Dynamic orientation disorder in rubidium fluorotantalate. Synchronous Ta–O and Ta–F vibrations

The oxygen-containing rubidium fluorotantalate Rb2TaOxF7−2x (x = 0.3) was synthesized in a single crystal form and its RT and LT structures were determined by X-ray diffraction. The structures include dynamically disordered seven-coordinated tantalum anions in the form of capped trigonal prisms. The oxygen presence was revealed by the appearance of the infrared band in the range of 700–800 cm−1 assigned to totally synchronous Ta–O and Ta–F vibrations as a result of the distances equalization due to fast reorientations of a seven-coordinated polyhedron. The phase transition at 145 K is accompanied by the syngony change from tetragonal to orthorhombic and appearance of the 920 cm−1 vibration assigned to a common terminal tantalum–oxygen triple bond.