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

Context 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. Methods 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.