In O-annealed La2CuO4+δ, where  was approximately equal to 0.03, bulk magnetization measurements revealed an increase of 4K in the superconducting temperature when the sample was cooled slowly through a narrow temperature range near to 195K. At this temperature, the 139La NQR spectra revealed the concomitant appearance of an anomalous feature, associated with the metallic phase, whose spectral weight increased with cooling rate. It was suggested that there was a distribution of local structures and an annealing-dependent volume fraction in this part of the sample. The results were interpreted in terms of O diffusion in a phase-separated material, and a model which was consistent with the observed changes in the superconducting temperature was developed. The activation energy for O diffusion (0.25eV) which was deduced from the model was of the same order of magnitude as the result which was obtained for La2-xSrxCuO4.

A.P.Reyes, P.C.Hammel, E.T.Ahrens, J.D.Thompson, P.C.Canfield, Z.Fisk, J.E.Schirber: Journal of the Physics and Chemistry of Solids, 1993, 54[10], 1393-402