First-principles investigation of A₂GeBeBr₆ (A = Rb, K) halide double perovskites for advanced optoelectronic and photonic devices

We present a first-principles DFT study of lead-free halide double perovskites A₂GeBeBr₆ (A = Rb, K) using the TB-mBJ functional. Both compounds stabilize in a cubic Fm-3m structure with lattice parameters of 10.758 Å (Rb) and 10.632 Å (K). They exhibit indirect band gaps suitable for visible-light harvesting and show ductile mechanical behavior. The calculated optical properties, including strong UV absorption and moderate dielectric constants, suggest potential for optoelectronic applications. This work provides fundamental insights into this unexplored material class and supports the design of lead-free perovskites for renewable energy technologies.