Effect of Ag doping on structure andcritical temperature of Bi2Sr2CaCu2O8+δ superconductors

The effects of Ag substitution for Bi, Sr and Cu on the c-axis lattice parameter and critical temperature (T_c) were determined for the Bi_2−xSr_2−yCaCu_2−zO_8+δ (BSCCO-2212) superconducting phase. Two distinct regimes of behavior were observed: fast cooling represented the regime where the uptake of excess oxygen was the controlling factor and slow cooling represented the regime where the cation content was the controlling factor on the structure and properties. Samples showed higher T_c values and longer c-axes with increased cooling rate. A linear increase in T_c with increase in c-axis length was observed for the faster-cooled samples. For the slow-cooled samples, increased c-axis length was observed when Ag was substituted for Bi and Sr, whereas substituting Ag for Cu caused no change in c-axis length. In addition, any deviation from the ideal 2212 stoichiometry was shown to reduce the T_c values of samples that were slowly cooled and fully oxygenated. Preliminary results obtained by high-resolution transmission electron microscopy coupled with electron energy loss spectroscopy indicated that relatively small amounts of Ag (≈1 at.%) can occupy Bi sites and a larger amount (≈10 at.%) of Ag can substitute for Cu.