Metal-insulator transition induced by a spin-state transition in TbBaCo2O5+δ (δ=0.5)

Temperature variation of the crystal structure has been investigated for TbBaCo2O5+delta (delta=0.5), which shows a metal-insulator (MI) transition at T(MI)approximate to 340 K. The compound has an oxygen-deficient perovskite structure, with alternating CoO6 octahedral and CoO5 pyramidal chains along the crystallographic b axis. We have found that both the pyramids and octahedra are significantly distorted in the insulating phase below T-MI, suggesting a d(3x2-r2)/d(3y2-r2)-type orbital ordering.