STUDY OF BI2SR2CACU2O8-CRYSTALS ANNEALED IN VACUUM(DELTA SINGLE)

High-quality Bi-2212 single crystals with a transition temperature of 91 K were annealed in vacuum at 550 degrees C for 10, 20, 40 and 72 h. AC susceptibility measurements showed that T-c and Delta T-c increase d with increasing annealing time t(a) in the case of short-time anneal ing (t(2) less than or equal to 40 h). However as the annealing time w as further increased to 72 h, the samples became non-superconducting. The results of X-ray diffraction showed that the lattice parameter of the crystal in c-axis increased with the annealing time, indicating th e decrease of the oxygen content. Raman spectra revealed that the inte nsities of the peaks at 290 and 650 cm(-1) corresponding to the O-Bi v ibration modes were gradually enhanced with the increase of annealing time as well as with the decrease of oxygen content, which supported t he model that the excess oxygen atoms were located in the bismuth laye r interstitially.