Structural and Magnetic Studies of Ordered Oxygen-Deficient PerovskitesLnBaCo2O5+delta, Closely Related to the "112" Structure

The systemLnBaCo2O5+delta, related to the "112" type structure (ap×ap×2ap), has been studied forLn=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,ap×2ap×2apand 3ap×3ap×2ap, arising from different orderings between oxygen and vacancies in the [LnOdelta] 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: thechiʹ(T) curves exhibit a cusp atTcuspwhich coincides approximately withTMR, the appearance of the MR.