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

The MgZrF₆ · n H₂O (n = 5, 2 and 0) compounds were studied by the methods of X‐ray diffraction and ¹⁹F, MAS ¹⁹F, and ¹H NMR spectroscopy. At room temperature, the compound MgZrF₆ · 5H₂O has a monoclinic C‐centered unit cell and is composed of isolated chains of edge‐sharing ZrF₈ dodecahedra reinforced with MgF₂(H₂O)₄ octahedra and uncoordinated H₂O 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 MgZrF₆ · 2H₂O comprises a three‐dimensional framework consisting of chains of edge‐sharing ZrF₈ dodecahedra linked to each other through MgF₄(H₂O)₂ octahedra. The compound MgZrF₆ belongs to the NaSbF₆ type and is built from regular ZrF₆ and MgF₆ octahedra linked into a three‐dimensional framework through linear Zr–F–Mg bridges. The peaks in ¹⁹F MAS spectra were attributed to the fluorine structural positions. The motions of structural water molecules were studied by variable‐temperature ¹H NMR spectroscopy.