Microscopic Insights into the Reconstructive Phase Transition of KNaNbOF₅with ¹⁹F NMR Spectroscopy

The centrosymmetric polymorph of KNaNbOF5 undergoes a reconstructive transition from a rare A-site vacancy ordered perovskite to a unique nonperovskite high-temperature phase. Previously, the high temperature phase was proposed to exhibit dynamical disorder with mixed occupancy of O and F on the anion sites owing to hopping rotations of the [NbOF5]2- octahedra. Here, we utilize 19F NMR techniques to assess the dynamic anion configurations through this reconstructive transition. We conclude from these measurements that the high-temperature phase exhibits Cmcm symmetry with hopping rotations of the [NbOF5]2- octahedra occurring only about the Fapical-Nb-O axis of the octahedra such that there are no O/F mixed occupancy anion sites. We use these results to refine the microscopic description of the reconstructive transition and its driving force and examine changes in the electronic structure of the d0 oxyfluoride through the transition. Our work demonstrates the efficacy of 19F NMR techniques combined with electronic structure calculations to understand thermodynamically driven changes in heteroanionic materials.