UNSTABLE BEHAVIOR OF THE YBA2CU4O8 SUPERCONDUCTOR UNDER PRESSURE

The effect of applied pressure on the YBa2Cu4O8 (Y 1:2:4) superconductor with a T(c) of 78 K up to 50 kbar at ambient temperature is studied. It is observed that Y 1:2:4 undergoes a phase transformation resulting in the YBa2Cu3O7 (Y 1:2:3) orthorhombic phase with an increased T(c) of 92 K. However, beyond 20 kbar, the Y 1:2:3 orthorhombic phase is further converted into the Y 1:2:3 tetragonal phase. The results show that when the pressurized specimens were heat treated again in flowing oxygen at 815-degrees-C, the Y 1:2:4 phase with increased superconducting volume fraction is present, as evidenced by ac susceptibility measurements. These results are discussed in terms of solid-state reactions and the structural changes taking place at high pressure.