Tuning A-Site Cations in Cubic ACaH 3 (A = Be, Mg, Sr, Ba) Perovskite Hydrides: A First-Principles Study of Structural, Electronic, Mechanical and Hydrogen Storage Properties
First-principles density functional theory calculations were performed using CASTEP to investigate the physical and hydrogen storage properties of cubic ACaH 3 (A = Be, Mg, Sr, and Ba) hydrides and to determine how A-site cation identity influences their behavior. The optimized lattice constants of BeCaH 3 , MgCaH 3 , SrCaH 3 , and BaCaH 3 are 4.091, 4.193, 4.362, and 4.464 Å, respectively, while the corresponding formation energies are -0.702, -1.031, -1.408, and -1.455 eV/atom, indicating energetically favorable formation relative to the constituent elemental states. All four hydrides exhibit metallic electronic structures, with bands crossing the Fermi level, whereas charge density and population analyses reveal predominantly ionic metal hydrogen bonding with a limited covalent contribution. The cubic elastic stability criteria are satisfied throughout the series, while phonon calculations reveal composition dependent soft modes in the ideal cubic phases at 0 K. BeCaH 3 is the softest and most elastically anisotropic member and is the only ductile compound, whereas SrCaH 3 exhibits the highest shear resistance and stiffness. The gravimetric and volumetric hydrogen storage capacities are, respectively, 5.770 wt.% and 72.93 g H 2 L -1 for BeCaH 3 , 4.470 wt.% and 67.90 g H 2 L -1 for MgCaH 3 , 2.295 wt.% and 60.04 g H 2 L -1 for SrCaH 3 , and 1.663 wt.% and 55.99 g H 2 L -1 for BaCaH 3 . The corresponding lowest estimated desorption temperatures are 82.680, 150.596, 693.382, and 716.070 K, respectively. Across the series, the increasingly negative formation energy is accompanied by decreasing hydrogen storage capacity and a progressively higher temperature requirement for hydrogen release. Overall, A-site substitution strongly influences the hydrogen storage capacity and thermodynamic requirement for hydrogen release, supporting further experimental evaluation of these ACaH 3 hydrides particularly BeCaH 3 as solid state hydrogen storage materials.