Thermodynamic modeling of superconducting phases in the yttrium–barium–copper–oxygen system

The following main results are presented:
• a critical assessment of the available thermodynamic data from different experiments for two solid solutions, YBa2Cu3O6+z (0 ⩽ z ⩽ 1) and Y2Ba4Cu7O14+w (0 ⩽ w ⩽ 1), and for one stoichiometric compound YBa2Cu4O8;
• an analytical representation of the Gibbs energy functions for these phases;
• a computer-assisted optimization analysis to obtain model parameters which best fit experimental data and are consistent with thermodynamic relations.The conditions for the existence of the other ordered superconducting phase, which can be formed as a stable superstructure of the YBa2Cu3O6+z phase are also outlined. More than 3500 experimental points obtained in a variety of experiments published in 110 papers have been processed in order to obtain the most reliable thermodynamic functions of phases over the temperature range from 250 to 1300 K and at pressures up to 100 kbar. The models with four sublattices were developed to describe the properties of the YBa2Cu3O6+z and Y2Ba4Cu7O14+w solid solutions. For the former solution, the model and the optimization procedure based on the maximum likelihood method with the linear error model is described in detail earlier.