Solid State Equilibria in the Ba-Cu-O System

Abstract

Thermodynamic modeling is performed for the Ba-Cu-O system, which is essential to a good understanding of phase and chemical equilibria in the Y-Ba-Cu-O and some other oxide systems containing high-temperature superconductors. A self-consistent set of thermodynamic functions of the phases BaO 2, BaCu 2O 2, BaCuO 2, Ba 2Cu 3O 5+ y and Ba 2CuO 3+ q is obtained. A variety of phase equilibria in the Ba-Cu-O system are calculated for a wide range of oxygen pressures and temperatures. The present thermodynamic data can be readily used for computing the phase equilibria and conditions for thermodynamic stability of oxide superconductors. It is detected that both BaCuO 2 and Ba 2CuO 3+ q have two stability boundaries, one at low temperatures and high oxygen pressures, and the other at high temperatures and low oxygen pressures. Critical analysis of phase equilibria in the Ba-Cu-O system makes it possible to explain a number of conflicting results encountered in the literature. These contradictions arise from solid state reactions between phases, which may be very slow due to kinetic problems.