Tetragonal phases of Ba2YCu3O7-yand Ba2EuCu3O7-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 Tcand oxygen-defect concentration is observed.
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