Intramolecular mobility and phase transitions in ammonium oxofluoroniobates (NH4)2NbOF5 and (NH4)3NbOF6, a NMR and DFT study

► Dynamic disorder is characteristic of (NH4)2NbOF5 and (NH4)3NbOF6. ► NbOF52− is rigid while NbOF63− is fluxional in terms of NMR. ► Pentagonal bipyramid coverts into capped trigonal prism at LT phase transition. ► NH4 groups and polyhedra reorient concordantly in (NH4)3NbOF6.