Thermal analysis and thermal expansion studies on high density $YBa_{2−x}La_xCu_3O_{7- \delta}, 0 \leq x \leq 0.2$

The compositions in the $YBa_{2-x}La_xCu_3O_{7- \delta}$ $(0 \leq x \leq 0.2)$ system were prepared by the solid state reaction, employing a novel hightemperature oxygen sintering route. The modified sintering route yields dense slab like microstructures with large grains. The decomposition (incongruent melting) temperature of the $YBa_2Cu_3O_{7-\delta}$ (Y-123) phase was found to shift to higher temperatures with increasing oxygen partial pressure and lanthanum content. Structure remained orthorhombic up to x = 0.2 with a decrease in the orthorhombic strain ((ba)/b). Iodometric titration indicated a systematic increase in the oxygen content with increasing lanthanum content. Thermo-gravimetric studies in various oxygen partial pressures revealed that the oxygen diffusion in to the $YBa_2Cu_3O_{7-\delta}$ $(\delta > 0.5)$ lattice is an exothermic event and takes place at temperatures not less than 573 K. High-temperature thermal-expansion measurements in air indicated that the nonlinearity in thermal expansion behaviour was reduced by the substitution of lanthanum