Large oxygen deficiency in a n=2 member of the RP series:: Sr3FeCoO7-x (x≤1.55)

A very large oxygen deficiency has been detected in an n = 2 member of the Ruddlesden and Popper (RP) series, Sr3FeCoO7-x, with x reaching 1.55. For any x, the compounds are tetragonal I4/mmm with a approximate to 3.85 Angstrom and c approximate to 20.2 Angstrom, a and c decreasing with x. The neutron powder diffraction (NPD) study of the most oxygen deficient phase, x = 1.55, shows that the oxygen vacancies are located in the common apex and in the basal plane of the double perovskite layers, leading for cobalt and iron to local tetrahedral and pyramidal coordinations. XANES spectroscopy reveals a divalent oxidation state for cobalt with strongly distorted tetrahedral coordination, whereas iron exhibits a trivalent state with a square pyramidal environment. This observation associated with the low dimensional character of the structure lead to complex magnetic properties. When the oxygen content is increased to x = 1.1, the formal charge of cobalt first increases to Co3+ whereas its coordination changes from tetrahedral to square pyramidal, Fe3+ being unchanged. For an oxygen stoichiometry close to 7 (x = 0.1), both cations are tetravalent and sit in an octahedral environment. Physical properties are also discussed.