Understanding the Structural and Electronic Properties of Bismuth Trihalides and Related Compounds

Bismuth trihalides, BiX₃ (X = F, Cl, Br, and I), have been thrust into prominence recently because of their close chemical relationship to the halide perovskites of lead, which exhibit remarkable performance as active layers in photovoltaic cells and other optoelectronic devices. In the present work, we have used calculations based on density functional theory to explore the energetics and electronic properties of BiX₃ in a variety of known and hypothetical structure types. The results for BiX₃ are compared with those obtained for the halides of the later rare earths, represented by YX₃ and LuX₃. The relative thermodynamic stabilities of the known and hypothetical structures are calculated, along with their band gaps. For the BiX₃ systems, we have explored the role of lone-pair effects associated with bismuth(III), and for BiI₃, we have compared the predicted structural behavior as a function of pressure with the available experimental data. We have also attempted to synthesize LuF₃ in the perovskite-related ReO₃-type structure, which is predicted to be only ∼7.7 kJ mol^-1 above the convex hull. This attempt was unsuccessful but led to the discovery of a new hydrated phase, (H₃O)Lu₃F₁₀H₂O, which is isomorphous with the known ytterbium analogue.