Electronic and optical properties of halogen-substituted LaBi2Cl1-y X y O4: a promising candidate for energy-efficient devices.

Layered bismuth oxyhalides exhibit significant promise for photocatalytic and optoelectronic applications owing to their diverse functional characteristics. Here, we report the structural stability and electronic and optical properties of LaBi2ClO4 and its halogen-substituted derivatives LaBi2Cl1-y X y O4 (X = Br, I; y = 0.25, 0.5, 0.75) by means of a density functional theory approach. Both pristine and halogen-substituted compounds are thermodynamically stable, as evidenced by their negative formation energies, and the pristine phase satisfies the mechanical stability criteria. LaBi2ClO4 is identified as an indirect band gap semiconductor based on electronic structure calculations, with gap values between 1.18 eV (GGA) and 1.98 eV (mBJ). The dominant contributions near the valence and conduction band edges originate from O-2p and Bi-6p electronic states, respectively. Halogen substitution systematically narrows the band gap, with iodine doping inducing the most pronounced reduction, attributed to enhanced lattice distortions and modified orbital hybridization. Optical properties, computed via complex dielectric functions, exhibit pronounced anisotropy and tunability; doping shifts absorption edges toward lower photon energies and increases dielectric constants and refractive indices. These tunable optical responses lead to enhanced optical conductivity and reflectivity across the visible to ultraviolet spectrum, underscoring halogen-doped LaBi2ClO4 as a versatile platform for next-generation optoelectronic devices and photocatalysts.