Cs2AsAuBr6 and Cs2AsAuCl6 lead-free double perovskites for energy applications.

This study presents a first-principles investigation of Cs2AsAuX6 (X = Br, Cl) double perovskites using density functional theory within generalized gradient approximation (GGA) and modified Becke-Johnson approaches. Structural optimization confirms stable cubic phases, with lattice parameters of 11.0757 Å for Cs2AsAuBr6 and 10.567 Å for Cs2AsAuCl6. Goldschmidt tolerance factors of 0.996 and 1.01 further indicate excellent structural stability. Electronic calculations reveal direct band gaps of 0.387 eV and 0.935 eV, supporting near-infrared and visible-light applications. Both compounds satisfy Born-Huang mechanical stability criteria and exhibit ductile behavior. Optical analysis shows strong visible-light absorption, refractive indices of 2.65 and 2.28, and absorption coefficients above 104 cm-1. Thermoelectric figures of merit reach approximately 0.3-0.4 at elevated temperatures, indicating promise for waste-heat recovery. Thermal calculations confirm thermodynamic stability across a broad temperature range. These findings may also support the sustainable utilization of mineral resources through ore characterization, mineral processing, beneficiation, and improved management of mining-related tailings.