Unveiling the structural, electronic, optical, and transport properties of Cs₂SeCl₆ double perovskite for clean energy

The growing demand for environmentally friendly energy materials has intensified interest in lead-free halide double perovskites for photovoltaic and optoelectronic applications. In this work, the structural, electronic, optical, and transport properties of the Cs₂SeCl₆ double perovskite are systematically investigated using density functional theory within the WIEN2k framework. Structural optimization confirms a stable cubic phase, with equilibrium lattice parameters obtained from total energy-volume fitting. The calculated elastic constants satisfy mechanical stability criteria and indicate a ductile nature with moderate stiffness, suitable for thin-film device fabrication. Electronic band structure calculations using GGA and mBJ-GGA functionals reveal an indirect semiconducting bandgap. Charge density analysis indicates mixed ionic-covalent bonding, characterized by strong Se-Cl covalency and ionic Cs-Cl interactions that enhance structural stability. Optical properties, including the dielectric function, absorption coefficient, refractive index, reflectivity, and energy-loss function, show strong absorption in the visible and ultraviolet regions, dominated by transitions from Cl 3p and Se 4p valence states to Se 5s conduction states. Phonon dispersion confirms dynamical stability, while transport calculations indicate a potentially favorable thermoelectric response with a notable Seebeck coefficient. An extended thermoelectric assessment, including the power factor, lattice and total thermal conductivities, and the figure of merit ZT, was also performed. Overall, Cs₂SeCl₆ emerges as a promising, environmentally benign alternative to lead-based perovskites for sustainable optoelectronic applications.