New A-site Ordered Perovskite Cobaltite LaBaCo_2O_6 : Synthesis, Structure, Physical Property and Cation Order-Disorder Effect(Condensed Matter : Electronic Structure, Electrical, Magnetic and Optical Properties)

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By means of annealing LaBaCo_2O_5 under high pressure oxygen, we have successfully synthesized a new A-site ordered perovskite cobaltite, LaBaCo_2O_6. The structure and electromagnetic properties of LaBaCo_2O_6 have been investigated and compared to those of the ordinary A-site disordered La_<0.5>Ba_<0.5>-CoO_3. Both LaBaCo_2O_6 and La_<0.5>Ba_<0.5>CoO_3 have mixed states of low-spin and intermediate-spin states and show metallic behaviors generated by double exchange interaction. LaBaCo_2O_6 has a ferromagnetic transition at T_C=175 K, which is lower by 15 K than T_C of La_<0.5>Ba_<0.5>CoO_3. At around 140 K, some structural changes are observed in both compounds and below the temperature they exhibit magnetic glassy (cluster glass) behaviors, accompanied by the gradual change to insulating or simiconductive behaviors. The refined structural data and magnetic and electrical properties suggest a partial d_<x^2-y^2> orbital ordering for LaBaCo_2O_6 below 140 K, in contrast to a partial d_<3z^2-r^2> orbital ordering for La_<0.5>Ba_<0.5>CoO_3.
社団法人日本物理学会の論文
2005-05-15