THERMODYNAMICS OF SUPERCONDUCTING PHASES IN THE Y-BA-CU-O SYSTEM

A thermodynamic phenomenological model is presented to describe thermodynamic properties, oxygen content and ordering in solid solutions from the family of phases Y2Ba4Cu6+nO14+n. All available thermodynamic and structural data on YBa2Cu3O6+z (123) are critically evaluated with the help of this model resulting in a self-consistent set of thermodynamic functions of the 123 tetragonal (t) and orthorhombic (o) phases. This can be used for T < 100 K and for all compositions. The phase diagram of the pseudobinary system YBa2Cu3O6-YBa2Cu3O7 is calculated. The orthorhombic-to-tetragonal second-order phase transition curve terminates in a tricritical point where it bifurcates into two solvus lines that encircle the low-temperature two-phase (t+o) field. Thermodynamic properties of YBa2Cu4O8 (124), Y2Ba4Cu7O14+z (247) and some other phases from the homologous series Y2Ba4Cu6+nO14+n are estimated. The phase boundaries between the superconducting phases are calculated. They are plotted on an Ellingham and compared with data from the literature. The present thermodynamic data can be readily used for computing the conditions for thermodynamic stability of superconductors in various chemical environments.