First-principles insights into the structural, optoelectronic, and thermoelectric properties of lead-free halide double perovskites Rb2AlAgX6 (X = Cl, Br, I) for energy applications.

This work presents a comprehensive first-principles investigation of the structural, electronic, optical, mechanical, and thermoelectric properties of the lead-free halide double perovskites Rb2AlAgX6 (X = Cl, Br, I) using density functional theory within the FP-LAPW method. Structural optimization confirms that all compounds crystallize in a stable cubic Fm-3m phase with lattice constants increasing from 10.76 Å (Cl) to 11.04 Å (I) and negative formation energies indicating thermodynamic stability. Electronic band structure calculations using the TB-mBJ potential reveal indirect bandgaps of 3.88 eV, 2.67 eV, and 1.21 eV for the chloride, bromide, and iodide phases, respectively, demonstrating tunable electronic behavior through halide substitution. Mechanical analysis confirms elastic stability and ductile character for all compounds. Optical calculations show strong absorption in the UV-visible region, with Rb2AlAgI6 exhibiting the highest dielectric constant (∼4.1) and strongest visible absorption. Thermoelectric analysis indicates temperature-activated transport behavior from 300 to 800 K, with Rb2AlAgI6 achieving a maximum Seebeck coefficient of ∼265 μV/K and the lowest thermal conductivity (∼0.18 W/m·K), highlighting the potential of this material family for optoelectronic and thermoelectric energy applications.