A systematic computational study on Na-based complex hydrides NaMXH6 (M = Sr, Ba; X = Co, Rh, Ir): A promising class of hydrogen storage materials

Abstract

In this work, density functional theory (DFT) calculations are employed to explore the structural, mechanical, dynamical, optical, thermal, and hydrogen storage properties of NaMXH₆ complex hydrides (M = Sr and Ba; and X = Co, Rh, and Ir). Our findings revealed that all the investigated compounds crystallize in a cubic structure with space group F$\overline{4}3$m (No. 216). Phonon dispersion analysis confirms the dynamic stability of the materials, while ab initio molecular-dynamics (AIMD) simulations demonstrate their thermal stability, with no structural deformation. All compounds exhibit indirect band gap semiconductor behavior based on their electronic properties. The band gap decreases when M and/or X cations of NaMXH₆ are substituted. Optical property analysis reveals dual absorption regions in visible and ultraviolet spectra, with absorption coefficients reaching up to 1.4 × 10⁶ cm⁻¹, indicating strong potential for optoelectronic applications and light energy harvesting. To assess mechanical characteristics, the elastic constants were calculated and found to satisfy the mechanical stability criteria, confirming that these hydrides are generally brittle, hard, and stiff. Among them, NaSrCoH₆ shows the highest stiffness and resistance to deformation, while Ba-based hydrides exhibit enhanced ductility. The Quasi-Harmonic Debye model was employed to analyze thermodynamic behavior across a range of temperatures, with results aligning well with fundamental thermodynamic principles. The hydrogen storage capacities wt% of NaSrCoH₆, NaSrRhH₆, NaSrIrH₆, NaBaCoH₆, NaBaRhH₆, and NaBaIrH₆ are 3.44, 2.76, 1.96, 2.68, 2.25, and 1.69, respectively. Overall, these results demonstrate that NaMXH₆ hydrides are multifunctional materials with promising hydrogen storage capacity, mechanical resilience, and semiconducting behavior, making them suitable for clean energy technologies.