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

Area of Science:

• Materials Science
• Solid-State Chemistry
• Crystallography

Background:

• Disordered materials with mixed-occupancy sites are key for tuning physicochemical properties.
• Understanding crystal structures is essential for designing functional materials.

Purpose of the Study:

• Investigate the crystal structure of RbM2O5F (M = Nb, Ta) pyrochlore-type oxyfluorides.
• Clarify the distribution of oxygen and fluorine anions and Rb occupancy.
• Determine if short-range anion ordering exists in these materials.

Main Methods:

• Multimodal approach combining experimental and computational techniques.
• Rietveld structural refinement of powder X-ray powder diffraction (PXRD) data.
• Solid-state Nuclear Magnetic Resonance (NMR) spectroscopy (19F MAS, 87Rb and 93Nb (CT)MAS and 3QMAS).
• Density Functional Theory (DFT) calculations using the supercell approach and PAW/GIPAW methods.

Main Results:

• RbM2O5F (M = Nb, Ta) oxyfluorides are isostructural with disordered average crystal structures.
• The anionic site is co-occupied by O and F, and the Rb site is 25% occupied.
• Experimental and computed NMR parameters indicate distributed local environments.
• DFT models best matching experimental NMR data reveal preferential short-range anion ordering.

Conclusions:

• RbM2O5F pyrochlores exhibit preferential short-range anion ordering, not random distribution.
• The identified ordered structures involve specific arrangements of [MO5F] octahedra and Rb-centered cages.
• This detailed structural understanding is vital for the rational design of novel oxyfluoride materials.