New Dion−Jacobson-Type Layered Perovskite Oxyfluorides, ASrNb₂O₆F (A = Li, Na, and Rb)

New Dion−Jacobson-type layered perovskite oxyfluorides, ASrNb 2 O 6 F (A = Li, Na, and Rb), have been prepared by conventional solid-state reaction, and their fluorine distribution in an anion sublattice was studied by performing 19 F MAS NMR spectroscopic analyses and lattice energy calculations to probe the evolution of interlayer cationic conductivitiy upon fluorine substitution. From these comparative studies, it is found that the fluorine anions are randomly distributed in two distinct crystallographic sites among three possible sites in the oxyfluoride lattice, that is, the equatorial site of NbO 6 octahedra (O equatorial ) and the central site of two corner-sharing NbO 6 octahedra along the c axis (O center ). On the other hand, the substituted fluorine does not prefer to occupy the O apex site, which interacts ionically with an alkali metal cation (A + ) in the interlayer, because the (Nb−O apex ) bond is too strongly covalent to be replaced by the ionic Nb−F bond. We have also compared systematically the Li and Na ion conductivities for the oxyfluorides, ASrNb 2 O 6 F, with those for the corresponding oxides, ALaNb 2 O 7 (A = Li and Na), and found that the fluorination of the perovskite layer gives rise to an enhancement of ionic transport in the interlayer space.