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

Abstract

AbstractBismuthates 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 ⋅ 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.