First-Principles HSE06 Study of Band-Gap Engineering in Cs ₂ MBiX ₆ (M = Au, Li; X = Cl, Br) for Optoelectronic and Thermoelectric Applications

Herein, the stability, electronic, optical, thermoelectric, and mechanical properties of Cs 2 MBiX 6 (M = Au, Li; X = Cl, Br) are examined using density functional theory (DFT). Thermodynamic and structural stability, as well as the degree of distortion in Cs 2 MBiX 6 , are evaluated using formation and cohesive energies together with tolerance factor analysis. The dynamic stability of these perovskites is confirmed by the positive phonon frequencies obtained from dispersion curves. By applying the Heyd–Scuseria–Ernzerhof 2006 (HSE06) hybrid functional, the optoelectronic properties are analyzed, revealing semiconducting behavior for all studied compounds. An indirect band gap (Eg) of 1.336 eV is obtained for Cs 2 AuBiCl 6 while wider band gaps of 3.913 eV and 4.575 eV are found for Cs 2 LiBiBr 6 and Cs 2 LiBiCl 6 , respectively. The Cs 2 MBiX 6 compounds demonstrate strong light absorption in the ultraviolet (UV) region, indicating their potential for UV-based optoelectronic applications such as photodetectors and UV sensors. To evaluate mechanical stability, the elastic constants are calculated. The Poisson’s ratio (ν) and Cauchy pressure (CP) further indicate ductile behavior for Cs 2 LiBiBr 6 and Cs 2 LiBiCl 6 , whereas Cs 2 AuBiCl 6 shows borderline ductile characteristics. The temperature-dependent electrical conductivity of Cs 2 LiBiCl 6 , Cs 2 AuBiCl 6 , and Cs 2 LiBiBr 6 increases significantly at elevated temperatures, resulting in thermoelectric figure of merit (ZT) values exceeding 0.7 over a wide temperature range. These findings provide useful insight into the potential of Cs 2 MBiX 6 compounds for UV-optoelectronic and thermoelectric applications.