Abstract
Three new oxides Sm 2SrCo 2O 7, Sm 2BaCo 2O 7 and Gd 2SrCo 2O 7 have been successfully synthesized by a solid state reaction method. The X — Ray diffraction spectra show that they are all isostructural with Sr 3Ti 2O 7. Ln 2SrCo 2O 7 (Ln=Sm, Gd) crystallized in the tetragonal system, Sm 2BaCo 2O 7 in the orthorhombic system. The Co-O bonds in CoO 2 planes of Ln 2SrCo 2O 7 are shorter than those of LnSrCoO 4 (Ln=Sm, Gd), thus their d electrons are more delocalized and their electrical resistivities are smaller. The electrical resistivities versus temperature in the range 300− 1100K showed that the five oxides represent weakly localized systems. In the lower temperature range, the magnetic behavior of Gd 2SrCo 2O 7 and GdSrCoO 4 fits the Curie-Weiss law well. The magnetic exchange reaction in the CoO sublattice of Gd 2 SrCo 2O 7 is ferromagnetic, but in GdSrCoO 4 is antiferromagnetic. The other three oxides with Sm +3 showed complex magnetic behavior which is perhaps related to the complexity of Sm +3.
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