Preparation and properties of superconducting ${\mathrm{La}}_{1+\mathrm{x}}$${\mathrm{Ba}}_{2\mathrm{-}\mathrm{x}}$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathrm{y}}$ (0≤x≤0.5) ceramics sintered in ${\mathrm{N}}_2$ gas atmosphere

A study of the preparation and properties of ${\mathrm{La}}_{1+\mathrm{x}}$ ${\mathrm{Ba}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{\mathrm{y}}$ (0≤x≤0.5) system was performed on two kinds of samples. One with 0≤x≤0.2 was sintered at 970–980 °C in ${\mathrm{N}}_2$ gas and postannealed at 300 °C in ${\mathrm{O}}_2$ gas atmosphere and the other with 0≤x≤0.5 was sintered at 900 °C in ${\mathrm{N}}_2$ gas and postannealed under the same conditions. The samples sintered at 970–980 °C were orthorhombic and their oxygen content(y) was nearly 6.95, smaller than that of those sintered at 900 °C, which was about 7.15. Thermogravimetric study revealed that samples sintered at 900 °C contained a large amount of oxygen; the oxygen content was not substantially reduced at elevated temperature, even in ${\mathrm{N}}_2$ gas atmosphere. Resistivity and dc magnetic-susceptibility-measurements revealed that the superconducting properties of the samples sintered at 970–980 °C were much better than those of the samples sintered at lower temperature. The $T_c$’s in the ${\mathrm{La}}_{1+\mathrm{x}}$ ${\mathrm{Ba}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{\mathrm{y}}$ system with y≊7.0 decreased linearly from 93 to 80 K with excess La content, x. Differential scanning calorimetry in ${\mathrm{O}}_2$ gas atmosphere showed that the orthorhombic-tetragonal structural phase transition temperatures were lowered monotonically from 485 °C (x=0) to 443 °C (x=0.3). We address the effect of La substitution for Ba in ${\mathrm{LaBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7.0}$ samples on their superconducting properties. We discuss the relation between the oxygen content in ${\mathrm{La}}_{1+\mathrm{x}}$ ${\mathrm{Ba}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{\mathrm{y}}$ samples and their superconducting properties. Finally, we discuss the reasons why the superconducting properties of the samples which were sintered in ${\mathrm{N}}_2$ gas are better than those of the samples sintered in ${\mathrm{O}}_2$ gas or air.