Thermodynamic Properties of Nd1+xBa2-xCu3Oy Compounds and Their Application for Optimizing the Synthesis of Superconducting Materials

The standard enthalpies of formation from oxides of Nd1+xBa2-xCu3Oy, where x = 0-0.8 and y = 6.65-7.24, were determined by using dissolution enthalpies of these compounds measured by solution calorimetry. The temperature dependence of the heat capacity of NdBa2Cu3O6.87 was measured over the range 5-320 K by adiabatic calorimetry. The absolute entropy, S degrees(T), the enthalpy of heating, H degrees(T K) - H degrees(0 K), and the reduced Gibbs energy, Phi degrees(T) = S degrees(T) - [H degrees(T) - H degrees(0)]/T, were calculated by using the temperature dependence of C-p in the interval 0-320 K. The heat capacity and absolute entropy of NdBa2Cu3O6.87 over the temperature range 320-1380 K were estimated by methods of quantum chemistry with use of the temperature dependence of C. at low temperatures. A thermodynamic simulation was performed on the basis of the obtained data, and the conditions for synthesis of the NdBa2Cu3O6.9 phase were determined. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)