A study of the composition, ionic and molecular mobility, and thermal properties of ammonium pentafluoridozirconate hydrates

The effect of hydrate number on the structural changes, thermal properties, and ionic (molecular) mobility character in NH$_{4}$ZrF$_{5}$ $\cdot$ H$_{2}$O и NH$_{4}$ZrF$_{5}$ $\cdot$ 0.75H$_{2}$O crystal hydrates have been investigated by the methods of IR, Raman, nuclear magnetic resonance (NMR) ($^{1}$H, $^{19}$F, including $^{19}$F MAS), and TG-DTA spectroscopy. Differences in crystal hydrate structures–anion structure, molecular state of water, and O–H $\cdots$ F, N–H $\cdots$ F hydrogen bond strengths–have been corroborated by IR and Raman spectroscopy data. Isotropic chemical shifts of magnetic inequivalent positions have been determined and attributed to crystal structures of the studied compounds by the method of $^{19}$F MAS NMR. It has been established that the removal of water molecules from NH$_{4}$ZrF$_{5}$ $\cdot$ H$_{2}$O and NH$_{4}$ZrF$_{5}$ $\cdot$ 0.75H$_{2}$O results in the transformation of chain or layered structures accompanied by the increase of the number of bridge bonds while retaining or increasing the dimensionality of the anion structural motif. According to the $^{1}$H NMR data, the NH$_{4}^{+}$ cation diffusion in NH$_{4}$ZrF$_{5}$ occurs only in the temperature range of 370–520 K.