Thermodynamic Properties of Nd 1+ x Ba 2– x ­Cu 3 O y Compounds and Their Application for Optimizing the Synthesis of Superconducting Materials

The standard enthalpies of formation from oxides of Nd 1+ x Ba 2– x Cu 3 O y , 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 NdBa 2 Cu 3 O 6.87 wasmeasured over the range 5–320 K by adiabatic calorimetry. The absolute entropy, S° ( T ), the enthalpy of heating, H° ( T K) – H° (0 K), and the reduced Gibbs energy, Φ ° ( T ) = S° ( T ) – [ H° ( T ) – H° ( 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 NdBa 2 Cu 3 O 6.87 over the temperature range 320–1380 K were estimated by methods of quantum chemistry with use of the temperature dependence of C p at low temperatures. A thermodynamic simulation was performed on the basis of the obtained data, and the conditions for synthesis of the NdBa 2 Cu 3 O 6.9 phase were determined.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)