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

The MgZrF6·nH2O (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.

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