Magnesium Hexafluoridozirconates MgZrF₆·5H₂O, MgZrF₆·2H₂O, and MgZrF₆: Structures, Phase Transitions, and Internal Mobility of Water Molecules

Abstract

The MgZrF 6 · n H 2 O ( n = 5, 2 and 0) compounds were studied by the methods of X‐ray diffraction and 19 F, MAS 19 F, and 1 H NMR spectroscopy. At room temperature, the compound MgZrF 6 · 5H 2 O has a monoclinic C‐centered unit cell and is composed of isolated chains of edge‐sharing ZrF 8 dodecahedra reinforced with MgF 2 (H 2 O) 4 octahedra and uncoordinated H 2 O molecules and characterized by a disordered system of hydrogen bonds. In the temperature range 259 to 255 K, a reversible monoclinic ↔ two‐domain triclinic phase transition is observed. The phase transition is accompanied with ordering of hydrogen atoms positions and the system of hydrogen bonds. The structure of MgZrF 6 · 2H 2 O comprises a three‐dimensional framework consisting of chains of edge‐sharing ZrF 8 dodecahedra linked to each other through MgF 4 (H 2 O) 2 octahedra. The compound MgZrF 6 belongs to the NaSbF 6 type and is built from regular ZrF 6 and MgF 6 octahedra linked into a three‐dimensional framework through linear Zr–F–Mg bridges. The peaks in 19 F MAS spectra were attributed to the fluorine structural positions. The motions of structural water molecules were studied by variable‐temperature 1 H NMR spectroscopy.