Benefit of high-pressure structure on sodium transport properties: example with NaFeF3 post-perovskite

Abstract In the search of promising Na-ion cathode materials, high-pressure synthesis method was applied to obtain sodium iron fluoride NaFeF3 with the post-perovskite lamellar structure. A two-step synthesis was performed: first, the Pnma orthorhombic perovskite structure NaFeF3 form (or Pv-NaFeF3, GdFeO3 type) was synthesized at 1270 ​K and 7.7 ​GPa, then the Cmcm orthorhombic post-perovskite structure (or pPv-NaFeF3, CaIrO3 type) was obtained using a second step at 700 ​K and 15 ​GPa. The post-perovskite form was stabilized under ambient conditions with a low amount of remaining perovskite phase (5 ​mol%). The Mossbauer analysis of pPv-NaFeF3 was firstly investigated with a paramagnetic structure obtained at 300 and 77 ​K, opposed to Pv-NaFeF3 magnetism reflecting the different [FeF6] polyhedra arrangements. Finally, the sodium migration within the framework of both structures was evaluated with the use of Bond Valence Energy Landscape (BVEL) calculations, resulting with an enhanced Na+ mobility within the pPv-NaFeF3 structure.