Structural and Magnetic Chemistry of NdBaCo2O5+δ

The crystallographic and magnetic structures of the oxygen-deficient perovskites NdBaCo₂O_5+δ (δ=0, 0.38, 0.5, 0.69) have been studied as a function of temperature by neutron powder diffraction. Long-range G-type antiferromagnetic order is realized for all samples apart from that with δ=0.5. The lack of magnetic order for δ=0.5 can be understood on the basis of a crystal-field-induced spin-state ordering between low-spin and high-spin Co³⁺. Contrary to studies of similar materials with smaller lanthanides, the δ=0 material exhibits no evidence of long-range charge ordering. No evidence of a spin-state transition as observed in YBaCo₂O₅ is found for any of our samples.