Effect of Alkali Metals on Physical Properties of Dynamically Stable XHfH ₃ (X = K, Rb, and Cs) Hydrides for Hydrogen Storage Applications: A Density Functional Theory and Ab Inito Molecular Dynamics Investigation

First‐principles and ab initio molecular dynamics (AIMD) simulations were employed to investigate the physical properties and hydrogen storage properties of novel cubic XHfH 3 (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, KHfH 3 exhibits the highest hydrogen storage performance, with a gravimetric capacity of 1.35 wt.% and a volumetric capacity of 72 g H 2 /L. These findings highlight XHfH 3 hydrides as promising candidates for hydrogen storage and multifunctional energy‐related applications.