STERIC EFFECTS ON SUPERCONDUCTIVITY IN (ND1-YYY)(2-X)CEXCUO4-DELTA

We present here the results of SQUID magnetization and powder X-ray di ffraction measurements for polycrystalline samples of (Nd1-yYy)(2-x)Ce xCuO4-delta for various x and y. The lattice constants and superconduc ting fractions were determined for each sample. The lattice contracts as more ytttium is substituted, and superconductivity is reduced. The trend is compared with the spacing of Gd2-xCexCuO4-delta to separate s teric from magnetic causes for the lack of a superconducting phase in Gd2-xCexCuO4-delta. The lack of superconductivity in (Nd0.7Y0.3)(2-x)C exCuO4-delta, which is almost the same size as Gd2-xCexCuO4-delta, ind icates that a small lattice spacing is the primary reason for the fail ure of Gd2-xCexCuO4-delta to be a superconductor. This contradicts a r ecent theoretical proposal which emphasized magnetic pair breaking.