First-principles study on the structural, electronic and optical properties of BiOX (X=Cl, Br, I) crystals

In order to investigate systematically the structural, electronic and optical properties of bismuth oxyhalides BiOX (X=Cl, Br, I) semiconductors, the lattice constants, structural characteristics, band structures, densities of states, atomic charge populations and optical properties of BiOX crystals have been calculated using first-principles based on DFT. The calculated indirect band gaps of BiOCl, BiOBr and BiOI crystals are 2.50, 2.10 and 1.59 eV, respectively. The analysis of densities of states and atomic charge populations for BiOX crystals indicates that, (a) the valance band maximum is mainly contributed to O 2p and X np states and the Bi 6p states dominate the conduction band minimum; (b) the contribution of X ns states obviously increases with the increase of X atomic numbers, and the dispersive energy level becomes more and more significant and (c) the sequence of covalent bonding strength between atoms is Bi–O >Bi–I>Bi–Br>Bi–Cl. In addition, the calculated absorption edges of the absorption coefficients I(ω) for BiOCl, BiOBr and BiOI crystals are 355, 448 and 645 nm, respectively, which agree well with our experimental measurements of 376, 442 and 628 nm and the previous reported results of 370, 440 and 670 nm.