DFT studies of the role of anion variation in physical properties of Cs2NaTlBr6-x Cl x (x = 0, 1, 2, 3, 4, 5 and 6) mixed halide double perovskites for optoelectronics.

Halide double perovskites have various benefits over lead-based perovskites due to their suitable optical absorption efficiency, higher stability, tunable bandgap, large carrier mobility, easy availability and low cost. The structural, electrical, optical and mechanical characteristics of the lead-free halide double perovskites Cs2NaTlBr6-x Cl x (x = 0, 1, 2, 3, 4, 5 and 6) are investigated by utilizing the first-principles density functional theory (DFT). The structural properties were computed at equilibrium, revealing that the crystals undergo structural phase transitions as the doping concentration varies. However, crystal stability was confirmed through the evaluation of the tolerance factor. The Heyd-Scuseria-Ernzerhof (HSE06) functional is used to correct the bandgap underestimation by generalized gradient approximation with Perdew-Burke-Ernzerhof (GGA-PBE). The band edge profile and electron density of states confirm the direct-bandgap semiconducting nature of the compounds. The bandgap increases approximately linearly with Cl incorporation, sharply tuned from 0.80 to 1.75 eV (GGA-PBE) and 1.78 to 2.98 eV (HSE06), making them highly suitable for photovoltaic and optoelectronic applications. The effective mass of the electron ranges from 0.283 to 0.449 m0, and the carrier mobility is from 83.913 to 305.485 cm2 V-1 s-1, respectively, suggesting excellent charge transport characteristics, essential for high-performance solar cells and photodetectors. The considered NaTl-based double perovskites have strong optical absorption in the visible and UV spectrum, with enhanced conductivity as Cl content increases. Binding energy analysis confirms strong lattice stability, with values decreasing from -3.193 eV (Br-rich) to -3.559 eV (Cl-rich), indicating improved thermodynamic robustness as Cl concentration increases. Mechanical analysis shows the ductile behaviour of the considered perovskites, supported by Poisson's ratio and Pugh's modulus. The rising bulk modulus and Debye temperature with Cl incorporation enhance rigidity and thermal stability. These insights advance the anion-engineered design of stable, high-efficiency, lead-free NaTl-based perovskites for next-generation optoelectronic applications.