• Both KCsSiH6 and RbCsSiH6 crystallize in a stable cubic symmetry and exhibit both thermal and dynamical stability, confirming their structural integrity.
• Gravimetric hydrogen storage capacities are 2.94 wt% for KCsSiH6 and 2.40 wt% for RbCsSiH6, indicating moderate storage potential for practical applications.
• Indirect band gaps of 3.14 eV and 3.17 eV combined with pronounced ultraviolet optical activity make these hydrides promising for optoelectronic and UV-filtering devices.
• Comprehensive thermodynamic analyses up to 800 K reveal favorable entropy, heat capacity, and Gibbs free energy changes, underscoring their thermal stability for energy applications.