First-Principles Exploration of Magnetic, Thermal and Hydrogen Storage Properties in Alkali-Based XHfH3 Perovskites

The development of effective hydrogen storage materials is vital for the implementation of sustainable and carbon-neutral energy solutions. Solid-state hydrides have emerged as an attractive option because of their enhanced safety and volumetric efficiency compared to traditional compressed and liquefied hydrogen systems. Modest storage performance and elevated desorption temperatures remain key barriers to practical implementation. In this study, a comprehensive first-principles analysis based on density functional theory (DFT) was conducted to evaluate the structural, electronic, phonon, magnetic, thermal, and hydrogen storage properties of XHfH3 (X = Li, Na, and Rb) perovskite hydrides. Structural optimization confirmed thermodynamic stability, with lattice parameters exhibiting a gradual increase from Li to Rb, consistent with the corresponding increase in ionic radii. Electronic band structure and density of states investigations reveal a metallic nature for all compounds. Following structural relaxation, NaHfH3 and RbHfH3 retain magnetic moments with integral spin densities of approximately 2.05 μB and 2.08 μB, respectively. Mechanical stability is confirmed by computed elastic constants satisfying the Born stability criterion. Phonon dispersion simulations demonstrate dynamic stability exclusively for RbHfH3, whereas LiHfH3 and NaHfH3 exhibit imaginary frequencies indicative of lattice instabilities. The high-frequency vibrational modes are largely associated with hydrogen atoms, significantly influencing lattice dynamics and thermodynamic behavior. Furthermore, molecular dynamics simulations confirm the thermal stability of the materials. Collectively, the XHfH₃ family exhibits tunable magnetic, vibrational, thermal, and hydrogen-storage-related behavior, making these compounds useful model systems for understanding alkali-site effects in hafnium-based hydride perovskites, although their high predicted desorption temperatures limit immediate practical hydrogen-storage applicability.