Mechanical Stability and Energy Gap Evolution in Cs-Based Ag, Bi Halide Double Perovskites under High Pressure: A Theoretical DFT Approach

Due to their intrinsicstability and reduced toxicity, lead-freehalide double perovskite semiconductors have become potential alternativesto lead-based perovskites. In the present study, we used density functionaltheory simulations to investigate the mechanical stability and bandgap evolution of double perovskites Cs2AgBiX6 (X = Cl and Br) under an applied pressure. To investigate the pressure-dependentproperties, the hydrostatic pressure induced was in the range of 0-100GPa. The mechanical behaviors indicated that the materials under studyare both ductile and mechanically stable and that the induced pressureenhances the ductility. As a result of the induced pressure, the covalentbonds transformed into metallic bonds with a reduction in bond lengths.Electronic properties, energy bands, and electronic density of stateswere obtained with the hybrid HSE06 functional, including spin-orbitcoupling (HSE06 + SOC) calculations. The electronic structure studyrevealed that Cs2AgBiX6 samples behave as X-& UGamma;indirect gap semiconductors, and the gap reduces with the appliedpressure. The pressure-driven samples ultimately transform from thesemiconductor to a metallic phase at the given pressure range. Also,the calculations demonstrated that the applied pressure and spin-orbitcoupling of the states pushed VBM and CBM toward the Fermi level whichcaused the evolution of the band gap. The relationship between thestructure and band gap demonstrates the potential for designing lead-freeinorganic perovskites for optoelectronic applications, including solarcells as well as X-ray detectors.