Correlation Between in Situ Growth Conditions and Thermodynamic Stability Criteria for Y1Ba2Cu3Oy

Abstract

In a recent publication data from the literature on the successful insitu growth of YBCO thin films have been compared with thermodynamic data, in terms of oxygen pressure and temperature.(1) Here we expand on some of the issues raised there and report on some more recent results. The thermodynamic data for reference 1 were determined on bulk material in a galvanic cell,(2) in which the reversible equilibrium between Y1Ba2Cu3O6 and the products Y2BaCuO5, BaCuO2, and Cu2O were observed. The data are represented in Fig. 1 by the line marked y = 6.0, the oxygen concentration of the just stable tetragonal insulating compound. The other straight lines are schematic only. The one at y = 6.5 indicates roughly where the tetragonal to orthorombic transformation occurs, and the one at 6.9 indicates the fully oxygenated (Tc = 90 K) superconductor. There is not full agreement on the exact position of all of these lines, and also of the YBCO decomposition products.(3,4) A recent study at 850°C over a limited pressure range (atmospheric to 4 × 10−4 atm) has shown that the phase diagram has the complication of there being many phases that change with oxygen pressure.(4) In addition, it appears possible that other phases are nucleated at the growing surface as will be discussed later.