Selective spin-state switch and metal-insulator transition in GdBaCo2O5.5 -: art. no. 180405

Ultrahigh resolution synchrotron diffraction data for GdBaCo2O5.5 throw new light on the metal-insulator transition of Co3+ Ba-cobaltites. An anomalous expansion of CoO6 octahedra is observed at the phase transition on heating, while CoO5 pyramids show the normal shrinking at the closing of the gap. The insulator-to-metal transition is attributed to a sudden excitation of some electrons in the octahedra (t(2g)(6) state) into the Co e(g) band (final t(2g)(4)e(g)(2) state). The t(2g)(5)e(g)(1) state in the pyramids does not change and the structural study also rules out a d(3x)(2)-r(2)/d(3y)(2)-r(2) orbital ordering at T-MI.