A Comprehensive First‐Principles Investigation of the Structural and Physical Properties of the Complete Bismuth Oxyhalide Family BiOX (X = F, Cl, Br, I)

First‑principles DFT calculations systematically investigate the structural, electronic, elastic, thermodynamic, and optical properties of the complete BiOX (X = F, Cl, Br, I) family. All compounds adopt a tetragonal P 4/ nmm layered structure, with lattice constants increasing monotonically with halogen size, in excellent agreement with experiments. Electronic structure reveals a progressive bandgap narrowing from 3.132 eV (BiOF) to 1.577 eV (BiOI), driven by the upward shift of halogen np orbitals at the valence band maximum, while the conduction band minimum remains Bi 6 p in character. Elastic constants satisfy all mechanical stability criteria; BiOF exhibits the highest bulk (118 GPa) and shear (40 GPa) moduli, yet Vickers hardness increases monotonically from 1.93 GPa (BiOF) to 7.30 GPa (BiOI) due to a halogen‑induced ductile‑to‑brittle transition (Pugh's G/B ratio). Quasi‑harmonic thermodynamic calculations show decreasing Debye temperatures (285–235 K) and composition‑dependent Grüneisen parameters. Optically, the absorption edge red‑shifts from BiOF to BiOI, with BiOI showing the strongest visible‑light response. This unified work establishes halogen‑tunable trends across multiple physical properties, providing a robust theoretical foundation for designing BiOX‑based photocatalysts and optoelectronic devices. The results highlight the critical role of halogen substitution in simultaneously modulating lattice, electronic, mechanical, thermal, and optical behavior.