Structural, Electronic, and Optical Properties of BiOX(1−x)Y(x) (X, Y = F, Cl, Br, and I) Solid Solutions from DFT Calculations

Six BiOX(1−x)Y(x) (X, Y = F, Cl, Br, and I) solid solutions have been systematically investigated by density functional theory calculations. BiOCl(1−x)Br(x), BiOBr(1−x)I(x), and BiOCl(1−x)I(x) solid solutions have very small bowing parameters; as such, some of their properties increase almost linearly with increasing x. For BiOF(1−x)Y(x) solid solutions, the bowing parameters are very large and it is extremely difficult to fit the related calculated data by a single equation. Consequently, BiOX(1−x)Y(x) (X, Y = Cl, Br, and I) solid solutions are highly miscible, while BiOF(1−x)Y(x) (Y = Cl, Br, and I) solid solutions are partially miscible. In other words, BiOF(1−x)Y(x) solid solutions have miscibility gaps or high miscibility temperature, resulting in phase separation and F/Y inhomogeneity. Comparison and analysis of the calculated results and the related physical–chemical properties with different halogen compositions indicates that the parameters of BiOX(1−x)Y(x) solid solutions are determined by the differences of the physical–chemical properties of the two halogen compositions. In this way, the large deviation of some BiOX(1−x)Y(x) solid solutions from Vegard’s law observed in experiments can be explained. Moreover, the composition ratio of BiOX(1−x)Y(x) solid solutions can be measured or monitored using optical measurements.