Preparation and superconducting properties of tetragonal ba₂ycu₃o_7-yand ba₂eucu₃o_7-ywith low oxygen-defect concentration (0.05<y<0.20)

Tetragonal phases of Ba₂YCu₃O_7-yand Ba₂EuCu₃O_7-ywith low oxygen-defect concentration (0.05<y<0.20) are prepared by synthesis under high oxygen pressure (3–8 atms). As oxygen pressure increases during sintering, the crystal structure of synthesized samples changes from the orthorhombic to tetragonal phase and superconductivity is suppressed. substitution of Y and Eu for Ba is observed to accompany the change to the tetragonal phase. Disordering of oxygen vacancies and such substitution may be vital for the absence of superconductivity because no strong correlation between T_cand oxygen-defect concentration is observed.