Positron annihilation at defects in sintered high-T_c perovskite superconductors

We report positron lifetime measurements in sintered superconducting YBa₂Cu₃O_7-x and GdBa₂Cu₃O_7-x oxides. It is shown that the thermal behaviour of the positron lifetime spectra strongly depends on the preparation of the ceramics. A lifetime of 190±3 ps is attributed to oxygen deficient regions. Two lifetimes of 251±7 ps and 225 ±5 ps are attributed to a cation vacancy presenting a temperature dependent atomic arrangement. The lifetime transition (251→225ps) occurs during decreases in temperature across the resistivity superconducting transition. This lifetime change indicates that the volume of the cation vacancy decreases in the superconducting state.