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

► Dynamic orientational disorder is characteristic of crystal structure of Rb2TaOxF7−2x. ► Oxygen is partially substituted for fluorine. ► Fast reorientations of the anions lead to equalization of the Ta-O and Ta-F bond distances. ► Synchronous Ta-O and Ta-F vibrations appear at 700-800 cm−1. ► The 700-800 cm−1 infrared band can serve as an oxygen sensor.