Double perovskite-based optoelectronic devices are gaining attention due to their unique characteristics, including a simple and stable crystal structure. This study employs density functional theory (DFT) with the full-potential linearized augmented plane-wave (FP-LAPW) method to investigate the structural, electronic, optical, mechanical, and thermodynamic properties of A₂BIrCl₆ (A = Cs, Rb; B = Na, K) double perovskite halides. The primary aim is to assess their potential applicability in optoelectronic devices and renewable energy technologies. The cubic stability of the predicted compounds was confirmed through the Goldsmith tolerance factor, octahedral factor, and a new tolerance factor. Additionally, to confirm their thermodynamic stability, we assessed the formation energy, binding energy, and phonon dispersion curves. We used the TB-mBJ potential to accurately predict the optoelectronic properties. The calculations of the electronic band structure indicated that the examined double perovskites exhibit a direct band gap semiconducting nature, with the following band gap values: 1.927 eV for Cs₂NaIrCl₆ 1.991 eV for Cs₂KIrCl₆, 2.025 eV for Rb₂NaIrCl₆, and 2.102 eV for Rb₂KIrCl₆. The A₂BIrCl₆ (A = Cs, Rb; B = Na, K) compounds demonstrate impressive optical properties, including low reflectivity and high light absorption coefficients (10⁴ cm^-1) in the visible spectrum. Their spectral response extends from the visible to the UV range, making them ideal candidates for applications in solar cells and optoelectronic devices. The mechanical stability of the titled compounds was confirmed through the Born-Huang stability conditions based on their stiffness constants. The brittle nature of all the examined perovskites is confirmed by Pugh's ratio, Cauchy pressure, and Poisson's ratio. Finally, the Helmholtz free energy (F), internal energy (E), entropy (S), and specific heat capacity (C _v) are calculated based on the phonon density of states.

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