Structural and magnetic studies of ordered oxygen-deficient perovskites LnBaCo2O5+δ, closely related to the "112" structure

The system LnBaCo(2)O(5+delta), related to the "112" type structure (a(p) x a(p) x 2a(p)), has been studied for Ln = Pr, Nd, Sm, Eu, Gd, Tb, Dy, and Ho, with various oxygen stoichiometries, According to the size of the lanthanide and to the oxygen stoichiometry, two kinds of supercells, a(p) x 2a(p) x 2a(p) and 3a(p) x 3a(p) x 2a(p), arising from different orderings between oxygen and vacancies in the [LnO(delta)] layer, have been evidenced by electron diffraction and HREM techniques. The influence of the oxygen content on the metal-insulator transition and the corresponding magnetic susceptibility anomaly is studied. Magnetoresistant (MR) effects are observed for all of the as-synthesized samples and are very sensitive to the oxygen content. They are related to strong competitions between ferromagnetism and antiferromagnetism: the chi' (T) curves exhibit a cusp at T-cusp which coincides approximately with T-MR, the appearance of the MR. (C) 1999 Academic Press.