Theoretical investigation of structural, electronic, and optical features of lead-free Rb 2 LiAsX 6 (X = F, Cl, Br, I) double perovskites

Abstract Lead-free double perovskites have attracted growing attention as environmentally benign materials for optoelectronic and energy-related applications. In this work, the structural, electronic, and optical properties of R b 2 L i A s X 6 X = F , C l , B r , I double perovskites are systematically investigated using density functional theory (DFT). Structural optimization, phonon dispersion, and formation energy analyses confirm the thermodynamic and dynamical stability of all studied compounds. Electronic structure calculations within the GGA–PBE framework reveal indirect band gaps of approximately 4.61, 2.96, 2.32, and 1.31 eV for R b 2 L i A s F 6 , R b 2 L i A s C l 6 , R b 2 L i A s B r 6 , and R b 2 L i A s I 6 , respectively, demonstrating a clear monotonic reduction in band gap with decreasing halide electronegativity. To achieve a more accurate description of the band gaps, additional calculations were performed using the HSE06 hybrid functional and the modified Becke–Johnson (mBJ) potential, which preserve the same compositional trend while yielding more reliable gap values. Optical property analyses further show that the Br- and I-based compounds exhibit strong absorption and enhanced optical conductivity in the visible region, making them promising candidates for photovoltaic and visible-light optoelectronic devices, whereas R b 2 L i A s C l 6 displays pronounced activity in the near-ultraviolet region, suggesting potential for ultraviolet photodetection applications.