Chemical, Physical, and Thermoelectric Properties of Lead-Free Double Perovskite Halides A2SbAgBr6 (A = Cs, Rb) A First-Principles Study

Double perovskite halides A 2 SbAgBr 6 (A = Cs, Rb) are promising materials for renewable energy applications, particularly in optoelectronic and thermoelectric devices. They crystallize in a stable cubic structure, as confirmed by calculated Goldschmidt tolerance factors ( τ G = 0.93 for Cs 2 SbAgBr 6 and τ G = 0.89 for Rb 2 SbAgBr 6 ), octahedral factor (μ = 0.49), and modified tolerance factors (τ = 3.96 for Cs 2 SbAgBr 6 and τ = 4.11 for Rb 2 SbAgBr 6 ), all within the stability criteria for perovskite structures. Negative formation energies (−2.981 eV/atom for Cs 2 SbAgBr 6 and −2.948 eV/atom for Rb 2 SbAgBr 6 ) further confirms thermodynamic stability. Furthermore, the calculated elastic constants (C 11 , C 12 , and C 44 ) satisfy the mechanical stability criteria for cubic crystals. In addition, the electronic properties were analyzed to assess semiconducting behavior. Electronic structure analysis shows that both compounds are indirect band-gap semiconductors with gaps of 0.874 eV (Rb 2 SbAgBr 6 ) and 0.924 eV (Cs 2 SbAgBr 6 ), slightly reduced when DFT-D dispersion corrections are included. Density of states indicates strong Br-p–Ag-d hybridization in the valence band and dominant Sb-d and Ag-d contributions in the conduction band, confirming semiconducting behavior. Building on these findings, the optical response of the materials was examined. Optical properties reveal high static dielectric constants (ε 1 (0) ≈ 6.42–6.43) and refractive indices (n (0) ≈ 2.54–2.55), strong absorption in the visible and UV regions, low reflectivity (∼0.19), and pronounced extinction coefficients, highlighting efficient light–matter interaction suitable for optoelectronic and photovoltaic applications. Moreover, the interplay of electronic and optical properties suggests potential for thermoelectric applications, prompting a detailed transport analysis. Thermoelectric analysis demonstrates favorable electrical conductivity, Seebeck coefficients, and power factors. Our results suggest that these compounds are highly promising candidates for both optoelectronic and thermoelectric devices.