Features of the ion mobility and phase transitions in hexafluoro complex compounds of tantalum(V), niobium(V), and titanium(IV) with the tetramethylammonium cation

Abstract

¹⁹F, ¹H NMR and DSC methods are used to study the ion mobility and phase transitions in hexafluoro complex compounds of tantalum(V), niobium(V), and titanium(IV) with the tetramethylammonium cation. A transformation of the NMR spectra observed in a temperature range 77(130)-450 K is found to be associated with a change in the type of ion motions in the anion and cation sublattices of the studied compounds. In a temperature range 170-450 K the main types of ion motions are isotropic reorientations of TaF₆, NbF₆, TiF₆ octahedra and tetramethylammonium ions. Two endothermic effects with the maxima at 232.5 K and 256.5 K for [N(CH₃)₄]TaF₆ and 235 K and 250 K for [N(CH₃)₄]NbF₆, which are observed in the DSC curve in the range 170-400 K correspond to phase transitions that have almost no effect on the NMR spectral parameters the a temperature range 230-260 K. For the [N(CH₃)₄]₂TiF₆ compound, an endothermic effect at 422 K is observed, corresponding to the phase transition from the rhombohedral to the cubic modification.