Ammonium tantalum oxide fluorides: new features of dynamic disorder

Ammonium fluoride salts of seven- and eight-coordinated tantalum were grown in the form of well shaped single crystals at different pH of solutions, and their crystal structures were determined by X-ray diffraction. The first one, (NH₄)₃TaOF₆, belongs to elpasolite-type structure (Fm{\overline 3}m, Z = 4) and is characterized by strong dynamic disorder. The ligand atoms (O and F) are each distributed in three positions (one 24e and two 96j). The tantalum atom is seven-coordinated in the form of a pentagonal bipyramid (PB) and occupies the 4a position without leaving the symmetry center of the polyhedron during its fast reorientation. One of the ammonium groups is tetrahedrally disordered (the nitrogen atom is shifted from the 8c into the 32f site), while the other group in the 4b site forms eight spatial orientations due to disordering of hydrogen atoms in the 96k and 32f positions. Strong dynamic disorder of [TaOF₆]^3- as a flexible unit is reflected in the IR spectrum at 736 cm^-1 and in the ¹⁹F MAS NMR spectrum at -51 ppm, indicating a state with synchronous stretching vibrations of Ta-O and Ta-F bonds. Two double ammonium fluoride salts of tantalum, which are formed concomitantly, differ slightly in composition but their crystal structures contain the same polyhedra in the form of a mono-capped trigonal prism (CTP or TPRS-7) and a distorted cube (CU or CU-8) with one vacant vertex. The latter tantalum polyhedron is detected for the first time. The CU polyhedron in one of the modifications of the tantalum double salt splits into two CTPs during a phase transition with decreasing temperature. The presence of oxygen in the compounds is confirmed by vibrational spectroscopy.