Orbital order-disorder transition with volume collapse in HoBaCo2O5.5:: A high-resolution neutron diffraction study

The crystal structure of the layered cobaltite HoBaCo2O5.5 (substituted with both O-16 and O-18) was studied across an insulator to metal (MI) transition at T-MI=305 K employing high-resolution neutron diffraction. We have found that the transition at T-MI is of the first-order type, accompanied by an abrupt negative change of the unit cell volume (similar to 0.15%) and melting of the orbital order in the pyramids. The existence of an isotope effect on T-MI suggests that the structure changes are caused by the electron delocalization above the transition.