The series of band gap reductions observed is NaPH₆ > NaAsH₆ > NaSbH₆. At room temperature, NaPH₆ shows the highest value of electrical and thermal conductivity. AIMD simulations have been extensively used to verify the thermal stability. Elastic properties show that NaPH₆ has the highest length flexibility, and shape elasticity also shows more resistance to change in volume and has the strongest resistance against deformation among the three compounds. Negative values of Cauchy pressure show covalent bonding in these compounds, while the B/G ratio indicates that all three compounds are brittle. Furthermore, the calculated Debye temperature (Ѳ_D) progressively decreases when passing from NaPH₆ to NaSbH₆, indicating weaker lattice vibrations and lower phonon frequencies in heavier counterparts. These outcomes offer significant insights for designing and optimizing the performance of hydrogen storage materials, leading to the advancement of hydrogen storage technologies.