Thermochemical investigation of YBa2Cu3O7−δ superconductor doped by lutetium

The synthesis of Y0.72Lu0.28Ba2Cu3O6.91 has been performed by solid-state reaction from Y2O3, Lu2O3, BaCO3 and CuO. The X-ray measurements confirmed the orthorhombic structure (space group is Pmmm). For the first time the standard molar enthalpy of formation of Y0.72Lu0.28Ba2Cu3O6.91 has been determined by solution calorimetry combining the enthalpies of dissolution of Y0.72Lu0.28Ba2Cu3O6.91 in 2 M HCl and literature data. It is found out that at room temperature this complex oxide is thermodynamically stable with respect to a decomposition into binary oxides or a mixture of individual phases (YBa2Cu3O6.91 and LuBa2Cu3O6.91). It could be established that substitution of Lu increases the stability of the Y123 phase.