Comprehensive first-principles investigation of electronic, optical, thermoelectric, and mechanical properties of Cs₂ZAuF₆ (Z = In, Tl) double perovskites for sustainable energy conversion

Cesium-based perovskites are gaining attention as non-toxic, thermally stable, and structurally robust materials for optoelectronic applications. In this work, we investigate Cs₂ZAuF₆ (Z = In, Tl) using density functional theory (DFT) with PBE-GGA, TB-mBJ, and spin–orbit coupling (SOC). The computed elastic constants comply with Born's stability criteria, while hardness calculations indicate strong resistance to mechanical stress. Debye temperatures of 232.5 K and 186.7 K, and melting points of 1094 K and 1010 K for Cs₂InAuF₆ and Cs₂TlAuF₆, respectively, demonstrate high-temperature resilience. Electronic structure analysis shows direct bandgaps of 3.47 eV (Cs₂InAuF₆) and 1.06 eV (Cs₂TlAuF₆), suitable for ultraviolet optoelectronics, corroborated by substantial UV absorption. Thermoelectric assessments reveal high Seebeck coefficients (∼124 μV/K and ∼ 186 μV/K), large power factors (∼11.4 × 10¹⁰ W/mK²s at 250 K and 28 × 10¹⁰ W/mK²s at 850 K), ZT values of 1.48 and 1.99, and low thermal conductivity for Cs₂InAuF₆ and Cs₂TlAuF₆, respectively, highlighting efficient energy conversion. Overall, Cs₂ZAuF₆ (Z = In, Tl) emerge as promising candidates for advanced optoelectronic and thermoelectric devices.