Strong band bowing in BiOX (X = Cl, Br, I) due to halogen alloying

Bismuth oxyhalides (BiOX, X = Cl, Br, and I) are promising photocatalytic materials whose electronic and optical properties can be systematically tuned through halogen alloying. This study presents a comprehensive computational and experimental investigation of halogen anion alloying effects on the electronic structure and optical properties of BiOCl_1-xBr_x, BiOCl_1-xI_x, and BiOBr_1-xI_x alloy systems. Using density functional theory calculations combined with the generalized quasichemical approximation, we systematically investigated band gaps and density of states for all symmetrically non-equivalent configurations in 24-atom supercells. Our calculations reveal significant band gap bowing behavior with bowing parameters of 0.98, 2.23, and 2.70 eV for BiOCl_1-xBr_x, BiOBr_1-xI_x, and BiOCl_1-xI_x alloys, respectively, representing substantial deviations from Vegard's law. Point defect formation energy calculations demonstrate that although halogen substitution is endothermic, the formation energies remain sufficiently low (2-143 meV per dopant atom) to enable experimental synthesis. The mixing enthalpies remain below 10 meV per formula unit across the entire composition range for all three systems. At typical synthesis temperatures, the configurational entropy contribution easily overcomes the enthalpy penalty, stabilizing the random solid solution. We successfully synthesized BiOCl_1-xI_x nanoparticles across the complete composition range (x = 0-1) with yields exceeding 90%, experimentally validating our predictions. UV-visible spectroscopy of the synthesized alloys confirms the predicted red-shift in absorption onset with increasing iodine content. While the electronic band structures exhibit strong bowing effects, the absorption spectra are reasonably captured by a linear interpolation between pure end-members, providing a practical approximation for targeted optical design applications. The minimum band gaps occur at approximately 75-78% heavier halogen content, offering optimal visible light absorption. These findings provide fundamental insights into halogen alloying mechanisms in bismuth oxyhalides and establish clear design principles for enhanced photo-absorption or photo-catalytic applications.