The Y–Ba–Cu–O–Cl system: thermodynamic characteristics of 1:2:3 phases, thermodynamic stability

In this study, the solution enthalpy for the YBa2Cu3O6.5Cl0.4 phase has been measured by solution calorimetry in 6 N HCl at 323 K. On the basis of experimental and literature data the following values were obtained: the formation enthalpy of YBa2Cu3O6.5Cl0.4 from the mixture of 0.433 Y2O3 + 0.133 YCl3 + 3 CuO + 2 BaCO3 (ΔrH0 (323.15 K) = + 309.20 ± 7.79 kJ mol−1); the standard formation enthalpy of YBa2Cu3O6.5Cl0.4 (ΔrH0 (323.15 K) = − 2756.6 ± 11.5 kJ mol−1); the formation enthalpy of YBa2Cu3O6.5Cl0.4 from the mixture of 0.433 Y2O3 + 0.133 YCl3 + 2 BaCuO2 + CuO (ΔrH0 (323.15 K) = − 92.98 ± 8.24 kJ mol−1). The calculated values permitted us to conclude that YBa2Cu3O6.5Cl0.4 reacted with CO2 giving the mixture of Y2O3, YCl3, CuO, BaCO3. Comparing two values of reaction enthalpies of 1:2:3 phases (with chlorine or without chlorine) showed that the formation of the Y123 phase from the 0.5 Y2O3 + CuO + 2 BaCuO2 mixture was thermodynamically unfavourable, but the formation of the Y123 phase with chlorine from the 0.433 Y2O3 + 0.133 YCl3 + 2 BaCuO2 mixture was thermodynamically favourable. The reason of the stability increase has been discussed.