The Use of Curium as a Rare Earth Substitute in High T c -Related Compounds
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THE USE OF CURIUM AS A RARE EARTH SUBSTITUTE IN HIGH Tc-RELATED COMPORNDS L. SODERHOLM, C.W. WILLIAMS, AND U.WELP Chemistry and Materials Sciences Divisions, Argonne National Laboratory, Argonne, IL 60439, USA. Abstract We report the synthesis of Cm2CuO4. The lattice constants of this material, determined by x-ray diffraction, show it to be a new member of the isostructural series R2CuO 4 (R=Pr, Nd, Sm, Eu, and Gd). Analysis of magnetic measurements is consistent with a free-ion effective moment for Cm 3 +, with no contribution to the susceptibility from Cuions. Introduction The superconducting properties of YBa 2Cu 3 O7 are not greatly affected by replacing Y by most magnetic rare earth (R) ions. Cooperative behaviour in this isostructural series of superconducting compounds indicates little interaction between the magnetic f-electrons, and the conduction electrons. The exception is R=Pr, which is not a superconductor. Inelastic neutron scattering experiments [1], analysis of the magnetic behaviour of Pr12307 [2], molecular-orbitalcalculations on several f-elements embedded in the 12307 lattice [3], and a variety of other experiments [4-6], lead us to the conclusion that an interaction between Pr magnetic f-electrons and the electrons in the CuO planes inhibits superconductivity in this material. Interestingly, while Pr greatly influences the superconducting behaviour of Pr1230 7 , it has little effect on the cooperative properties of the recently reported electron superconductor Pr2-xCexCuO4 [7]. The series R2-xCexCuO4 (R=Pr,Nd,Sm, and Eu) all superconduct at similar temperatures. Therefore Pr does not exhibit the same influence on the cooperative properties within this isostructural series, even though it is structurally similar to the R1230 7 series. There are two features special to Pr among the rare-earths that could account for its unique behaviour within the R1230 7 series. The first feature is the ease with which Pr is oxidized to the tetravalent state, and therefore the energy of its localized states with respect to those of the Cu-O network. The second is, as a result of the lanthanide contraction, Pr is the rare-earth with the most extended, occupied, forbitals of all the R1230 7 compounds. Therefore, we expect these forbitals of Pr to most influence, and to be most influenced by, their environment. Mat. Res. Soc. Symp. Proc. Vol. 169. ©1990 Materials Research Society
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In an attempt to find other examples of an f-element which influences the superconducting properties of these high-Tc materials, we Our search previously proved looked to the actinide 5f-elements. successful with the synthesis of Cm12307 (curium is element 96). Cm12307 is isostructural with Y1230 7 , but is not a superconductor [8]. A Curie-Weiss plot of the inverse susceptibility vs temperature shows This an indication of an antiferromagnetic transition at 22 K [8]. compares with recent reports that the Pr moments order antiferromagnetically in Pr12307 at 17 K [9,10]. We now extend this work by synthesizing Cm2CuO4, a new member of
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