Preparation of a YBa_2Cu_4O_8 high-temperature superconductor from a spray-dried nitrate precursor

摘要

The high-temperature superconductor YBa_2Cu_4O_8 (1-2-4) with Tc ～ 80 K was detected for the first time by Zandbergen et al. [1] as a lattice defect in partly decomposed powder of YBa2Cu3O7-v (1-2-3). More recently [2, 3] it has been prepared as a distinct phase in thin films. Using a thin film containing 85% of this phase, Marsh et al. [4] have determined its stoichiometry and crystal structure. It has been found that the 1-2-4 structure differs from 1-2-3 by replacement of a single linear Cu-0 chain parallel to the b-axis by a double Cu-O chain with edge-sharing, square planar oxygen co-ordination. The bond between the oxygen and the copper in the double chains is stronger, and so Yl^C^Og does not change its oxygen content up to 850 ℃ [5]. For that reason, the usually applied annealing in oxygen at about 500 °C in order to optimize the superconducting properties of YBa2Cii3O7_v becomes superflous for YBa2Cu4Og. Since, in contrast to the case of the 1-2-3 phase, the 1-2-4 phase undergoes no tetragonal-orthorhombic transition; neither a twinning effect nor essential oxygen inhomogeneities are observed, making this phase promising for applications.