Open frameworks in the NaxMny(P2O7)mFn fluoro-pyrophosphates system

Three new sodium manganese fluoro-pyrophosphate compounds, namely, Na5.5Mn1.5(P2O7)2F0.5 (I), Na7Mn0.75(P2O7)2F2 (II), and Na9Mn3(P2O7)4F (III), have been synthesized by heating a mixture of NaPF6, Na2PO3F or NaH2PO4 with different Mn sources in NaNO3 and KNO3 fluxes. The structures of the title compounds were characterized via single-crystal X-ray diffraction (XRD). II is characteristic of a shell of Na+ ions that encloses one [Mn0.75(P2O7)2F2]7- unit, whereas I and III reveal three-dimensional (3D) frameworks that consist of MnO6, Mn/NaO5F0.5 octahedra or MnO6 octahedra and distorted MnO5 square pyramids with P2O7 units, where Na+ cations reside in different-membered ring one-dimensional (1D) tunnels. The estimated total Na+ ion conductivity for a pellet of III in open air is 10-5 S cm-1 at 400 degrees C, which is lower than that of many NASICON-type compounds at room temperature, with a higher activation energy of 0.89 eV for III compared to the value of similar to 0.4 eV for high-performance sodium ion conductors. The analysis of Na+ diffusion pathways revealed that percolation occurs through a zig-zag chain along the b axis via the bond valence energy landscape approach. Detailed characterization, such as spectroscopic and magnetic properties and specific heat for III, is also reported.