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

Abstract

Bismuth trihalides, BiX 3 (X F, Cl, Br and I) have been thrust into prominence recently due to 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 BiX 3 in a variety of known and hypothetical structure types. The results for BiX 3 are compared with those obtained for the halides of the later rare-earths, represented by YX 3 and LuX 3 . The relative thermodynamic stabilities of the known and hypothetical structures are calculated, along with their band gaps. For the BiX 3 systems we have explored the role of 1 lone pair effects associated with Bi(III), and for BiI 3 we have compared the predicted structural behaviour as a function of pressure with the available experimental data. We have also attempted to synthesize LuF 3 in the perovskite-related ReO 3 -type structure, which is predicted to be only ∼ 7.7 kJ mol −1 above the convex hull. This attempt was unsuccessful, but lead to the discovery of a new hydrated phase, (H 3 O)Lu 3 F 10 H 2 O, which is isomorphous with the known ytterbium analogue.

Why are Double Perovskite Iodides so Rare?

“…One factor is certainly the tendency of the Bi(III) compounds to crystallize as low-dimensional 3:2:9 type phases due to their high covalency, with bismuth halide compounds notably favoring layered-type structures. 104 Most of our predictions, however, relate to rare-earth phases, and we believe that the challenges of synthesizing these lanthanide compounds are much greater than those encountered with post-transition metals. In particular, both the starting materials and products are likely to be air-sensitive and require rigorous exclusion of moisture.…”

“…The case of Cs 2 NaErI 6 , for example, involved a solid state reaction between carefully dried Csl, Nal, Er metal, and I 2 in a sealed tantalum tube, rather than a simple reaction in aqueous solution. The rare earth metal iodides are more ionic than their post-transition-metal analogues, and we would expect their band gaps to be wider than, for example, bismuth-containing compounds . They are therefore less likely to be useful for PV applications and more suitable for use in areas such as X-ray detection and magnetic and optical applications.…”

“…The rare earth metal iodides are more ionic than their post-transitionmetal analogues, and we would expect their band gaps to be wider than, for example, bismuth-containing compounds. 104 They are therefore less likely to be useful for PV applications and more suitable for use in areas such as X-ray detection and magnetic and optical applications. We are confident that efforts along these lines will lead to the discovery of many of the compounds listed in Table 2.…”

Stacking Faults Assist Lithium-Ion Conduction in a Halide-Based Superionic Conductor

“…However, YCl 3 contains (002) planes of octahedral cation voids, in contrast to LYC, which has Y 3+ and Li + ions distributed throughout its (001) and (002) planes. Deng et al 52 recently demonstrated that YCl 3 is amenable to polymorphism, especially with respect to the related BiI 3 -type structure which shows an alternate ABCABC anion stacking arrangement. According to their calculations, the energy difference between the two structure-types is ≈0.1 kJ mol −1 (≈1 meV atom −1 ), suggesting that alternate stackings, or even stacking faults, are possible in YCl 3 -type structures.…”

Halide Double-Perovskite Semiconductors beyond Photovoltaics

“…Thus, investigations on the synthesis products as a function of the milling conditions (i.e., frequency and time) and the effects of introducing chemical additives to control the kinetics of the crystallization should be conducted to make full use of this synthetic strategy. Another challenge in the solution-based fabrication of such halide double perovskites is that the low-dimensional, non-conductive, 3:2:9 phase (e.g., Cs 3 Bi 2 Br 9 ) is thermodynamically favored, 30 thus competing with the elpasolite phase (i.e., Cs 2 AgBiBr 6 ). The 3:2:9 phase consists of face-sharing double-layered [Bi 2 Br 9 ] 3– octahedra.…”