Theoretical Design and Characterization of Rb ₂ ZnRuCl ₆ and Cs ₂ ZnRuCl ₆ Novel Halide Perovskites

The exploration of lead‐free halide double perovskites has intensified due to their tunable structural and electronic properties combined with enhanced environmental compatibility. In this work, we introduce two novel Ru‐based halide double perovskites, Rb ₂ ZnRuCl ₆ and Cs ₂ ZnRuCl ₆ , investigated through first‐principles calculations within the framework of density functional theory (DFT). Structural analysis confirmed the cubic (no. 225) phase with stable tolerance and octahedral factors. The cohesive energies of –4.47 and –4.34 eV/atom, together with formation energies of –7.66 and –7.72 eV for Rb ₂ ZnRuCl ₆ and Cs ₂ ZnRuCl ₆ , respectively, indicate strong energetic and thermodynamic stability. Both compounds exhibit direct bandgaps of 1.54 and 1.53 eV at the X point, with density of states analysis highlighting the dominant contribution of Ru d and Cl p orbitals near the Fermi level. Phonon dispersions revealed no imaginary modes, confirming dynamical stability, while mechanical analysis verified elastic stability with moderate bulk and shear moduli. Notably, Cs ₂ ZnRuCl ₆ presents reduced elastic anisotropy compared to its Rb counterpart, suggesting a more isotropic mechanical response. These results establish Rb ₂ ZnRuCl ₆ and Cs ₂ ZnRuCl ₆ as promising candidates for next‐generation optoelectronic and energy‐related applications.