The Influences of Cool- and Hot-Pressure Treatment on the Phase Transition and Superconductivity of YBa2Cu3O7-

In this paper, the influence of cool- and hot-pressure treatment were studied on the phase transition region from or thorhombic to tetragonal and superconductivity in YBa2Cu3O7-. After the cool pressure treatment of 2.2~5.2 GPa, the samples remained the orthorhombic structure, but lost their superconductivities and kept up the semiconductivity in the region from 98 K to room temperature. If the samples are sintered in air, their zero resistance states will be regained. When the samples were sealed hermetically into a Cu bowl, there was no separation of Cu in the samples within the region of hot-pressure temperature from 250 ℃ up to 950 ℃. After the hot-pressure treatment (2.2 GPa, 250~950 ℃), the samples were in semiconductiviting state in the region of 98 K to room temperature. If the samples are followed by an after-treatment in oxygen, the zero resistance state can be observed (at about 86 K). In the process of hot-pressure treatment, it was found that the transition region of orthorhombic to tetragonal is in the 450~950 ℃, 950 ℃ is the end temperature of transition, which is higher than that of samples obtained by sintering in oxygen, air, nitrogen and vacuum. And about 400 ℃ is the onset temperature of transition, which is lower than that in nitrogen, air and oxygen. Therefore the phase transition region is widened, and the end temperature of T0-t raises. These are related to that the Cu bowl restrains the reduction role under high pressure. If the quenching rate under high pressure is slowed (102 ℃/s), or the samples are followed by an after-treatment in O2, the YBa2Cu3O7- both with high density and Tc can be obtained. The deficiency and hole of oxygen are also discussed here.