First-principles investigation of structural, mechanical, electronic, optical, and thermodynamic properties of perovskite hydrides XRuH3 (X=Mg, Ca, Sr, Ba) for hydrogen storage

Perovskite hydrides have recently gained attention as solid-state hydrogen storage materials owing to their high density and tunable thermodynamic stability. In this work, we investigate the structural, mechanical, electronic, optical, thermodynamic and hydrogen storage properties of alkaline earth metal-ruthenium perovskite hydrides XRuH 3 (X = Mg, Ca, Sr, Ba) using density functional theory (DFT) as implemented in WIEN2k. Formation enthalpies confirm the thermodynamic stability of all XRuH 3 hydrides. The optimized lattice parameters of MgRuH 3 , CaRuH 3 , SrRuH 3 and BaRuH 3 are 3.47 Å, 3.63 Å, 3.76 Å and 3.91 Å, respectively. Mechanical analysis shows compliance with the Born criteria, while Pugh's ratios above 2.3 classify all compounds as ductile. Phonon dispersion curves confirm dynamical stability for CaRuH 3 , SrRuH 3 , and BaRuH 3 , whereas MgRuH 3 exhibits imaginary modes. AIMD simulations at 300 K further support the finite-temperature stability of CaRuH 3 and SrRuH 3 , while MgRuH 3 exhibits large temperature fluctuations consistent with its soft phonons. Electronic structure calculations show metallic character dominated by Ru 4d states near the Fermi level. Hydrogen storage analysis indicates gravimetric capacities range from 2.36 wt% for MgRuH 3 to 1.25 wt% for BaRuH 3 , while volumetric capacities (42.0 to 59.8 g H 2 /L) are higher than the U.S Department of Energy target of 30 g H 2 /L. Thermodynamic analysis shows heat capacities approaching the Dulong-Petit limit at higher temperatures while BaRuH 3 has a very low thermal expansion. Among the studied compounds, CaRuH 3 and SrRuH 3 display excellent structural integrity and volumetric performance, suggesting their suitability as frameworks for further development and optimization. Strategies to tailor the desorption thermodynamics toward practical hydrogen storage applications like partial Ru-site substitution or nanostructuring may help fine tune desorption thermodynamics toward practical hydrogen storage applications.