The polymorphic nature of M${}_3$BiBr${}_6$ halides (M=Cs, Rb) and their reversible intercalation with water to isomorphous hydrates at room temperature

Bismuthates with composition M${}_3$BiBr${}_6$ (M=Rb, Cs) form series of polymorphs capable of reversible H${}_2$O incorporation at room temperature. The host compounds were synthesized by annealing mixtures of MBr/BiBr${}_3$, 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-Rb${}_3$BiBr${}_6$ (space group Pbnm) is known from previous work, mP-Rb${}_3$BiBr${}_6$ crystallizes with a new type of structure (space group P2${}_1$/c). For Cs${}_3$BiBr${}_6$, the oP-phase crystallizes isostructural to Cs${}_3$NdCl${}_6$ (space group Pbcm), the mC-phase isostructural to Cs${}_3$BiCl${}_6$ (space group C2/c) and the tP-phase isostructural to Tl${}_3$BiCl${}_6$ (space group P4${}_2$/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 M${}_3$BiBr${}_6$ transform to semi-hydrates M${}_3$BiBr${}_6$ ⋅ 1/2 H${}_2$O. 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-Cs${}_3$BiBr${}_6$ features a crystal structure with higher density and does not accommodate water. (© 2020 The Authors. Zeitschrift für anorganische und allgemeine Chemie published by Wiley-VCH GmbH)