Effect of Oxygen Pressure on the Phase Transformation in the Y-Ba-Cu-O and Related Superconducting Oxides

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Abstract
The orthorhombic-tetragonal YBa2Cu3O7-δ phase boundary in the δ-T diagram was determined by the detailed isobar TG measurement under the oxygen partial pressures of 1 to 0.01 atm. The phase boundary was found not to be located at 5 = const, but to have a slope of dδ/dT∼−4.7 × 10-4K-1. Using the same method, the transformation temperatures of a series of lanthanide substituted systems were measured at 1 atm. O2. The transformation temperature increased with increasing the atomic number of the lanthanide element. All these experimental results are well explained in terms of the order-disorder transformation caused by the repulsion energy between the nearest neighbor oxygen atoms on the Cu plane sandwiched by Ba planes.