E0588

STRUCTURAL ANOMALY INDUCED IN YBa₂Cu₃O_x (6<x<7) BY H₂O ABSORPTION. S. Edo and T. Takama^*, Department of Mechanical System Technology, Hokkaido Polytechnic College, Zenibako 3-190, Otaru 047-02, Japan; *Department of Applied Physics, Faculty of Engineering, Hokkaido University, Kita-ku, Sapporo 060, Japan.

The present authors have found that heat-treatment in air between 373 K and 473 K transforms the structure of oxygen-reduced YBa₂Cu₃O_x (6<x<7) into an anomalous structure having an extraordinary long spacing (>12.5 A) along the c-axis (Takama and Edo, Physica C 235-240, 1994, 401-402). In the present paper, the cause of the transformation is investigated. Polycrystalline samples of YBa₂Cu₃O_6.95 with T_c of 91 K were annealed at 998 K for 6 hr in air and quenched into liquid nitrogen to obtain oxygen content x of 6.40. The samples were pulverized and annealed at 473 K in atmospheres of several kinds of gases (air, N₂, Ar, O₂ and the mixture of Ar and H₂O) at pressure of 1 atm for 30-230 hr. The X-ray diffraction profiles were measured by step scanning. Ac magnetic susceptibility was measured using the Hartshon bridge circuit. The X-ray profiles for the samples annealed in N₂, Ar and O₂ were analyzed by the Rietveld method assuming a space group of Pmmm. For annealings in N₂ and Ar, the lattice parameters slightly varied without a detectable weight change. For annealing in O₂, the lattice parameters were consistent with those for an as-quenched sample with x=6.71. The oxygen content x estimated from the weight gain increased from 6.40 to 6.73 through oxygen absorption. The sample annealed in Ar+H₂O, however, showed a drastic change in the profile as did that for the sample annealed in air. The profile change indicated an increase of the lattice parameter c by about 18 % compared with that for the as-quenched sample. The weight gain during the annealing was 3.1 %. Similar changes were also observed for the fully oxidized sample with x=6.95. The two samples with different concentrations turned to normal conductors after annealing in Ar+H₂O. These observations suggest that the anomalous structure is induced by absorption of H₂O molecules in the atmospheres regardless of the oxygen content. When the annealed samples were re-heated to temperature above 673 K, the structure reverted to the orthorhombic structure and superconductivity was recovered along with a reduction in weight. It seems that the recovery takes place with desorption of water.