Abstract

A scheme is proposed to describe the spin state of the Co³⁺ ions in the layered TbBaCo₂O_5.5 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 T_C ~ 280 K). The proposed scheme agrees with the well-known structural data obtained for the TbBaCo₂O_5.5 cobaltites. As follows from volumetric and linear expansion, the transition takes place over a wide temperature range T ≈ T_MI ± 50 K. The study of thermal expansion shows that an LS/IS state is retained down to T = 80 K.

中文翻译：

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金属-绝缘体转变范围内层状TbBaCo 2 O 5.5钴矿中Co 3+离子的自旋态

摘要

提出了一种描述金属-绝缘体转变附近层状TbBaCo ₂ O _5.5钴中Co ³⁺离子自旋态的方案。金属相中Co ³⁺离子的自旋态对应于HS（\（t _ {{2g}} ^ {4} \）\（e_ {g} ^ {2} \）的混合物，S = 2）和LS（\（t _ {{2g}} ^ {6} \）\（e_ {g} ^ {0} \），S = 0）的状态大致相同。过渡到非金属状态是由于八面体中HS状态转换为LS状态，部分LS状态转换为IS（\（t _ {{2g}} ^ {5} \）\（e_ {g } ^ {1} \），S= 1）的状态在金字塔（近Ť _ç〜280 K）。拟议的方案与TbBaCo ₂ O _5.5钴获得的众所周知的结构数据一致。如从体积和线性膨胀如下，该过渡发生在很宽的温度范围内Ť ≈ Ť _MI ±50 K的热膨胀表明，一个LS / IS状态被保持至该研究Ť = 80 K.