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, 48<i>f</i>, is co-occupied by O and F, while the Rb site, 32<i>e</i>, 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 "<i>supercell</i>" 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.