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. (copyright 2017 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)