Orbital order-disorder transition with volume collapse in HoBa Co2 O5.5: A high-resolution neutron diffraction study

Abstract

The crystal structure of the layered cobaltite HoBa Co2 O5.5 (substituted with both O16 and O18) was studied across an insulator to metal (MI) transition at TMI =305 K employing high-resolution neutron diffraction. We have found that the transition at TMI is of the first-order type, accompanied by an abrupt negative change of the unit cell volume (∼0.15%) and melting of the orbital order in the pyramids. The existence of an isotope effect on TMI suggests that the structure changes are caused by the electron delocalization above the transition. © 2006 The American Physical Society.