Crystal structure and migration paths of alkaline ions in NaVPO4F

The sample with a nominal composition ‘NaVPO4F’ is prepared by the mechanochemically assisted solid-state synthesis using quenching. The detailed study of its crystal and local structure is conducted by means of XRD, FTIR and solid-state 31P NMR spectroscopy in comparison with Na3V2(PO4)2F3. It is shown that the as-prepared ‘NaVPO4F’ has a multiphase composition, including NaVPO4F as a main phase and Na3V2(PO4)2F3 and Na2.57V4P4O17F as side products. The crystal structure of NaVPO4F is described in the monoclinic C2/c space group. It is characterized by a negligible V3+/V4+ oxidation with the corresponding F-/O2- substitution and the presence of structural disordering. Using the Voronoi-Dirichlet partition (VDP) method, the Na+ and Li+ migration pathways in Tavorite-like NaVPO4F and closely related LiVPO4F (with the triclinic structure, P-1 S.G.) were analyzed. While the Na+ migration is suppressed in both cases, the Na+/Li+ ion exchange in NaVPO4F with the formation of monoclinic LiVPO4F could occur, but is difficult due to the sodium immobility rather than instability of the lithium derivatives as was concluded from the DFT calculations.