Thermodynamic studies on YBa 2Cu 3O 7−δ and Y 2BaCuO 5 by solution calorimetry

Abstract

We have measured calorimetrically the enthalpies of solution of superconducting YBa 2Cu 3O 7−δ ( δ = 0.09) which has a T c( off- set) of 90 K and semiconducting Y 2BaCuO 5 in IN HCl aqueous solution at 298 K. The standard enthalpies of formation of YBa 2Cu 3O 7−δ and Y 2BaCuO 5 have been calculated in combination with supplementary values from the literature as follows: Δ f H m o ( YBa 2 Cu 3 O 6.91) = − (3027 ± α) kJ mol −1 and Δ f H o m ( Y 2 BaCuO 5) = − (2716 ± α) kJ mol −1 (α is from the supplementary values and experimental error and is less than 5). An increase in the stability of YBa 2Cu 3O 7− δ is expected relative to the parent oxides (Y 2O 3, BaO and CuO) from 300 up to 1156 K by applying the entropy value to the calculation of the free energy change of formation for YBa 2Cu 3O 7−δ, which is the reason why the high- T c superconductor YBa 2Cu 3O 7−δ has a tendency to decompose above 1200 K.