HIGH-T_c SUPERCONDUCTING PROPERTIES OF YBa₂Cu_3-xSn_x O_7-y SYSTEM

The effect of Sn concentration on the superconducting properties and structure of YBa₂Cu_3-xSn_x O_7-y system were systematically investigated. The zero resistance temperature was found to be above 90K as z ranges from 0 to 1.1. For x > 1.3, the system exhibited no superconducting properties at liquid nitrogen temperature. The results of X-ray diffraction showed a regular pattern of mixed structures of 123 phase and a new phase in the system with the increase of Sn concentration. TEM analysis suggested that the new phase might be BaSnO₃ with deformed perovskite structure and about 6% of Cu were substituted by Sn in the 123 phase at x = 1.0. It is therefore likely that the new phase modulates the structure of 123 phase and stabilize the 123 phase. Although the Sn atoms enter 123 phase and occupy the Cu sites in the Cu-O chain and break the chain, the system remains with T_c > 90K. Consequently, the Cu-O chain plays a less important role in the high-T_c superconductivity.