Kinetics and Thermodynamics of the Reaction of Molecular Oxygen with YBa3Cu3Ox

Abstract

The sensitivity of the properties of RBa2Cu3Ox materials to oxygen stoichiometry has been widely reported.1 This is a serious problem in the synthesis and processing of these compounds since many operations must be carried out at temperatures and oxygen partial pressures where bulk oxygen is in dynamic equilibrium with gas phase molecular oxygen. Therefore it is crucial for the rational design of processing strategies to understand how the equilibria and kinetics of oxygen stoichiometry in this material vary with temperature and pressure. In this paper we report work on two problems in this area: first, we describe the development of accurate methods for the determination of oxygen stoichiometry in experimental samples, and second, the use of these methods to investigate and analyze equilibrium oxygen contents over a range of temperatures and oxygen partial pressures. Equilibrium oxygen contents from 350 – 900°C at oxygen pressures of. 05, .21, and 1.00 atmospheres have been determined. The pressure dependence has also been examined over a wider range at selected temperatures. Most of this data can be adequately fit to a model which assumes that a single chemical equilibrium constant can describe the entire stoichiometry range from x=6 to 7. However, kinetic data suggests that the detailed mechanism must be more complex, and that the overall thermochemistry may reflect contributions from a number of interacting processes rather than any single elementary step. In concurrent work we have used these methods to develop sample series that can be used to examine magnetic properties2 and far IR spectra3 of these materials as a systematic function of oxygen content.