Characterization of Electron Superconducting and Related Nd_<2-x>M_xCuO_<4+y> (M=Ce and Th) Compounds

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Ce- and Th-doped Nd_2CuO_<4+y> superconducting and related compounds were systematically synthesized and characterized. An X-ray diffraction analysis of the variation of lattice dimensions of the Nd_<2-x>M_xCuO_<4+y> compounds revealed that Ce had an average ionic radius of 0.998 Å and an effective valence of +3.84, significantly different from those of the formal Ce^<3+> and Ce^<4+> ions. An iodometric titration analysis detected excess oxygen above the nominal value of 4 in both the Ce- and Th-doped materials. The analysis also showed that the average [Cu-O]^p charge reversed from positive to negative as x increased, and reached a negative maximum x=0.15 where superconductivity appeared. Using effective valences of +3.84 for Ce and +4 for Th ions, the values of p were found to be -0.067 and -0.048 for the 21 K Nd_<1.85<Ce_<0.15>CuO_<4.030> and the 15 K Nd_<1.85>Th_<0.15>CuO_<4.048> electron superconductors, respectively.
社団法人応用物理学会の論文
1989-10-20