Spin state of Co$^{3+}$ ions in layered GdBaCo$_2$O$_{5.5}$ cobaltite in the paramagnetic phase

A new scheme interpreting the changes in the spin state of Co$^{3+}$ ions in GdBaCo$_2$O$_{5.5}$ in the course of the metal-insulator transition is proposed. The transition occurs gradually within a wide ($\sim100$ K) temperature range. The changes in the spin state of Co$^{3+}$ ions are revealed using the data on the linear thermal expansion. In the metallic state, less than one-half of Co$^{3+}$ ions are in the high-spin (HS, $S=2$) state in octahedra, whereas the remaining ions are in the low-spin (LS, $S=0$) state. The transition to the nonmetallic state occurs owing to the transformation of the HS state to the LS state in octahedra and to the transformation of some part of LS Co$^{3+}$ in pyramids to the intermediate-spin (IS, $S=1$) state.