Investigation of structural, elastic, electronic, and optical properties of lead-free double perovskites Cs₂XBeBr₆ (X = Ge, Sn): a first-principles DFT study

In this study, the structural, elastic, electronic, and optical properties of Cs₂GeBeBr₆ and Cs₂SnBeBr₆ halide double perovskites (HDPs) were investigated using density functional theory (DFT) calculations. Notably, the Tran-Blaha modified Becke-Johnson (TB-mBJ) method was employed to predict indirect band gaps of 2.434 eV for Cs₂GeBeBr₆ and 2.855 eV for Cs₂SnBeBr₆. The stability of these compounds in a cubic (Fm-3m) structure was confirmed through formation energy, cohesive energy, tolerance factor, and elastic constants. Furthermore, the ductile nature of the materials was demonstrated through Poisson's and Pugh's ratios. Our optical property analysis, spanning the 0–13 eV energy range, revealed key insights into the dielectric functions, extinction coefficient, electron energy loss, refractive index, optical conductivity, reflectivity, and absorption coefficient. These results highlight the potential of Cs₂GeBeBr₆ and Cs₂SnBeBr₆ for future optoelectronic and photovoltaic applications.

In this investigation, we employed density functional theory (DFT), implemented using the Wien2k package. The exchange and correlation potentials were treated using the generalized gradient approximation (GGA) and the Tran-Blaha modified Becke-Johnson (TB-mBJ) method. To evaluate dynamic stability, we analyzed the phonon band structures using the CASTEP code.