Structural instability study of a Bi-based superconductor by the positron lifetime spectrum technique

The variations of positron lifetimes with time at room temperature were measured for the Bi-based 2212 phase superconductor. It is found that lifetimes τ1 and τ2, intensity I1, mean lifetime τm and bulk lifetime τb, all keep on decreasing for 130 days, and then remain roughly unchanged. This result confirms that the structure of oxide superconductors is instable even at room temperature. The decrease of the annihilation parameters with time is explained in terms of the intercalation of the excess oxygen in the region between the Bi-O layers. Possible correlations with the electron structure, oxygen content, hole density and transition temperature are also discussed