Ultra-high 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 (t2g6 state) into the Co eg band
(final t2g4eg2 state). The t2g5eg1 state in the pyramids does
not change and the structural study also rules out a
d3x2−r2/d3y2−r2 orbital ordering at TMI.Comment: Phys. Rev. B (to appear