Effect of Alkali Metals on Physical Properties of Dynamically Stable XHfH₃ (X = K, Rb, and Cs) Hydrides for Hydrogen...

First-principles and ab initio molecular dynamics (AIMD) simulations were employed to investigate the physical properties and hydrogen storage properties of novel cubic XHfH₃ (X = K, Rb, and Cs) perovskite hydrides. To the best of our knowledge, this is the first theoretical study of these compounds. Negative formation energies (-1.60 to -1.35 eV/atom) confirm their thermodynamic stability, while AIMD simulations demonstrate structural integrity at 300 K and elevated temperatures over 10 ps. Phonon dispersion calculations reveal the dynamical stability of the Rb- and Cs-based compounds through the absence of imaginary phonon modes. Electronic band structure calculations using both GGA-PBE and HSE06 functionals indicate metallic behavior for all hydrides. Mechanical analysis confirms elastic stability and reveals a gradual transition from brittle to ductile behavior with increasing alkali cation size. Optical calculations show metallic characteristics with tunable plasmonic response and absorption behavior. Thermodynamic properties exhibit strong temperature-dependent phonon contributions, reflected in the evolution of enthalpy, entropy, and Gibbs free energy. Among the studied compounds, KHfH₃ exhibits the highest hydrogen storage performance, with a gravimetric capacity of 1.35 wt.% and a volumetric capacity of 72 g H₂/L. These findings highlight XHfH₃ hydrides as promising candidates for hydrogen storage and multifunctional energy-related applications.