Spin State of the Co3+ Ions in the Layered TbBaCo2O5.5 Cobaltite in the Metal–Insulator Transition Range

A scheme is proposed to describe the spin state of the Co3+ ions in the layered TbBaCo2O5.5 cobaltite near the metal–insulator transition. The spin state of the Co3+ ions in the metallic phase corresponds to a mixture of the HS( $$t_{{2g}}^{4}$$ $$e_{g}^{2}$$ , S = 2) and LS( $$t_{{2g}}^{6}$$ $$e_{g}^{0}$$ , S = 0) states taken at approximately the same proportion. The transition into a nonmetallic state occurs due to the transformation of the HS state into the LS state in octahedra and part of the LS state into the IS( $$t_{{2g}}^{5}$$ $$e_{g}^{1}$$ , S = 1) state in pyramids (near TC ~ 280 K). The proposed scheme agrees with the well-known structural data obtained for the TbBaCo2O5.5 cobaltites. As follows from volumetric and linear expansion, the transition takes place over a wide temperature range T ≈ TMI ± 50 K. The study of thermal expansion shows that an LS/IS state is retained down to T = 80 K.