STRUCTURAL AND MAGNETIC STUDIES OF ORDERED OXYGEN-DEFICIENT PEROVSKITES LnBaCo2O5+delta, CLOSELY RELATED TO THE '112' STRUCTURE

摘要

The system LnBaCo2O5+delta, related to the "112" type structure (apxapx2ap), has been studied for Ln=Pr, Nd, Sm, Eu, Gd, Tb, Dy, and Ho, with various O stoichiometries. According to the size of the lanthanide and to the O stoichiometry, two kinds of supercells, apx2apx2ap and 3apx3apx2ap, arising from different orderings between O and vacancies in the [LnOdelta] layer, have been evidenced by electron diffraction and HREM techniques. The influence of the O 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 O content. They are related to strong competitions between ferromagnetism and antiferromagnetism: the chi'(T) curves exhibit a cusp at Tcusp which coincides approximately with TMR, the appearance of the MR. 13 refs.