Comprehensive first-principles study of Na2NiH6 and K2NiH6 double perovskite hydrides: Structural Stability, Optoelectronic Response and Hydrogen Storage Properties

The increasing demand for efficient solid-state hydrogen storage materials has stimulated significant interest in hydrogen-rich double perovskite hydrides due to their structural versatility and tunable physicochemical properties. In this work, the structural, electronic, optical, and hydrogen-storage properties of Na 2 NiH 6 and K 2 NiH 6 were investigated using density functional theory (DFT) within the GGA-PBE and HSE06 frameworks. Structural stability was confirmed by Goldschmidt tolerance factors ( t = 0.91 for Na 2 NiH 6 and t = 0.96 for K 2 NiH 6 ), negative formation energies, phonon dispersion calculations, and ab initio molecular dynamics simulations at 400 K. Formation energies of −0.182 and −0.196 eV/atom indicate thermodynamic stability for Na 2 NiH 6 and K 2 NiH 6 , respectively. Both compounds exhibit indirect semiconducting behavior with HSE06 band gaps of 3.17 eV and 1.33 eV. Optical calculations reveal strong ultraviolet absorption for Na 2 NiH 6 , whereas K 2 NiH 6 displays enhanced activity in the visible region. Regarding hydrogen storage, Na 2 NiH 6 achieves a gravimetric capacity of 5.46 wt% and a volumetric density of 79.86 g H 2 L −1 , while K 2 NiH 6 reaches 4.23 wt% and 65.75 g H 2 L −1 . Estimated desorption temperatures of 403 K and 432 K suggest favorable intermediate thermodynamic stability. Overall, Na 2 NiH 6 offers the best balance between storage capacity and hydrogen release, whereas K 2 NiH 6 exhibits superior visible-light optoelectronic performance.