Synthesis and Selective Topochemical Fluorination of the Cation and Anion-Vacancy Ordered phases Ba₂YCoO₅ and Ba₃YCo₂O_7.5

Abstract

The synthesis and characterization of two cation-ordered, anion-vacancy ordered phases, Ba₂YCoO₅ and Ba₃YCo₂O_7.5, is described. Neutron powder diffraction data reveal both phases adopt structures in which octahedral Y³⁺ and tetrahedral Co³⁺ centers are ordered within a “cubic” perovskite lattice. The unusual ordered pattern adopted by the cations can be attributed to the large concentration of anion vacancies within each phase. Reaction of Ba₂YCoO₅ with CuF₂ under flowing oxygen topochemically inserts fluorine into the host material to form Ba₂YCoO₅F_0.42(1). In contrast Ba₂YCoO₅ does not intercalate oxygen, even under high oxygen pressure. The selective insertion of fluorine, but not oxygen, into Ba₂YCoO₅ is discussed and rationalized on the basis of the lattice strain of the resulting oxidized materials. Magnetization and neutron diffraction data reveal Ba₃YCo₂O_7.5 and Ba₂YCoO₅F_0.42 adopt antiferromagnetically ordered states at low-temperature, while in contrast Ba₂YCoO₅ shows no sign of long-range magnetic order