The Crystal Structure and Ordering of Nb5+ Doped High Tc Superconductors Ba3−yYyCu3−xNbxO9−z

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Abstract
The crystal structure and ordering of Nb 5+ doped high Tc compounds has been identified by XRD and TEM. As examples, we have studied Ba2Y1Cu2.6Nb0.4O9−z and Ba2Y1Cu2.5Nb0.5O9−z , which consist of -two main phases, respectively. A face-centered cubic superlattice perovskite structure with a=8.4513 (Å) is common phase to both samples. In this phase, Y, Nb, and Cu are on B-sites with ordered Y and Nb in direction. However, the second phase is different in these two samples. One contains a well-known ordered Ba-Y-Ba structure on A-sites as an orthorhombic perovskite superstructure, but another sample contains disordered Ba and Y on A-sites as a tetragonal perovskite structure with a=b=3.8667 (Å) and c=3.9429 (Å). Both samples are high Tc superconductors with Tc=90K .

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