Selective spin-state and metal-insulator transitions in GdBaCo2O5.5

By means of ultra-high resolution synchrotron diffraction and calorimetry measurements we have studied the metal-insulator transition in GdBaCo2O5.5. The appearance of the metallic state is attributed to a sudden excitation of some electrons in the octahedra (t2g6 state) into the Co eg band (final t2g4eg2 state). In contrast, the t2g5eg1 state in the pyramids does not change at the transition. Calorimetry measurements show that the insulator-to-metal transition is first order and the entropy change estimated from the latent heat corroborates the pictured scheme