Experimental Visualization of the Interstitialcy Diffusion of Anions in the LaOF-Based Oxyfluoride La_0.9Sr_0.1O_0.45F₂

Fluoride-ion and oxide-ion conductors are attractive materials due to their wide applications such as next-generation fluoride-ion batteries and solid oxide fuel cells. Crucial to the development of these anion conductors is the knowledge of crystal structures and the ion-diffusion mechanism at an atomic scale. Recently, mixed-anion compounds have attracted much attention, but experimental visualization of anion-diffusion pathways is very rare in mixed-anion compounds. Lanthanum oxyfluoride LaOF-based materials are mixed-anion compounds and exhibit high anion (fluoride-ion and oxide-ion) conductivities; however, their high-temperature crystal structures and anion-diffusion mechanism are not known satisfactorily. Herein, we report detailed information on the crystal structure and structural disorder of La0.9Sr0.1O0.45F2 and LaOF from -243 °C (30 K) to 600 °C. Trigonal R3¯ m β-LaOF undergoes a first-order phase transition into a cubic α-phase with the Fm3¯ m fluorite-type structure around 490 °C on heating, while La0.9Sr0.1O0.45F2 is a cubic Fm3¯ m α-phase between -243 and 600 °C. Neither significant amounts of interstitial anions at the 32f site nor significant anion vacancies at the lattice 8c site are observed in cubic LaOF at 600 °C, while both interstitial anions at the 32f site and anion vacancies at the lattice 8c site exist in cubic La0.9Sr0.1O0.45F2 from -243 to 600 °C. We have succeeded in experimental visualization of anion-diffusion pathways in La0.9Sr0.1O0.45F2. It was found that the anions migrate through both the interstitial 32f and lattice 8c sites (8c-32f-32f-8c anion-diffusion pathways), indicating an interstitialcy diffusion mechanism. The existence of interstitial anions and anion vacancies and the formation of the anion-diffusion pathways are the structural origins of the high anion conductivity of La0.9Sr0.1O0.45F2. The present elucidation of the crystal structure and anion-diffusion mechanism might provide useful knowledge for the design of superior anion conductors, which develop the next-generation fluoride-ion batteries and solid oxide fuel cells.