• • NaB8 achieves a superconducting critical temperature (Tc) of 24.4 K at ambient pressure, among the highest reported for boron cage compounds, enabling potential energy-efficient power applications.
• • ZrB8 exhibits the highest hardness of 21.9 GPa among the studied MB8 compounds, approaching superhard thresholds and offering mechanical robustness for industrial coatings.
• • s-block metals (e.g., Na, Mg) increase the electron-phonon coupling constant (λ) and Tc but reduce hardness, whereas d-block metals (e.g., Zr, Ti) enhance hardness at the expense of Tc, revealing a tunable trade-off.
• • The study systematically maps the influence of metal centers on p-orbital occupancy of boron bands, providing a design rule for balancing superconductivity and hardness in cage-like borides.