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The superexchange interaction between transition-metal ions via oxygen dianion was investigated by the ab initio molecular orbital (MO) method. It is found that the magnitude of the effective exchange integral (J ab) for the CuOCu unit is far larger than those of the NiONi, CrOCr and FeOFe units. Implications of this result are discussed in relation to the high T c superconductivity for Ba-La-Cu-O and R-Ba-Cu-O (R=Y, etc.).

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