Topochemical Fluoride Exchange Reaction with Anionic Electrons toward Fluoride-Ion Conduction in Layered Ba2–xAxNF1–x (A = Na, K)

Monovalent potassium substitution at the divalent barium sites of Ba2NF, a Ba2N electride analogue, led to fluoride-ion conduction in the two-dimensional (2D) interlayer space. Potassium substitution made the system chemically unstable for x ≥ 0.25 in Ba2–xKxNF1–x, and the nominal fluorine stoichiometry was not attained as the composition approached this region. Fluoride ions, fluoride vacancies, and interstitial anionic electrons (IAEs) coexisted in the 2D interlayer space of Ba1.8K0.2NF0.7·0.1e– because the missing charge of the absent fluoride ions was compensated for by IAEs. First-principles calculations demonstrated that the coexistence of these three species hindered smooth fluoride-ion diffusion on account of the IAEs acting as a scattering barrier. Chemical fluorination to replace these residual IAEs with fluoride ions via an exchange reaction led to the coexistence of only fluoride ions and their vacancies, resulting in fluoride-ion conduction. The obtained results demonstrate the first emergence of fluoride-ion conduction in an electride analogue, which is expected to motivate further exploration of this new family of fluoride-ion conductors. Our skeleton-retained anion exchange approach suggests that such 2D electrides may represent a fresh chemical space for realizing monovalent anionic conduction.