Structural and electronic effects of M- and X-site substitution in M4Ag2BiX9 (M = Cs, Rb; X = Br, I) layered Zintl halides.

Structural, electronic, and optical properties of Cs4Ag2BiX9 and Rb4Ag2BiX9 (X = Br, I) Zintl-phase halides have been investigated using density functional theory via the Vienna ab-initio simulation package. Structural analysis confirms orthorhombic symmetry (space group Pnnm, No. 58) for all four compounds, with optimized lattice parameters in line with known trends in halide substitution. Electronic band structure and density of states revealed that all compounds are indirect band gap semiconductors with band gaps ranging from 1.75 to 2.32 eV. Projected density of states and electronic charge density distribution analyses showed that valence band maxima originated mainly from Ag-X hybridized states, while conduction band minima is dominated by Bi-6p orbitals. The -COHP analysis confirmed strong bonding interactions, particularly in iodide-based compounds, supporting enhanced cohesion and stability. The optical properties including absorption spectra, dielectric function, and optical conductivity demonstrated less anisotropy and UV dominant absorption. The results indicated that halide substitution plays a more prominent role than A-site cation substitution in tuning the optoelectronic behavior. Optical properties reveal strong UV absorption (5-20 eV), making bromides suitable for deep-UV photo detectors, while iodides e.g., Rb4Ag2BiI9 show potential as tandem solar cell absorbers due to red shifted band gaps (∼1.75 eV).