First-principles insights into lead-free K2AgRhX6 (X = F, Cl, Br, I) halide double perovskites as stable platforms for next-generation optoelectronic and energy conversion devices.

CONTEXT: Double perovskites have emerged as promising candidates for renewable energy technologies due to their structural simplicity and thermodynamic stability. Among them, K2AgRhF6 is the most stable (-2.54 eV/atom), consistent with its highest bulk modulus (64.15 GPa), tolerance factor (0.85), and octahedral factor (0.86). Elastic analysis indicates ductile behavior for K2AgRhF6 (ν = 0.35, B/G = 3.13) and K2AgRhBr6 (ν = 0.28, B/G = 1.99), while K2AgRhCl6 (ν = 0.26, B/G = 1.75) and K2AgRhI6 (ν = 0.13, B/G = 2.37) lie near the brittle-ductile threshold. Band structure calculations reveal semiconducting gaps of 2.56 eV (F), 2.03 eV (Cl), 1.44 eV (Br), and 0.55 eV (I), with K2AgRhBr6 and K2AgRhI6 exhibiting strong optical absorption in the visible spectrum. Thermoelectric analysis yields figures of merit approaching 0.75 at room temperature across the series, highlighting their efficiency in energy conversion. Collectively, these findings position K2AgRhX6 halide double perovskites as robust, lead-free multifunctional materials with integrated structural stability, tunable optoelectronic response, and promising thermoelectric efficiency for next-generation optoelectronic devices.

METHOD: In this work, density functional theory (DFT) calculations within the WIEN2k framework were used to explore the structural, mechanical, electronic, optical, and thermoelectric properties of halide-based double perovskites K2AgRhX6 (X = F, Cl, Br, I). All compounds crystallize in the cubic Fm3-m (225) space group, and their negative formation energies confirm thermodynamic stability.