Effect of La3+ substitution for Sr2+ on the phase formation, structure andproperties of (Tl0.5Pb0.5)Sr2Ca2Cu3Oy compounds

Optimization of the hole concentration in (Tl0.5Pb0.5)Sr2Ca2Cu3Oy (Tl-1223) was attempted though partial replacement of Sr2+ by La (3+). Nine sample w ith nominal composition (Tl0.5Pb0.5)Sr2-xLaxCa2Cu3Oy were prepared, with x varies from 0 to 2. Magnetization measurements as a function of temperature showed that the transition temperature increased with increasing La conten t. These measurements also showed an increase of the diamagnetic signal as La was increased. X-ray diffraction patterns of the samples showed that La addition improved the phase purity of Tl-1223 for low level La substitution (x less than or equal to 0.03). Formation of impurity phases and a Phase t ransformation were also observed for high La contents. The microstructure a ppearance of the samples with different La content was different. Samples w ith x values 0.5, 1 and 2, were very brittle and the pellets dissociated in to fine powder in one week time. These results will be discussed in terms o f changes in the charge carrier concentration in the Cu-O planes of the ori ginally overdoped Tl-1223 high temperature superconductors.