Theoretical analysis of structural, electronic, optical, elastic and thermoelectric properties of double perovskites Cs2CuBiX6 (X=Br or I): DFT approach

Double perovskites have attracted significant attention for renewable energy applications due to their promising optoelectronic and thermoelectric properties. In this study, we investigate the structural, mechanical, optical, and thermoelectric properties of Cs₂CuBiX₆ (X = Br, I) using density functional theory (DFT). The compounds exhibit negative formation energies of −2.11 eV/atom and −2.08 eV/atom, respectively, confirming thermodynamic stability. Phonon dispersion analysis reveals no imaginary modes, indicating dynamical stability. Ground-state properties are obtained using the GGA, while electronic and optical properties are refined using the Tran-Blaha modified Becke-Johnson (TB-mBJ) potential, predicting indirect bandgaps of 0.903 eV and 0.648 eV for Cs₂CuBiBr₆ and Cs₂CuBiI₆, respectively. Optical analyses indicate strong polarization, highlighting the potential of these materials in photovoltaic applications. Mechanical property evaluation reveals brittle behavior. Thermoelectric analysis shows that Cs₂CuBiI₆ demonstrates superior performance, with higher Seebeck coefficient and electrical conductivity, making it a more promising candidate for energy conversion technologies.