Enhancing fluoride-ion conduction via doping-induced anionic ordering in an n = 1 Ba-based Ruddlesden-Popper oxyfluoride structure

Tuning the atomic arrangement in mixed-anion compounds is one of the key research areas in solid-state chemistry, as it enables the understanding of the structure-property relationship. In this study, we proposed a strategy for fluorine ordering in an oxyfluoride compound with an n = 1 Ruddlesden-Popper (RP) structure, A_n+1B_nO_3n+1-xF_2x, by aliovalent doping at the A-site. The framework structure based on Ba₂(Sn/Zr)O_4-xF_2x was selected, because oxygen and fluorine occupy all three anion positions—equatorial, apical, and interstitial. To analyze the materials, we used a combination of high-resolution neutron diffraction, synchrotron X-ray diffraction, electrochemical testing, and computational analyses. Our findings show that doping potassium leads to fluorine ordering at the interstitial sites. This ordering occurs to balance the electronic charge at the anion positions, following the electrostatic valence rule. As a result, the electrochemical properties of the compound change: the electrical conductivity increases and the activation energy decreases. These results offer new insights and suggest a strategy for controlling the atomic arrangement in mixed-anion compounds, opening possibilities for designing materials with tailored properties.