Oxygen ordering in the system YBa_1.5Sr_0.5Cu₃O_x under pressure

Measurements of the magnetization are used to determine the dependence of the superconducting transition temperature on the oxygen content in the system YBa_1.5Sr_0.5Cu₃O_x. It is shown that the plateau at 60 K depending on T_c(x) for YBa₂Cu₃O_x is shifted into the temperature region around 40 K for YBa_1.5Sr_0.5Cu₃O_x. The influence of the hydrostatic pressures up to 1 GPa on the critical parameters of the superconducting state in the ceramic samples YBa_1.5Sr_0.5Cu₃O_x with varying oxygen content (x=6.56-6.95) are investigated. The influence of pressure on the superconducting properties depends strongly on the oxygen content. After applying and removing pressure in samples with an oxygen deficit, a relaxation of the superconducting parameters is observed, associated with oxygen ordering in the Cu-O chains. The oxygen ordering effect (dT_c/dP)₀ grows as x decreases. Near the metal-insulator transition at x=6.56 the oxygen ordering effect (dT_c/dP)₀≈20K·GPa^-1 predominates over the true pressure effect (dT_c/dP)_i≈2 K·GPa^-1. The characteristic relaxation times of T_c under pressure are determined. The nonmonotonic behavior of M_min with time upon the application and removal of pressure is due to motion of twin boundaries.