STRUCTURAL-PROPERTIES OF THE NONSUPERCONDUCTOR (HO, CE)2(SR, BA, LA)2CU3O8+Z

The crystal structure of nonsuperconducting (Ho2/3Ce1/3)2(Sr1/3Ba1/3La1/3)2Cu3O8+z (z almost-equal-to 1.10) was analyzed by the Rietveld refinement of time-of-flight neutron powder diffraction data. This oxide is isomorphous with superconducting (Eu, Ce)2 (Ba, Eu)2Cu3O8+z with 14/mmm space group. Its tetragonal unit cell contains two fluorite blocks, i.e., [Ho, Ce)O2] and two "123" blocks, i.e., [(Ho, Ce) (Sr, Ba, La)2Cu3O6+z]. Lattice constants were refined to be a = 0.383046(6) nm and c = 2.82944(6) nm. Smaller Ho(Ce) and larger Sr(Ba, La) ions are ordered in two different sites in the fluorite and "123" blocks, respectively. A Cu atom between two (Sr, Ba, La)O planes and two oxygen atoms bonded to it are displaced from ideal positions. Oxygen nonstoichiometry in this oxide results from the partial occupation of oxygen sites on z = 0 and 1/2 planes. The structural parameters of the present compounds are compared with those of superconducting (Nd, Ce)2(Ba, Nd)2Cu3O8+z to clarify the reason for its nonsuperconductivity.