Synthesis and x-ray characterization of superconducting and related Ln_2-xCe_xCuO_4-y (Ln=Nd and Gd) compounds

In view of the recent attention focused on electron superconductivity, we have prepared and analyzed the crystalline structures of Ce substituted Ln2-xCexCuO4-y (Ln=Nd and Gd) compounds. X-ray powder diffraction analysis shows that the Ce ions have been succesfully doped into the Nd sites of the tetragonal Nd2CuO4 lattice and the maximum Ce solubility limit is xc = 0.20. Linear decreases in both the c lattice dimension and the unit cell volume, V, with increasing x up to the solubility limit indicate the smaller Ce4+ ions, not the larger Ce3+ ions are present. The tetragonal unit cell dimensions of the 21 K superconducting Nd1.85Ce0.15CuO4-y compound refined from a high quality powder diffraction pattern with a figure of merit F30=267.3 (0.003, 35) are: a = 3.9463 (1) A ̊, c = 12.0809 (2) A ̊ and V = 188.14 (1) A ̊. X-ray results also show that the maximum Ce solubility limit in Gd2CuO4 is significantly lower, with xc=0.15. Linear decreases in c and V with increasing x in Gd2-xCex CuO4-y have also been obtained indicating the presence of Ce4+ ions. The unit cell dimensions of the semiconducting Gd1.85Ce0.15CuO4-y compound are a = 3.9023 (1) A ̊, c = 11.8310 (2) A ̊ and V = 180.16 (1) A ̊3. © 1989.