Revealed Preferential Short-Range Anion Ordering in Disordered RbM₂O₅F (M = Nb, Ta) Pyrochlore-Type Oxyfluorides

Describing the crystal structure of disordered materials with mixed-occupancy crystallographic sites is essential for understanding their physicochemical properties and designing new materials tuned to their targeted functionalities. Here, we investigate the structure of RbM₂O₅F (M = Nb, Ta) pyrochlore-type oxyfluorides using a multimodal approach that combines experimental and computational techniques. Rietveld structural refinement of powder X-ray powder diffraction (PXRD) data confirmed that these oxyfluorides are isostructural, and their average crystal structure is disordered. The anionic site, 48f, is co-occupied by O and F, while the Rb site, 32e, is occupied at 25%. The shapes of the high-field solid-state ¹⁹F MAS, and ⁸⁷Rb and ⁹³Nb (CT)MAS and 3QMAS NMR spectra indicate that the local environment of these nuclei is distributed. Using the "supercell" approach, models incorporating different anion arrangements and Rb atoms distributed in their crystallographic sites were built and relaxed using density functional theory (DFT), and NMR parameters for ¹⁹F, ⁸⁷Rb, and ⁹³Nb, were computed using the PAW and GIPAW approaches. The models showing the best agreement between computed and experimental NMR parameters are made up exclusively of [MO₅F]^6- octahedra, [RbO₁₅F₃]^32-, [RbO₁₆F₂]^33-, and [RbO₁₄F₄]^31- cages, indicating the existence of a preferential short-range anion ordering in these pyrochlores instead of the expected random distribution.