Unraveling the Structure–Fluoride Transport Relationships of the Mechanochemically Synthesized Ba0.57M0.43F2.43 (M = Y, La, Nd, Sm, and Bi) Fluoride-Ion Conductors

Aliovalent-doped BaF₂ exhibits high ionic conductivity, making it a promising candidate as a solid electrolyte in high-energy-density fluoride-ion batteries. When Ba²⁺ is partially substituted with trivalent ions, such as La³⁺ or Bi³⁺, the introduction of additional fluorine yielded for the sake of the charge neutralization condition increases the conductivity by 4 orders of magnitude, reaching approximately 10^–4 S cm^–1 at 150 °C. However, the relationship between the position of the additional fluorine within the crystal structure and the electrical properties remains unclear. In this study, we explore it by varying the aliovalent dopants in the mechanochemically synthesized Ba_0.57M_0.43F_2.43 (M = Y, La, Nd, Sm, Bi) and investigating both the electrical properties and the structures. For these aliovalent-doped compounds, improved conductivity is observed, and for certain compounds, the electrochemical stability window extends beyond 5.5 V. We perform Rietveld refinement of neutron powder diffraction data and the maximum entropy method to investigate the fluorine positions. These crystal structures suggest the fluorine positions deviated from the ideal octahedral center 4b to the combinations of 24e, 32f, and 48i positions in the cubic BaF₂ crystal (space group Fm3̅m) to form a closer ionic conduction path.