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

Abstract

Three new sodium manganese fluoro-pyrophosphate compounds, namely, Na_5.5Mn_1.5(P₂O₇)₂F_0.5 (I), Na₇Mn_0.75(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 [Mn_0.75(P₂O₇)₂F₂]⁷⁻ unit, whereas I and III reveal three-dimensional (3D) frameworks that consist of MnO₆, Mn/NaO₅F_0.5 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 °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 ∼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.