First-principles insights into lead-free K₂AgRhX₆ (X = F, Cl, Br, I) halide double perovskites as stable platforms...

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.

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.