Effect of oxygen pressure on the phase transformation in the Y-Ba-Cu-O and related superconducting oxides

The orthorhombic-tetragonal YBa2Cu3O(7-delta) phase boundary in the delta-T diagram was determined by the detailed isobar thermogravimetry measurement under the oxygen partial pressures of 1 to 0.01 atm. 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 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.