Crystal structure and migration paths of alkaline ions in NaVPO4F

Abstract

A sample with a nominal composition ‘NaVPO₄F’ is prepared by mechanochemically assisted solid-state synthesis using quenching. A detailed study of its crystal and local structure is conducted by means of XRD and FTIR and solid-state ³¹P NMR spectroscopies in comparison with Na₃V₂(PO₄)₂F₃. It is shown that the as-prepared ‘NaVPO₄F’ has a multiphase composition, including NaVPO₄F as the main phase and Na₃V₂(PO₄)₂F₃ and Na_2.57V₄P₄O₁₇F as the side products. The crystal structure of NaVPO₄F is described in the monoclinic C2/c space group. It is characterized by 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 S.G.) are 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 the instability of the lithium derivatives as was concluded from the DFT calculations.