Equilibrium of 1:2:3 CLBLCO superconductors with oxygen: reaction equation, thermodynamics and improvement of homogeneity Original Research Article

Equilibrium of 1:2:3 superconductors (CaxLa1−x)(Ba1.75−xLa0.25+x)Cu3Oy (this compound has in the past variously denoted as CLBLCO, CLBCO or CaLaBaCuO) with oxygen was studied for x=0.1 and x=0.4 in the temperature range of 150–950 °C under 1 atm. O2. The main process is the reversible reaction −Cu32+O6.625+0.25O2=−Cu22+Cu3+O7.125 which is completed with the formation of one Cu3+. The enthalpy (in kJ/mol CLBLCO) and entropy (in J(mol CLBLCO)−1K−1) of this reaction were calculated from the temperature dependence of the equilibrium constant. The values are ΔH=−33.1 and ΔS=−29.9 for x=0.1 and ΔH=−49.4 and ΔS=−42.7 for x=0.4. It was found that the equilibrium of ceramic pellet of CLBLCO with oxygen cannot be practically achieved below 300 °C while the equilibrium for powder is achieved even at 200 °C. Low rate of reaction of CLBLCO with oxygen causes the problem in low temperature equilibration. In contrast, diffusion of oxygen ions in the ceramics is observed even at 200 °C. This diffusion proceeds without the change of the oxygen content and may be applied in order to improve the homogeneity of the distribution of oxygen ions.