The Polymorphic Nature of M₃BiBr₆ Halides (M=Cs, Rb) and their Reversible Intercalation with Water to Isomorphous Hydrates at Room Temperature

Abstract

Bismuthates with composition M3BiBr6 (M=Rb, Cs) form series of polymorphs capable of reversible H2O incorporation at room temperature. The host compounds were synthesized by annealing mixtures of MBr/BiBr3, or by precipitation from acidic solution and removing water molecules from the host framework of the hydrates in a thermal dehydration reaction. While the compound oP‐Rb3BiBr6 (space group Pbnm) is known from previous work, mP‐Rb3BiBr6 crystallizes with a new type of structure (space group P21/c). For Cs3BiBr6, the oP‐phase crystallizes isostructural to Cs3NdCl6 (space group Pbcm), the mC‐phase isostructural to Cs3BiCl6 (space group C2/c) and the tP‐phase isostructural to Tl3BiCl6 (space group P42/m). All crystal structures were refined from single crystal X‐ray diffraction data. In both systems, the crystal structures of orthorhombic and the monoclinic phases are polytypes and differ by stacking sequence only. By intercalation of water molecules, all polytypes of M3BiBr6 transform to semi‐hydrates M3BiBr6 ⋅ 1/2 H2O. Both, the incorporation of water and the reverse process proceed without the host structure being changed. The inclusion of crystal water was also confirmed by IR‐spectroscopy, thermal gravimetric analysis and mass spectrometry. The modification tP‐Cs3BiBr6 features a crystal structure with higher density and does not accommodate water.

Molecular and Supramolecular Structure of a New Luminescent Hybrid Compound: (C5N2H14)2[BiBr6]Br·H2O

“…The distortion is not severe and is less pronounced than in mononuclear bromobismuthates with organic cations [32,38,39]. However, comparisons between all-inorganic bromobismuthates and small organic cations [40] show that the latter feature almost undistorted BiBr 6 3− anions; for instance, the range of Bi-Br bond distances in Cs 3 BiBr 6 is very narrow: 2.83-2.85 Å [28]. This confirms that the 6s 2 lone pair on the Bi atom does not display stereochemical activity, which is normal unless very electronegative ligands like fluorine are present [41].…”

“…These octahedra share vertices, edges, or faces to form a variety of anions ranging from 0D to 2D [26,27]. Surprisingly, those cases where single BiX 6 3− anions do not combine into more complex structures are rare compared to other halometallate anions [28][29][30][31][32][33][34][35]. As a rule, such compounds form with large organic or inorganic cations and demonstrate high reactivity towards moisture.…”