First‐Principles Study of Relativistic Local Orbital Effects on the Electronic Transport Properties of BiOX (X = Cl, Br, I) Using WIEN2k and BoltzTraP

Bismuth oxyhalide (BiOX) is a group of layered materials known for their distinctive physicochemical and optical properties. This study investigates the electronic transport properties (thermoelectric performance) of bismuth oxyhalides (BiOX, where X = Cl, Br, I) using first‐principles calculations. Electronic structures were calculated using the full‐potential linearized augmented plane‐wave method implemented in WIEN2k, and the transport coefficients were evaluated using BoltzTraP code based on the semi‐classical Boltzmann transport theory. The focus is on the effect of relativistic local orbitals (RLO) employed for the accurate treatment of Bi 5d semicore states—on the electronic structure of these compounds. When RLO is not included, the indirect bandgaps of BiOCl, BiOBr, and BiOI are 2.76, 2.21, and 1.42 eV, respectively, and these values increase to 2.94, 2.31, and 1.67 eV with the inclusion of RLO. Effective masses near the band edges were also calculated to support the analysis of transport properties. Although lattice thermal conductivity κ ℓ was not calculated, the power factor ( S 2 σ / τ ) was used to assess the thermoelectric potential. Assuming inherently low κ ℓ due to layered structure, BiOCl and BiOI emerge as promising candidates, offering advantages in transport quality and carrier activation, respectively.